@article{wang_kota_he_blackburn_li_goshe_huber_clouse_2008, title={Sequential transphosphorylation of the BRI1/BAK1 receptor kinase complex impacts early events in brassinosteroid signaling}, volume={15}, ISSN={["1534-5807"]}, DOI={10.1016/j.devcel.2008.06.011}, abstractNote={Brassinosteroids (BRs) regulate plant development through a signal transduction pathway involving the BRI1 and BAK1 transmembrane receptor kinases. The detailed molecular mechanisms of phosphorylation, kinase activation, and oligomerization of the BRI1/BAK1 complex in response to BRs are uncertain. We demonstrate that BR-dependent activation of BRI1 precedes association with BAK1 in planta, and that BRI1 positively regulates BAK1 phosphorylation levels in vivo. BRI1 transphosphorylates BAK1 in vitro on specific kinase-domain residues critical for BAK1 function. BAK1 also transphosphorylates BRI1, thereby quantitatively increasing BRI1 kinase activity toward a specific substrate. We propose a sequential transphosphorylation model in which BRI1 controls signaling specificity by direct BR binding followed by substrate phosphorylation. The coreceptor BAK1 is then activated by BRI1-dependent transphosphorylation and subsequently enhances signaling output through reciprocal BRI1 transphosphorylation. This model suggests both conservation and distinct differences between the molecular mechanisms regulating phosphorylation-dependent kinase activation in plant and animal receptor kinases.}, number={2}, journal={DEVELOPMENTAL CELL}, author={Wang, Xiaofeng and Kota, Uma and He, Kai and Blackburn, Kevin and Li, Jia and Goshe, Michael B. and Huber, Steven C. and Clouse, Steven D.}, year={2008}, month={Aug}, pages={220–235} }
 @article{hernandez-sebastia_marsolais_saravitz_israel_dewey_huber_2005, title={Free amino acid profiles suggest a possible role for asparagine in the control of storage-product accumulation in developing seeds of low- and high-protein soybean lines}, volume={56}, ISSN={["1460-2431"]}, DOI={10.1093/jxb/eri191}, abstractNote={Several approaches were taken to examine the role of N-assimilate supply in the control of soybean (Glycine max) seed composition. In the first study, developing seeds were grown in vitro with D-[U-14C]sucrose (Suc) and different concentrations of Gln. Light stimulated carbon flux into oil and protein, and was required to sustain Suc uptake and anabolic processes under conditions of elevated nitrogen supply. High Gln supply resulted in higher transcript levels of beta-conglycinin and oleosin. In the second study, analyses of soluble amino acid pools in two genetically related lines, NC103 and NC106 (low- and high-seed protein, respectively) showed that, in the light, NC106 accumulated higher levels of Asn and several other amino acids in developing cotyledons compared with NC103, whereas at the seed coat and apoplast levels both lines were similar. In the dark, NC103 accumulated Gln, Arg, and its precursors, suggesting a reduced availability of organic acids required for amino acid interconversions, while NC106 maintained higher levels of the pyruvate-derived amino acids Val, Leu, and Ile. Comparing NC103 and NC106, differences in seed composition were reflected in steady-state transcript levels of storage proteins and the lipogenic enzyme multi-subunit acetyl CoA carboxylase. In the third study, a positive correlation (P < or = 0.05) between free Asn in developing cotyledons and seed protein content at maturity was confirmed in a comparison of five unrelated field-grown cultivars. The findings support the hypothesis that high seed-protein content in soybean is determined by the capacity of the embryo to take up nitrogen sources and to synthesize storage proteins. Asn levels are probably tightly regulated in the embryo of high-protein lines, and may act as a metabolic signal of seed nitrogen status.}, number={417}, journal={JOURNAL OF EXPERIMENTAL BOTANY}, author={Hernandez-Sebastia, C and Marsolais, F and Saravitz, C and Israel, D and Dewey, RE and Huber, SC}, year={2005}, month={Jul}, pages={1951–1963} }
 @article{wang_goshe_soderblom_phinney_kuchar_li_asami_yoshida_huber_clouse_2005, title={Identification and functional analysis of in vivo phosphorylation sites of the Arabidopsis BRASSINOSTEROID-INSENSITIVE1 receptor kinase}, volume={17}, ISSN={["1532-298X"]}, DOI={10.1105/tpc.105.031393}, abstractNote={Brassinosteroids (BRs) regulate multiple aspects of plant growth and development and require an active BRASSINOSTEROID-INSENSITIVE1 (BRI1) and BRI1-ASSOCIATED RECEPTOR KINASE1 (BAK1) for hormone perception and signal transduction. Many animal receptor kinases exhibit ligand-dependent oligomerization followed by autophosphorylation and activation of the intracellular kinase domain. To determine if early events in BR signaling share this mechanism, we used coimmunoprecipitation of epitope-tagged proteins to show that in vivo association of BRI1 and BAK1 was affected by endogenous and exogenous BR levels and that phosphorylation of both BRI1 and BAK1 on Thr residues was BR dependent. Immunoprecipitation of epitope-tagged BRI1 from Arabidopsis thaliana followed by liquid chromatography-tandem mass spectrometry (LC/MS/MS) identified S-838, S-858, T-872, and T-880 in the juxtamembrane region, T-982 in the kinase domain, and S-1168 in C-terminal region as in vivo phosphorylation sites of BRI1. MS analysis also strongly suggested that an additional two residues in the juxtamembrane region and three sites in the activation loop of kinase subdomain VII/VIII were phosphorylated in vivo. We also identified four specific BAK1 autophosphorylation sites in vitro using LC/MS/MS. Site-directed mutagenesis of identified and predicted BRI1 phosphorylation sites revealed that the highly conserved activation loop residue T-1049 and either S-1044 or T-1045 were essential for kinase function in vitro and normal BRI1 signaling in planta. Mutations in the juxtamembrane or C-terminal regions had only small observable effects on autophosphorylation and in planta signaling but dramatically affected phosphorylation of a peptide substrate in vitro. These findings are consistent with many aspects of the animal receptor kinase model in which ligand-dependent autophosphorylation of the activation loop generates a functional kinase, whereas phosphorylation of noncatalytic intracellular domains is required for recognition and/or phosphorylation of downstream substrates.}, number={6}, journal={PLANT CELL}, author={Wang, XF and Goshe, MB and Soderblom, EJ and Phinney, BS and Kuchar, JA and Li, J and Asami, T and Yoshida, S and Huber, SC and Clouse, SD}, year={2005}, month={Jun}, pages={1685–1703} }
 @article{tang_novitzky_griffin_huber_dewey_2005, title={Oleate desaturase enzymes of soybean: evidence of regulation through differential stability and phosphorylation}, volume={44}, ISSN={["1365-313X"]}, DOI={10.1111/j.1365-313X.2005.02535.x}, abstractNote={Summary}, number={3}, journal={PLANT JOURNAL}, author={Tang, GQ and Novitzky, WP and Griffin, HC and Huber, SC and Dewey, RE}, year={2005}, month={Nov}, pages={433–446} }
 @article{may_culpepper_cerny_coots_corkern_cothren_croon_ferreira_hart_hayes_et al._2004, title={Transgenic cotton with improved resistance to glyphosate herbicide}, volume={44}, DOI={10.2135/cropsci2004.0234}, abstractNote={Glyphosate [N-(phosphonomethyl)glycine] herbicide can be topically applied twice at rates as high as 0.84 kg a.e. (acid-equivalent) ha−1 to glyphosate-resistant cotton (Gossypium hirsutum L.) cultivars until the fourth true leaf stage, with the requirement of at least 10 d and two nodes of growth between applications. But, such cultivars are not reproductively resistant to glyphosate applied topically or imprecisely directed after the four-leaf stage because glyphosate can curtail pollen development and ovule fertilization, which potentially reduces yield. Extending glyphosate resistance past the four-leaf stage would provide growers with additional weed management options. Our objective was to test under field conditions glyphosate resistance of cotton germplasm transformed with gene constructs previously shown to impart extended glyphosate resistance in the greenhouse. Four or six transgenic cotton lines containing one of several constructs conferring extended glyphosate resistance, plus the current glyphosate-resistant control (‘Coker 312’-1445), were tested at nine U.S. locations in 2001. Within locations, treatment designs consisted of cross-classified arrangements of transgenic lines and glyphosate rates [0, 1.68, and 2.52 kg a.e. ha−1]. Treated plots received glyphosate over-the-top of cotton at four growth stages (3-, 6-, 10-, and 14-leaf crop stages). Compared with Coker 312-1445, extended glyphosate resistance was expressed as higher yields when glyphosate was applied topically at the four growth stages. Mature plant mapping confirmed extended glyphosate resistance of the new transgenic cotton through similar fruit distribution and weight with or without glyphosate treatment. The capability to apply glyphosate topically to cotton later in crop development will facilitate weed management and could reduce dependence on directed herbicides.}, number={1}, journal={Crop Science}, author={May, O. L. and Culpepper, A. S. and Cerny, R. E. and Coots, C. B. and Corkern, C. B. and Cothren, J. T. and Croon, K. A. and Ferreira, K. L. and Hart, J. L. and Hayes, R. M. and et al.}, year={2004}, pages={234–240} }
 @article{tang_hardin_dewey_huber_2003, title={A novel C-terminal proteolytic processing of cytosolic pyruvate kinase, its phosphorylation and degradation by the proteasome in developing soybean seeds}, volume={34}, ISSN={["0960-7412"]}, DOI={10.1046/j.1365-313X.2003.01711.x}, abstractNote={Summary}, number={1}, journal={PLANT JOURNAL}, author={Tang, GQ and Hardin, SC and Dewey, R and Huber, SC}, year={2003}, month={Apr}, pages={77–93} }
 @article{shen_clark_huber_2003, title={The C-terminal tail of Arabidopsis 14-3-3 omega functions as an autoinhibitor and may contain a tenth alpha-helix}, volume={34}, ISSN={["1365-313X"]}, DOI={10.1046/j.1365-313X.2003.01739.x}, abstractNote={Summary}, number={4}, journal={PLANT JOURNAL}, author={Shen, W and Clark, AC and Huber, SC}, year={2003}, month={May}, pages={473–484} }
 @article{athwal_huber_2002, title={Divalent cations and polyamines bind to loop 8 of 14-3-3 proteins, modulating their interaction with phosphorylated nitrate reductase}, volume={29}, ISSN={["1365-313X"]}, DOI={10.1046/j.0960-7412.2001.01200.x}, abstractNote={Summary}, number={2}, journal={PLANT JOURNAL}, author={Athwal, GS and Huber, SC}, year={2002}, month={Jan}, pages={119–129} }
 @article{long_zhao_rashotte_muday_huber_2002, title={Gravity-stimulated changes in auxin and invertase gene expression in maize pulvinal cells}, volume={128}, ISSN={["1532-2548"]}, DOI={10.1104/pp.010579}, abstractNote={Abstract}, number={2}, journal={PLANT PHYSIOLOGY}, author={Long, JC and Zhao, W and Rashotte, AM and Muday, GK and Huber, SC}, year={2002}, month={Feb}, pages={591–602} }
 @article{huber_mackintosh_kaiser_2002, title={Metabolic enzymes as targets for 14-3-3 proteins}, volume={50}, ISSN={["1573-5028"]}, DOI={10.1023/A:1021284002779}, abstractNote={The 14-3-3 proteins are binding proteins that have been shown to interact with a wide array of enzymes involved in primary biosynthetic and energy metabolism in plants. In most cases, the significance of binding of the 14-3-3 protein is not known. However, most of the interactions are phosphorylation-dependent and most of the known binding partners are found in the cytosol, while some may also be localized to plastids and mitochondria. In this review, we examine the factors that may regulate the binding of 14-3-3s to their target proteins, and discuss their possible roles in the regulation of the activity and proteolytic degradation of enzymes involved in primary carbon and nitrogen metabolism.}, number={6}, journal={PLANT MOLECULAR BIOLOGY}, author={Huber, SC and MacKintosh, C and Kaiser, WM}, year={2002}, month={Dec}, pages={1053–1063} }
 @article{huang_hardin_huber_2001, title={Identification of a novel phosphorylation motif for CDPKs: Phosphorylation of synthetic peptides lacking basic residues at P-3/P-4}, volume={393}, ISSN={["0003-9861"]}, DOI={10.1006/abbi.2001.2476}, abstractNote={The Ca(2+)-dependent protein kinases (CDPKs) are members of a large subfamily of protein kinases in plants that have been implicated in the control of numerous aspects of plant growth and development. One known substrate of the CDPKs is the ER-located ACA2 calcium pump, which is regulated by phosphorylation of Ser(45). In the present study, a synthetic peptide based on the known regulatory phosphorylation site (RRFRFTANLS(45)KRYEA) was efficiently phosphorylated in vitro by CDPKs but not a plant SNF1-related protein kinase. Phosphorylation of the Ser(45)-ACA2 peptide was surprising because the sequence lacks basic residues at P-3/P-4 (relative to the phosphorylated Ser at position P) that are considered to be essential recognition elements for CDPKs. We demonstrate that phosphorylation of the Ser(45)-ACA2 peptide is dependent on the cluster of basic residues found N-terminal (P-6 to P-9) as well as C-terminal (P + 1/P + 2) to the phosphorylated Ser. The results establish a new general phosphorylation motif for CDPKs: [Basic-Basic-X-Basic]-phi-X(4)-S/T-X-Basic (where phi is a hydrophobic residue). The motif predicts a number of new phosphorylation sites in plant proteins. Evidence is presented that the novel motif may explain the phosphorylation by CDPKs of Ser271 in the aquaporin PM28A.}, number={1}, journal={ARCHIVES OF BIOCHEMISTRY AND BIOPHYSICS}, author={Huang, JZ and Hardin, SC and Huber, SC}, year={2001}, month={Sep}, pages={61–66} }
 @article{huang_huber_2001, title={Phosphorylation of synthetic peptides by a CDPK and plant SNF1-related protein kinase. Influence of proline and basic amino acid residues at selected positions}, volume={42}, ISSN={["0032-0781"]}, DOI={10.1093/pcp/pce137}, abstractNote={Spinach (Spinacia oleracea L.) leaf sucrose-phosphate synthase (SPS) can be inactivated by phosphorylation of Ser-158 by calmodulin-like domain protein kinases (CDPKs) or SNF1-related protein kinases (SnRK1) in vitro. While the phosphorylation site sequence is relatively conserved, most of the deduced sequences of SPS from dicot species surrounding the Ser-158 regulatory phosphorylation site contain a Pro residue at P-4 (where P is the phosphorylated Ser); spinach is the exception and contains an Arg at P-4. We show that a Pro at P-4 selectively inhibits phosphorylation of the peptide by a CDPK relative to a SnRK1. The presence of a Pro at P-4, by allowing a tight turn in the peptide substrate, may interfere with proper binding of residues at P-5 and beyond. Both kinases had greater activity with peptides having basic residues at P-6 and P+5 (in addition to the known requirement for an Arg at P-3/P-4), and when the residue at P-6 was a His, the pH optimum for phosphorylation of the peptide was acid shifted. The results are used to predict proteins that may be selectively phosphorylated by SnRK1s (as opposed to CDPKs), such as SPS in dicot species, or may be phosphorylated in a pH-dependent manner.}, number={10}, journal={PLANT AND CELL PHYSIOLOGY}, author={Huang, JZ and Huber, SC}, year={2001}, month={Oct}, pages={1079–1087} }
 @article{kaiser_huber_2001, title={Post-translational regulation of nitrate reductase: mechanism, physiological relevance and environmental triggers}, volume={52}, ISSN={["0022-0957"]}, DOI={10.1093/jexbot/52.363.1981}, abstractNote={Assimilatory nitrate reductase (NR) of higher plants is a most interesting enzyme, both from its central function in plant primary metabolism and from the complex regulation of its expression and control of catalytic activity and degradation. Here, present knowledge about the mechanism of post-translational regulation of NR is summarized and the properties of the regulatory enzymes involved (protein kinases, protein phosphatases and 14-3-3-binding proteins) are described. It is shown that light and oxygen availability are the major external triggers for the rapid and reversible modulation of NR activity, and that sugars and/or sugar phosphates are the internal signals which regulate the protein kinase(s) and phosphatase. It is also demonstrated that stress factors like nitrate deficiency and salinity have remarkably little direct influence on the NR activation state. Further, changes in NR activity measured in vitro are not always associated with changes in nitrate reduction rates in vivo, suggesting that NR can be under strong substrate limitation. The degradation and half-life of the NR protein also appear to be affected by NR phosphorylation and 14-3-3 binding, as NR activation always correlates positively with its stability. However, it is not known whether the molecular form of NR in vivo affects its susceptibility to proteolytic degradation, or whether factors that affect the NR activation state also independently affect the activity or induction of the NR protease(s). A second and potentially important function of NR, the production of nitric oxide (NO) from nitrite is briefly described, but it remains to be determined whether NR produces NO for pathogen/stress signalling in vivo.}, number={363}, journal={JOURNAL OF EXPERIMENTAL BOTANY}, author={Kaiser, WM and Huber, SC}, year={2001}, month={Oct}, pages={1981–1989} }
 @misc{toroser_huber_2000, title={Carbon and nitrogen metabolism and reversible protein phosphorylation}, volume={32}, ISBN={["0-12-005932-0"]}, ISSN={["2162-5948"]}, DOI={10.1016/S0065-2296(00)32032-8}, abstractNote={Publisher Summary This chapter focuses on the regulation of activity of several enzymes involved in carbon- and nitrogen-metabolisms that are phosphorylated by either calmodulin-like domain protein kinases (CDPKs) or sucrose nonfermenting-1 (SNF1)-related protein. Coordination between carbohydrate metabolism and nitrogen assimilation is essential to avoid direct competition, and it potentially involves control at several levels including gene expression, membrane transport, and enzyme activity. One important mechanism that may impact all three levels is reversible protein phosphorylation. Recent studies have also identified several other enzymes, including trehalose-6-phosphate synthase and glutamine synthetases, as phosphoproteins because they interact with 14-3-3 proteins in a phosphorylation-dependent manner. Several transport activities that may impact metabolism either directly or indirectly may also be controlled by phosphorylation. Of particular importance is the possible regulation by phosphorylation of ion and solute transport, for example, the plasma membrane H + -adenosine triphosphate (ATP)ase, plasma membrane K + channel, and the sucrose transporter.}, journal={ADVANCES IN BOTANICAL RESEARCH INCORPORATING ADVANCES IN PLANT PATHOLOGY, VOL 32}, author={Toroser, D and Huber, SC}, year={2000}, pages={435–458} }
 @article{athwal_lombardo_huber_masters_fu_huber_2000, title={Modulation of 14-3-3 protein interactions with target polypeptides by physical and metabolic effectors}, volume={41}, DOI={10.1093/pcp/41.4.523}, abstractNote={The proteins commonly referred to as 14-3-3s have recently come to prominence in the study of protein:protein interactions, having been shown to act as allosteric or steric regulators and possibly scaffolds. The binding of 14-3-3 proteins to the regulatory phosphorylation site of nitrate reductase (NR) was studied in real-time by surface plasmon resonance, using primarily an immobilized synthetic phosphopeptide based on spinach NR-Ser543. Both plant and yeast 14-3-3 proteins were shown to bind the immobilized peptide ligand in a Mg2+-stimulated manner. Stimulation resulted from a reduction in KD and an increase in steady-state binding level (Req). As shown previously for plant 14-3-3s, fluorescent probes also indicated that yeast BMH2 interacted directly with cations, which bind and affect surface hydrophobicity. Binding of 14-3-3s to the phosphopeptide ligand occurred in the absence of divalent cations when the pH was reduced below neutral, and the basis for enhanced binding was a reduction in K(D). At pH 7.5 (+Mg2+), AMP inhibited binding of plant 14-3-3s to the NR based peptide ligand. The binding of AMP to 14-3-3s was directly demonstrated by equilibrium dialysis (plant), and from the observation that recombinant plant 14-3-3s have a low, but detectable, AMP phosphatase activity.}, number={4}, journal={Plant and Cell Physiology}, author={Athwal, G. S. and Lombardo, C. R. and Huber, J. L. and Masters, S. C. and Fu, H. A. and Huber, S. C.}, year={2000}, pages={523–533} }
 @article{oh_ray_huber_asara_gage_clouse_2000, title={Recombinant brassinosteroid insensitive 1 receptor-like kinase autophosphorylates on serine and threonine residues and phosphorylates a conserved peptide motif in vitro}, volume={124}, ISSN={["1532-2548"]}, DOI={10.1104/pp.124.2.751}, abstractNote={Abstract}, number={2}, journal={PLANT PHYSIOLOGY}, author={Oh, MH and Ray, WK and Huber, SC and Asara, JM and Gage, DA and Clouse, SD}, year={2000}, month={Oct}, pages={751–765} }
 @article{toroser_plaut_huber_2000, title={Regulation of a plant SNF1-related protein kinase by glucose-6-phosphate}, volume={123}, ISSN={["0032-0889"]}, DOI={10.1104/pp.123.1.403}, abstractNote={Abstract}, number={1}, journal={PLANT PHYSIOLOGY}, author={Toroser, D and Plaut, Z and Huber, SC}, year={2000}, month={May}, pages={403–411} }
 @misc{winter_huber_2000, title={Regulation of sucrose metabolism in higher plants: Localization and regulation of activity of key enzymes}, volume={19}, ISSN={["1549-7836"]}, DOI={10.1016/s0735-2689(01)80002-2}, abstractNote={Sucrose (Suc) plays a central role in plant growth and development. It is a major end product of photosynthesis and functions as a primary transport sugar and in some cases as a direct or indirect regulator of gene expression. Research during the last 2 decades has identified the pathways involved and which enzymes contribute to the control of flux. Availability of metabolites for Suc synthesis and ‘demand’ for products of sucrose degradation are important factors, but this review specifically focuses on the biosynthetic enzyme sucrose-phosphate synthase (SPS), and the degradative enzymes, sucrose synthase (SuSy), and the invertases. Recent progress has included the cloning of genes encoding these enzymes and the elucidation of posttranslational regulatory mechanisms. Protein phosphorylation is emerging as an important mechanism controlling SPS activity in response to various environmental and endogenous signals. In terms of Suc degradation, invertase-catalyzed hydrolysis generally has been associated with cell expansion, whereas SuSy-catalyzed metabolism has been linked with biosynthetic processes (e.g., cell wall or storage products). Recent results indicate that SuSy may be localized in multiple cellular compartments: (1) as a soluble enzyme in the cytosol (as traditionally assumed); (2) associated with the plasma membrane; and (3) associated with the actin cytoskel-eton. Phosphorylation of SuSy has been shown to occur and may be one of the factors controlling localization of the enzyme. The purpose of this review is to summarize some of the recent developments relating to regulation of activity and localization of key enzymes involved in sucrose metabolism in plants.}, number={1}, journal={CRITICAL REVIEWS IN PLANT SCIENCES}, author={Winter, H and Huber, SC}, year={2000}, pages={31–67} }
 @misc{winter_huber_2000, title={Regulation of sucrose metabolism in higher plants: Localization and regulation of activity of key enzymes}, volume={35}, ISSN={["1549-7798"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0033829914&partnerID=MN8TOARS}, DOI={10.1080/10409230008984165}, abstractNote={ABSTRACT Sucrose (Sue) plays a central role in plant growth and development. It is a major end product of photosynthesis and functions as a primary transport sugar and in some cases as a direct or indirect regulator of gene expression. Research during the last 2 decades has identified the pathways involved and which enzymes contribute to the control of flux. Availability of metabolites for Sue synthesis and 'demand' for products of sucrose degradation are important factors, but this review specifically focuses on the biosynthetic enzyme sucrose-phosphate synthase (SPS), and the degradative enzymes, sucrose synthase (SuSy), and the invertases. Recent progress has included the cloning of genes encoding these enzymes and the elucidation of posttranslational regulatory mechanisms. Protein phosphorylation is emerging as an important mechanism controlling SPS activity in response to various environmental and endogenous signals. In terms of Sue degradation, invertase-catalyzed hydrolysis generally has been associated with cell expansion, whereas SuSy-catalyzed metabolism has been linked with biosynthetic processes (e.g., cell wall or storage products). Recent results indicate that SuSy may be localized in multiple cellular compartments: (1) as a soluble enzyme in the cytosol (as traditionally assumed); (2) associated with the plasma membrane; and (3) associated with the actin cytoskeleton. Phosphorylation of SuSy has been shown to occur and may be one of the factors controlling localization of the enzyme. The purpose of this review is to summarize some of the recent developments relating to regulation of activity and localization of key enzymes involved in sucrose metabolism in plants.}, number={4}, journal={CRITICAL REVIEWS IN BIOCHEMISTRY AND MOLECULAR BIOLOGY}, author={Winter, H and Huber, SC}, year={2000}, pages={253–289} }
 @article{kaiser_weiner_huber_1999, title={Nitrate reductase in higher plants: A case study for transduction of environmental stimuli into control of catalytic activity}, volume={105}, DOI={10.1034/j.1399-3054.1999.105225.x}, abstractNote={In higher plants, cytosolic NAD(P)H‐nitrate reductase (NR) is rapidly modulated by environmental conditions such as light, CO2, or oxygen availability. In leaves, NR is activated by photosynthesis, reaching an activation state of 60–80%. In the dark, or after stomatal closure, leaf NR is inactivated down to 20 or 40% of its maximum activity. In roots, hypoxia or anoxia activate NR, whereas high oxygen supply inactivates NR. Spinach leaf NR is inactivated by phosphorylation of serine 543 and subsequent Mg2+‐dependent binding of 14‐3‐3 proteins at, or close to, this phosphorylation site. At least three different protein kinases (NR‐PK) have been identified in spinach leaves that are able to phosphorylate NR on serine 543. Two of them show up as calmodulin‐like domain protein kinases (CDPKs), and one as a SNF1‐like protein kinase. Dephosphorylation of serine 543 is catalyzed by a Mg2+‐dependent protein phosphatase and by a type 2A protein phosphatase (NR‐PP), which is regulated by a trimer/dimer interconversion. The NR‐PKs, NR‐PPs, and 14‐3‐3s are present even in NR‐depleted plant tissues. Artificial activation of NR in vivo is achieved by cellular acidification, by respiratory inhibitors, or by mannose feeding. As for anoxia, these treatments seem to act, at least in part, via cytosolic acidification, mediated by low cytosolic ATP levels. Activation is also achieved by ionophore‐induced release of divalent cations from the cytosol. In addition, cytosolic AMP and phosphate esters seem to regulate NR‐PK and NR‐PP activities, thereby adapting NR activity within minutes to the changing environment.}, number={2}, journal={Physiologia Plantarum}, author={Kaiser, W. M. and Weiner, H. and Huber, S. C.}, year={1999}, pages={385–390} }
 @article{toroser_mcmichael_krause_kurreck_sonnewald_stitt_huber_1999, title={Site-directed mutagenesis of serine 158 demonstrates its role in spinach leaf sucrose-phosphate synthase modulation}, volume={17}, ISSN={["0960-7412"]}, DOI={10.1046/j.1365-313X.1999.00389.x}, abstractNote={Summary}, number={4}, journal={PLANT JOURNAL}, author={Toroser, D and McMichael, R and Krause, KP and Kurreck, J and Sonnewald, U and Stitt, M and Huber, SC}, year={1999}, month={Feb}, pages={407–413} }
 @article{toroser_huber_1998, title={3-hydroxy-3-methylglutaryl-coenzyme a reductase kinase and sucrose-phosphate synthase kinase activities in cauliflower florets: Ca2+ dependence and substrate specificities}, volume={355}, ISSN={["0003-9861"]}, DOI={10.1006/abbi.1998.0740}, abstractNote={Plant 3-hydroxy-3-methylglutaryl-CoA reductase(HMGR; EC 1.1.1.34) and sucrose-phosphate synthase (SPS; EC 2.4.1.14) and synthetic peptides designed from the known phosphorylation sites of plant HMGR (SAMS*: KSHMKYNRSTKDVK), rat acetyl-CoA carboxylase (SAMS: HMRSAMSGLHLVKRR), spinach SPS (SP2: GRRJRRISSVEJJDKK), and spinach NADH:nitrate reductase (NR6: GPTLKRTASTPFJNTTSK) were used to characterize kinase activities from cauliflower (Brassica oleracea L. ) inflorescences. The three major peaks of protein kinase activity resolved by anion-exchange FPLC are homologs of those observed previously in spinach leaves and thus are designated PKI, PKIV, and PKIII, listed in order of elution. PKIV was the most active in terms of phosphorylation and inactivation of recombinant Nicotiana HMGR and was also strictly Ca2+ dependent. The novel aspects are that PKIII has not been detected in previous cauliflower studies, that SAMS* is a more specific peptide substrate to identify potential HMGR kinases, and that the major HMGR kinase in cauliflower is Ca2+ dependent. Of the three major kinases that phosphorylated the SP2 peptide only PKI (partially Ca2+ sensitive) and PKIII (Ca2+ insensitive) inactivated native spinach leaf SPS. Cauliflower extracts contained endogenous SPS that was inactivated by endogenous kinase(s) in an ATP-dependent manner and this may be one of the substrate target proteins for PKI and/or PKIII. The substrate specificity of the three kinase peaks was studied using synthetic peptide variants of the SP2 sequence. All three kinases had a strong preference for peptides with a basic residue at P-6 (as in SP2 and SAMS*; SAMS has a free amino terminus at this position) or a Pro at P-7 (as in NR6). This requirement for certain residues at P-6 or P-7 was not recognized in earlier studies but appears to be a general requirement. In plant HMGR, a conserved His residue at P-6 is involved directly in catalysis and this may explain why substrates reduced HMGR phosphorylation in vitro.}, number={2}, journal={ARCHIVES OF BIOCHEMISTRY AND BIOPHYSICS}, author={Toroser, D and Huber, SC}, year={1998}, month={Jul}, pages={291–300} }
 @article{athwal_huber_huber_1998, title={Biological significance of divalent metal ion binding to 14-3-3 proteins in relationship to nitrate reductase inactivation}, volume={39}, ISSN={["0032-0781"]}, DOI={10.1093/oxfordjournals.pcp.a029303}, abstractNote={In this report we address two questions regarding the regulation of phosphorylated nitrate reductase (pNR; EC 1.6.6.1) by 14-3-3 proteins. The first concerns the requirement for millimolar concentrations of a divalent cation in order to form the inactive pNR:14-3-3 complex at pH 7.5. The second concerns the reduced requirement for divalent cations at pH 6.5. In answering these questions we highlight a possible general mechanism involved in the regulation of 14-3-3 binding to target proteins. We show that divalent cations (e.g. Ca2+, Mg2+ and Mn2+) bind directly to 14-3-3s, and as a result cause a conformational change, manifested as an increase in surface hydrophobicity. A similar change is also obtained by decreasing the pH from pH 7.5 to pH 6.5, in the absence of divalent cations, and we propose that protonation of amino acid residues brings about a similar effect to metal ion binding. A possible regulatory mechanism, where the 14-3-3 protein has to be "primed" prior to binding a target protein, is discussed.}, number={10}, journal={PLANT AND CELL PHYSIOLOGY}, author={Athwal, GS and Huber, JL and Huber, SC}, year={1998}, month={Oct}, pages={1065–1072} }
 @article{winter_huber_huber_1998, title={Identification of sucrose synthase as an actin-binding protein}, volume={430}, ISSN={["1873-3468"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032479338&partnerID=MN8TOARS}, DOI={10.1016/S0014-5793(98)00659-0}, abstractNote={Several lines of evidence indicate that sucrose synthase (SuSy) binds both G‐ and F‐actin: (i) presence of SuSy in the Triton X‐100‐insoluble fraction of microsomal membranes (i.e. crude cytoskeleton fraction); (ii) co‐immunoprecipitation of actin with anti‐SuSy monoclonal antibodies; (iii) association of SuSy with in situ phalloidin‐stabilized F‐actin filaments; and (iv) direct binding to F‐actin, polymerized in vitro. Aldolase, well known to interact with F‐actin, interfered with binding of SuSy, suggesting that a common or overlapping binding site may be involved. We postulate that some of the soluble SuSy in the cytosol may be associated with the actin cytoskeleton in vivo.}, number={3}, journal={FEBS LETTERS}, author={Winter, H and Huber, JL and Huber, SC}, year={1998}, month={Jul}, pages={205–208} }
 @article{purcino_arellano_athwal_huber_1998, title={Nitrate effect on carbon and nitrogen assimilating enzymes of maize hybrids representing seven eras of breeding}, volume={43}, number={2}, journal={Maydica}, author={Purcino, A. A. C. and Arellano, C. and Athwal, G. S. and Huber, S. C.}, year={1998}, pages={83–94} }
 @article{athwal_huber_huber_1998, title={Phosphorylated nitrate reductase and 14-3-3 proteins - Site of interaction, effects of ions, and evidence for an AMP-binding site on 14-3-3 proteins}, volume={118}, ISSN={["0032-0889"]}, DOI={10.1104/pp.118.3.1041}, abstractNote={Abstract}, number={3}, journal={PLANT PHYSIOLOGY}, author={Athwal, GS and Huber, JL and Huber, SC}, year={1998}, month={Nov}, pages={1041–1048} }
 @article{toroser_athwal_huber_1998, title={Site-specific regulatory interaction between spinach leaf sucrose-phosphate synthase and 14-3-3 proteins}, volume={435}, ISSN={["0014-5793"]}, DOI={10.1016/S0014-5793(98)01048-5}, abstractNote={We report an Mg2+‐dependent interaction between spinach leaf sucrose‐phosphate synthase (SPS) and endogenous 14‐3‐3 proteins, as evidenced by co‐elution during gel filtration and co‐immunoprecipitation. The content of 14‐3‐3s associated with an SPS immunoprecipitate was inversely related to activity, and was specifically reduced when tissue was pretreated with 5‐aminoimidazole‐4‐carboxamide riboside, suggesting metabolite control in vivo. A synthetic phosphopeptide based on Ser‐229 was shown by surface plasmon resonance to bind a recombinant plant 14‐3‐3, and addition of the phosphorylated SPS‐229 peptide was found to stimulate the SPS activity of an SPS:14‐3‐3 complex. Taken together, the results suggest a regulatory interaction of 14‐3‐3 proteins with Ser‐229 of SPS.}, number={1}, journal={FEBS LETTERS}, author={Toroser, D and Athwal, GS and Huber, SC}, year={1998}, month={Sep}, pages={110–114} }
 @article{kaiser_huber_1997, title={Correlation between apparent activation state of nitrate reductase (NR), NR hysteresis and degradation of NR protein}, volume={48}, ISSN={["0022-0957"]}, DOI={10.1093/jxb/48.7.1367}, abstractNote={Nitrate reductase (NR) activity was measured in extracts from spinach leaves exposed to light or prolonged darkness, and to various treatments provoking an artificial activation of the enzyme in the dark. NR activity was determined immediately either in the presence of Mg 2+ , which gives an estimation of the putative (actual) activity in situ (NR act ), or in EDTA without preincubation, which gives an intermediate activity (NR int ), or after a 30 min preincubation with EDTA plus AMP plus Pi, which gives the maximum NR activity (NR max ). NR max is thought to reflect total NR protein contents. In the dark, NR act was usually very low. Dark inactivation was prevented or reversed by feeding AICAR (5-aminoimidazole-4-carboxiamide ribonucleoside), or by anaerobiosis, acid treatment or addition of uncoupler. During prolonged darkness, NR max decreased, indicating net protein degradation with a half-time of 21 h. Conditions which caused an activation (dephosphorylation) of NR in the dark, slowed down NR protein degradation. This was also confirmed by Western blotting. Blockage of cytosolic protein synthesis with cycloheximide (CHX) did not accelerate NR protein degradation. In contrast, after 5 h in the dark, NR act increased in CHX-treated leaves. As this increase was sensitive to PP2A-inhibitors, it was probably due to NR dephosphorylation. However, extractable NR kinase and NR phosphatase activities were not changed by CHX treatment. Apparently, CHX interacted with the NR regulatory system indirectly by affecting turnover of another protein. The increase from NR int to NR max which occurred during preincubation of the leaf extract with EDTA plus AMP plus Pi was insensitive to PP2A inhibitors and was interpreted as a hysteretic conversion of NR from an inactive into an active form. Hysteretic activation was positively correlated to the NR phosphorylation state. A model is presented to explain the hysteretic behaviour of NR in relation to NR phosphorylation/ dephosphorylation. Overall, the data indicate that NR protein phosphorylation not only controls the catalytic activity of NR, but also acts as a signal for NR protein degradation, with phospho-NR probably being a better substrate for protein degradation than the dephospho-form.}, number={312}, journal={JOURNAL OF EXPERIMENTAL BOTANY}, author={Kaiser, WM and Huber, SC}, year={1997}, month={Jul}, pages={1367–1374} }
 @article{petracek_dickey_huber_thompson_1997, title={Light-regulated changes in abundance and polyribosome association of ferredoxin mRNA are dependent on photosynthesis}, volume={9}, DOI={10.2307/3870586}, number={12}, journal={Plant Cell}, author={Petracek, M. E. and Dickey, L. F. and Huber, S. C. and Thompson, William}, year={1997}, pages={2291–2300} }
 @article{winter_huber_huber_1997, title={Membrane association of sucrose synthase: changes during the graviresponse and possible control by protein phosphorylation}, volume={420}, ISSN={["1873-3468"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031590792&partnerID=MN8TOARS}, DOI={10.1016/S0014-5793(97)01506-8}, abstractNote={Sucrose synthase (SuSy) plays an important role in sucrose degradation and occurs both as a soluble and as a membrane‐associated enzyme in higher plants. We show that membrane association can vary in vivo in response to gravistimulation, apparently involving SuSy dephosphorylation, and is a reversible process in vitro. Phosphorylation of SuSy has little effect on its activity but decreases its surface hydrophobicity as reported with the fluorescent probe bis‐ANS. We postulate that phosphorylation of SuSy (and perhaps other membrane proteins) is involved in the release of the membrane‐bound enzyme in part as a result of decreased surface hydrophobicity.}, number={2-3}, journal={FEBS LETTERS}, author={Winter, H and Huber, JL and Huber, SC}, year={1997}, month={Dec}, pages={151–155} }
 @article{toroser_huber_1997, title={Protein phosphorylation as a mechanism for osmotic-stress activation of sucrose-phosphate synthase in spinach leaves}, volume={114}, ISSN={["0032-0889"]}, DOI={10.1104/pp.114.3.947}, abstractNote={Abstract}, number={3}, journal={PLANT PHYSIOLOGY}, author={Toroser, D and Huber, SC}, year={1997}, month={Jul}, pages={947–955} }
 @article{huber_huber_liao_gage_mcmichael_chourey_hannah_koch_1996, title={Phosphorylation of serine-15 of maize leaf sucrose synthase - Occurrence in vivo and possible regulatory significance}, volume={112}, ISSN={["1532-2548"]}, DOI={10.1104/pp.112.2.793}, abstractNote={Abstract}, number={2}, journal={PLANT PHYSIOLOGY}, author={Huber, C and Huber, JL and Liao, PC and Gage, DA and McMichael, RW and Chourey, PS and Hannah, LC and Koch, K}, year={1996}, month={Oct}, pages={793–802} }
 @misc{huber_bachmann_huber_1996, title={Post-translational regulation of nitrate reductase activity: A role for Ca2+ and 14-3-3 proteins}, volume={1}, ISSN={["1878-4372"]}, DOI={10.1016/S1360-1385(96)10046-7}, abstractNote={The control of nitrate reductase (NR) activity by reversible protein phosphorylation is a two-stage process: the enzyme is reversibly phosphorylated, and the phosphorylated form then binds an inhibitor protein. Recent results indicate that the phosphorylation reaction is Ca2+-dependent and that the inhibitor is a member of the 14-3-3 family of proteins. These results suggest the convergence of several signal transduction pathways in the control of nitrate assimilation. Recent results also implicate metabolites and 5′-AMP as possible effectors regulating the phosphorylation status of NR in vivo.}, number={12}, journal={TRENDS IN PLANT SCIENCE}, author={Huber, SC and Bachmann, M and Huber, JL}, year={1996}, month={Dec}, pages={432–438} }
 @misc{huber_huber_1996, title={Role and regulation of sucrose-phosphate synthase in higher plants}, volume={47}, ISSN={["1040-2519"]}, DOI={10.1146/annurev.arplant.47.1.431}, abstractNote={ ▪ Abstract  Sucrose-phosphate synthase (SPS; E.C. 2.4.1.14) is the plant enzyme thought to play a major role in sucrose biosynthesis. In photosynthetic and nonphotosynthetic tissues, SPS is regulated by metabolites and by reversible protein phosphorylation. In leaves, phosphorylation modulates SPS activity in response to light/dark signals and end-product accumulation. SPS is phosphorylated on multiple seryl residues in vivo, and the major regulatory phosphorylation site involved is Ser158 in spinach leaves and Ser162 in maize leaves. Regulation of the enzymatic activity of SPS appears to involve calcium, metabolites, and novel “coarse” control of the protein phosphatase that activates SPS. Activation of SPS also occurs during osmotic stress of leaf tissue in darkness, which may function to facilitate sucrose formation for osmoregulation. Manipulation of SPS expression in vivo confirms the role of this enzyme in the control of sucrose biosynthesis. }, journal={ANNUAL REVIEW OF PLANT PHYSIOLOGY AND PLANT MOLECULAR BIOLOGY}, author={Huber, SC and Huber, JL}, year={1996}, pages={431–444} }
 @article{huber_huber_1995, title={Metabolic activators of spinach leaf nitrate reductase: Effects on enzymatic activity and dephosphorylation by endogenous phosphatases}, volume={196}, DOI={10.1007/bf00193232}, number={1}, journal={Planta}, author={Huber, S. C. and Huber, J. L.}, year={1995}, pages={180} }
 @article{huber_huber_kaiser_1994, title={Differential response of nitrate reductase and sucrose-phosphate synthase-activation to inorganic and organic salts, in vitro and in situ}, volume={92}, DOI={10.1034/j.1399-3054.1994.920216.x}, number={2}, journal={Physiologia Plantarum}, author={Huber, S. C. and Huber, J. L. and Kaiser, W. M.}, year={1994}, pages={302} }
 @article{huber_huber_campbell_redinbaugh_1992, title={Apparent dependence of the light activation of nitrate reductase and sucrose-phosphate synthase activities in spinach leaves on protein synthesis}, volume={33}, DOI={10.1093/oxfordjournals.pcp.a078299}, abstractNote={The activity of spinach leaf nitrate reductase (NR) responds rapidly and reversibly to light/dark transitions by a mechanism which is correlated with changes in protein phosphorylation (J.L. Huber, S.C. Huber, W.H. Campbell and M.G. Redinbaugh (1992) Arch. Biochem. Biophys. in press). Phosphorylation of the NR protein appears to increase sensitivity of the enzyme to Mg2+ inhibition, without affecting activity in the absence of Mg2+. In the present study, we have compared the light/dark modulation of sucrose-phosphate synthase (SPS; also known to be regulated by protein phosphorylation) and NR (assayed±Mg2+) in spinach leaves exposed to extended darkness and treated with inhibitors of gene expression. When intact plants were subjected to extended darkness, the capacity for light-dependent increase in NR protein and activation, inferred (indirectly) from the enzyme activities with and without Mg2+ declined even though the capacity for photosynthesis and activation of SPS remained constant. The component of the in situ light activation of both enzymes that appears to involve dephosphorylation could be prevented and reversed by inhibitors of transcription (cordycepin) and cytoplasmic protein synthesis (cycloheximide), in the absence of a large effect on photosynthetic activity. As expected, okadaic acid, a potent and specific inhibitor of type 1 and 2A protein phosphatases, also prevented and reversed the activation of both enzymes in vivo. The results are taken to suggest that the protein phosphatase(s) that dephosphorylates and activates both enzymes is itself activated by light in a process that involves protein synthesis. Overall, NR and SPS share many common features of control, but are not identical in terms of regulation.}, number={5}, journal={Plant and Cell Physiology}, author={Huber, S. C. and Huber, J. L. and Campbell, W. H. and Redinbaugh, M. G.}, year={1992}, pages={639} }
 @article{huber_hanson_1992, title={CARBON PARTITIONING AND GROWTH OF A STARCHLESS MUTANT OF NICOTIANA-SYLVESTRIS}, volume={99}, ISSN={["0032-0889"]}, DOI={10.1104/pp.99.4.1449}, abstractNote={We have further characterized the photosynthetic carbohydrate metabolism and growth of a starchless mutant (NS 458) of Nicotiana sylvestris that is deficient in plastid phosphoglucomutase (Hanson KR, McHale NA [1988] Plant Physiol 88: 838-844). In general, the mutant had only slightly lower rates of photosynthesis under ambient conditions than the wild type. However, accumulation of soluble sugars (primarily hexose sugars) in source leaves of the mutant compensated for only about half of the carbon stored as starch in the wild type. Therefore, the export rate was slightly higher in the mutant relative to the wild type. Starch in the wild type and soluble sugars in the mutant were used to support plant growth at night. Growth of the mutant was progressively restricted, relative to wild type, when plants were grown under shortened photoperiods. When grown under short days, leaf expansion of the mutant was greater during the day, but was restricted at night relative to wild-type leaves, which expanded primarily at night. We postulate that restricted growth of the mutant on short days is the result of several factors, including slightly lower net photosynthesis and inability to synthesize starch in both source and sink tissues for use at night. In short-term experiments, increased "sink demand" on a source leaf (by shading all other source leaves) had no immediate effect on starch accumulation during the photoperiod in the wild type or on soluble sugar accumulation in the mutant. These results would be consistent with a transport limitation in N. sylvestris such that not all of the additional carbon flux into sucrose in the mutant can be exported from the leaf. Consequently, the mutant accumulates hexose sugars during the photoperiod, apparently as the result of sucrose hydrolysis within the vacuole by acid invertase.}, number={4}, journal={PLANT PHYSIOLOGY}, author={HUBER, SC and HANSON, KR}, year={1992}, month={Aug}, pages={1449–1454} }
 @article{huber_huber_campbell_redinbaugh_1992, title={COMPARATIVE-STUDIES OF THE LIGHT-MODULATION OF NITRATE REDUCTASE AND SUCROSE-PHOSPHATE SYNTHASE ACTIVITIES IN SPINACH LEAVES}, volume={100}, ISSN={["0032-0889"]}, DOI={10.1104/pp.100.2.706}, abstractNote={We recently obtained evidence that the activity of spinach (Spinacia oleracea L.) leaf nitrate reductase (NR) responds rapidly and reversibly to light/dark transitions by a mechanism that is strongly correlated with protein phosphorylation. Phosphorylation of the NR protein appears to increase sensitivity to Mg(2+) inhibition, without affecting activity in the absence of Mg(2+). In the present study, we have compared the light/dark modulation of sucrose-phosphate synthase (SPS), also known to be regulated by protein phosphorylation, and NR activities (assayed with and without Mg(2+)) in spinach leaves. There appears to be a physiological role for both enzymes in mature source leaves (production of sucrose and amino acids for export), whereas NR is also present and activated by light in immature sink leaves. In mature leaves, there are significant diurnal changes in SPS and NR activities (assayed under selective conditions where phosphorylation status affects enzyme activity) during a normal day/night cycle. With both enzymes, activities are highest in the morning and decline as the photoperiod progresses. For SPS, diurnal changes are largely the result of phosphorylation/dephosphorylation, whereas with NR, the covalent modification is super-imposed on changes in the level of NR protein. Accumulation of end products of photosynthesis in excised illuminated leaves increased maximum NR activity, reduced its sensitivity of Mg(2+) inhibition, and prevented the decline in activity with time in the light seen with attached leaves. In contrast, SPS was rapidly inactivated in excised leaves. Overall, NR and SPS share many common features of control but are not identical in terms of regulation in situ.}, number={2}, journal={PLANT PHYSIOLOGY}, author={HUBER, SC and HUBER, JL and CAMPBELL, WH and REDINBAUGH, MG}, year={1992}, month={Oct}, pages={706–712} }
 @article{huber_huber_1992, title={ROLE OF SUCROSE-PHOSPHATE SYNTHASE IN SUCROSE METABOLISM IN LEAVES}, volume={99}, ISSN={["0032-0889"]}, DOI={10.1104/pp.99.4.1275}, abstractNote={Sucrose is formed in the cytoplasm of leaf cells from triose phosphates exported from the chloroplast. Flux control is shared among key enzymes of the pathway, one of which is sucrose-phosphate synthase (SPS). Regulation of SPS by protein phosphorylation is important in vivo and may explain diurnal changes in SPS activity and carbon partitioning. The signal transduction pathway mediating the light activation of SPS in vivo appears to involve metabolites and novel "coarse" control of the protein phosphatase that dephosphorylates and activates SPS. Regulation of the phosphorylation of SPS may provide a general mechanism whereby sucrose formation is coordinated with the rate of photosynthesis and the rate of nitrate assimilation. There are apparent differences among species in the properties of SPS that may reflect different strategies for the control of carbon partitioning. The SPS gene has recently been cloned from maize; results of preliminary studies with transgenic tomato plants expressing high levels of maize SPS support the postulate that SPS activity can influence the partitioning of carbon between starch and sucrose.}, number={4}, journal={PLANT PHYSIOLOGY}, author={HUBER, SC and HUBER, JL}, year={1992}, month={Aug}, pages={1275–1278} }
 @article{huber_huber_1991, title={INVITRO PHOSPHORYLATION AND INACTIVATION OF SPINACH LEAF SUCROSE-PHOSPHATE SYNTHASE BY AN ENDOGENOUS PROTEIN-KINASE}, volume={1091}, ISSN={["0006-3002"]}, DOI={10.1016/0167-4889(91)90205-c}, abstractNote={(1) Partially purified preparations of spinach (Spinacia oleracea L.) leaf sucrose-phosphate synthase (SPS) contain an endogenous protein kinase that phosphorylates and inactivates the enzyme with [gamma-32P]ATP. (2) The kinetic effect of phosphorylation is to alter affinities for substrates and the effector inorganic phosphate without affecting maximum velocity. (3) Two-dimensional peptide mapping of tryptic digests of in vitro labeled SPS yielded two phosphopeptides (designated sites 5 and 7). Labeling of the two sites occurred equally with time, and both correlated with inactivation. Maximum inactivation was associated with incorporation of 1.5 to 2.0 mol P/mol SPS tetramer, and about 70% of the phosphoryl groups were incorporated into one of the sites (phosphopeptide 7). (4) Phosphorylation and inactivation were strongly inhibited by NaCl, and the presence of salt alters some characteristics of the kinase reaction. In the absence of salt, the apparent Km for Mg.ATP was estimated to be 5 microM. (5) The dependence of the rate of phosphorylation on SPS concentration suggested that SPS and the protein kinase are distinct enzymes, but have some tendency to associate especially in the presence of ethylene glycol. (6) Ca2+/EGTA and polyamines have no effect on the rate of phosphorylation, whereas polycations (polylysine, polybrene and protamine) are inhibitory. (7) Of the metabolic intermediates tested, Glc 6-P inhibited phosphorylation and inactivation of the enzyme. The inhibition was not antagonized by inorganic phosphate, which suggests that Glc 6-P may be an effector of the kinase, rather than the target protein. Regulation by Glc 6-P may be of physiological significance.}, number={3}, journal={BIOCHIMICA ET BIOPHYSICA ACTA}, author={HUBER, SC and HUBER, JL}, year={1991}, month={Feb}, pages={393–400} }
 @article{huber_huber_1991, title={REGULATION OF MAIZE LEAF SUCROSE-PHOSPHATE SYNTHASE BY PROTEIN-PHOSPHORYLATION}, volume={32}, ISSN={["0032-0781"]}, DOI={10.1093/oxfordjournals.pcp.a078083}, abstractNote={Studies were conducted to determine the potential for regulation of maize leaf sucrose-phosphate synthase (SPS) by protein phosphorylation. Highly activated enzyme, in desalted crude leaf extracts prepared from illuminated leaves, was inactivated in vitro in a time- and ATP-de-pendent manner. Partial purification of SPS by polyethylene glycol fractionation and Mono Q chromatography yielded enzyme that was not ATP-inactivated, possibly due to elimination of contaminating protein kinase. We used the partially purified SPS as substrate to identify an endogenous protein kinase. The protein kinase catalyzed the time- and ATP-dependent inacti-vation of SPS, and the apparent Km for Mg-ATP was estimated to be approximately 10μM. The partially purified maize SPS protein was phosphorylated in vitro using [y-32P]ATP and either the endogenous protein kinase or the catalytic subunit of cAMP-dependent protein kinase. The incorporation of radiolabel was closely paralleled by inactivation of the enzyme. These results provide the first evidence for regulation of maize leaf SPS by protein phosphorylation, which we postulate is the mechanism of light-dark regulation in vivo.}, number={3}, journal={PLANT AND CELL PHYSIOLOGY}, author={HUBER, SC and HUBER, JL}, year={1991}, month={Apr}, pages={319–326} }
 @article{huber_huber_1991, title={SALT ACTIVATION OF SUCROSE-PHOSPHATE SYNTHASE FROM DARKENED LEAVES OF MAIZE AND OTHER C-4 PLANTS}, volume={32}, ISSN={["0032-0781"]}, DOI={10.1093/oxfordjournals.pcp.a078084}, abstractNote={Maize leaf sucrose-phosphate synthase (SPS) has been shown to be inactivated by protein phosphorylation in vitro, which appears to be the mechanism of light modulation in situ [Huber and Huber (1991) Plant Cell Physiol. 32: 319–326]. The catalytic activity of the inactivated enzyme (dark form or in vitro inactivated form) was strongly stimulated by high ionic strength in the assay mixture and at 0.4 M KC1 reached activities similar to those obtained from illuminated leaves. Numerous salts were effective, but for most studies, 0.3 M KC1 was used. The salt-stimulation of enzyme activity was rapid and readily reversible and was antagonized by the presence of ethylene glycol in the assay. The presence of salt was also found to reduce the IC50 (concentration required for 50% inhibition) for p-chloromercuribenzenesulfonic acid. We postulate that phosphorylation of maize SPS induces a conformational change in the protein (that affects both maximum catalytic activity and sensitivity to Pi either through electrostatic or hydrophobic interactions which are affected by high ionic strength. Salt stimulation of the deactivated enzyme extracted from darkened leaves was observed for a variety of C-4 plants, but not for any of the C-3 species tested.}, number={3}, journal={PLANT AND CELL PHYSIOLOGY}, author={HUBER, SC and HUBER, JL}, year={1991}, month={Apr}, pages={327–333} }
 @article{huber_huber_1990, title={ACTIVATION OF SUCROSE-PHOSPHATE SYNTHASE FROM DARKENED SPINACH LEAVES BY AN ENDOGENOUS PROTEIN PHOSPHATASE}, volume={282}, ISSN={["0003-9861"]}, DOI={10.1016/0003-9861(90)90138-O}, abstractNote={Sucrose-phosphate synthase (SPS; EC 2.4.1.14) extracted from darkened spinach (Spinacia oleracea L.) leaves has a low activation state, defined as the ratio of activity measured with limiting substrates (plus the inhibitor Pi) to activity with saturating substrates (maximum velocity). Preincubation at 25 degrees C of desalted crude extracts from darkened leaves resulted in a time-dependent increase in activation state that was inhibited by Pi [IC50 (concentration causing 50% inhibition) approximately 3 mM], molybdate, okadaic acid (IC50 approximately 25 nM) and vanadate, but was stimulated by fluoride. The "spontaneous activation" of SPS in vitro was enhanced slightly by exogenous MgCl2 (up to 5 mM) and exhibited a pH optimum of 7.0 to 7.5. Radioactive phosphate incorporated into SPS during labeling of excised leaves with [32P]Pi in the dark was lost with time when extracts were incubated at 25 degrees C. This loss in radiolabel was substantially reduced by vanadate. These results provide direct evidence for action of an endogenous protein phosphatase(s) using SPS as substrate. The spontaneous activation achieved in vitro could be reversed by subsequent addition of 1 mM Mg.ATP; the activation/inactivation achieved in vitro was similar in magnitude to the dark-light regulation observed in vivo. Moreover, feeding okadaic acid to excised leaves in the dark blocked subsequent light activation of SPS without affecting photosynthetic rate. These results are consistent with the notion that SPS contains phosphorylation site(s) that reduce enzyme activation state and that dephosphorylation of these residue(s) is the mechanism of light activation. Regulation of the protein phosphatase by Pi may be of physiological significance.}, number={2}, journal={ARCHIVES OF BIOCHEMISTRY AND BIOPHYSICS}, author={HUBER, SC and HUBER, JL}, year={1990}, month={Nov}, pages={421–426} }
 @article{huber_huber_1990, title={Regulation of spinach leaf sucrose-phosphate synthase by multisite phosphorylation}, volume={9}, journal={Current Topics in Plant Biochemistry and Physiology}, author={Huber, S. C. and Huber, J. L. A.}, year={1990}, pages={329} }
 @article{huber_1989, title={BIOCHEMICAL-MECHANISM FOR REGULATION OF SUCROSE ACCUMULATION IN LEAVES DURING PHOTOSYNTHESIS}, volume={91}, ISSN={["0032-0889"]}, DOI={10.1104/pp.91.2.656}, abstractNote={It is not known why some species accumulate high concentrations of sucrose in leaves during photosynthesis while others do not. To determine the possible basis, we have studied 10 species, known to differ in the accumulation of sucrose, in terms of activities of sucrose hydrolyzing enzymes. In general, acid invertase activity decreased as leaves expanded; however, activities remaining in mature, fully expanded leaves ranged from low (<10 micromoles per gram fresh weight per hour) to very high (>100 micromoles per gram fresh weight per hour). In contrast, sucrose synthase activities were low and relatively similar among the species (4-10 micromoles per gram fresh weight per hour). Importantly, leaf sucrose concentration, measured at midafternoon, was negatively correlated with acid invertase activity. We propose that sucrose accumulation in vacuoles of species such as soybean and tobacco is prevented by acid invertase-mediated hydrolysis. Initial attempts were made to characterize the relatively high activity of acid invertase from mature soybean leaves. Two apparent forms of the enzyme were resolved by Mono Q chromatography. The two forms had similar affinity for substrate (apparent K(m) [sucrose] = 3 millimolar) and did not interconvert upon rechromatography. It appeared that the loss of whole leaf invertase activity during expansion was largely the result of changes in one of the enzyme forms. Overall, the results provide a mechanism to explain why some species do not accumulate sucrose in their leaves. Some futile cycling between sucrose and hexose sugars is postulated to occur in these species, and thus, the energy cost of sucrose production may be higher than is generally thought.}, number={2}, journal={PLANT PHYSIOLOGY}, author={HUBER, SC}, year={1989}, month={Oct}, pages={656–662} }
 @article{huber_sugiyama_alberte_1989, title={PHOTOSYNTHETIC DETERMINANTS OF GROWTH IN MAIZE PLANTS - EFFECTS OF NITROGEN NUTRITION ON GROWTH, CARBON FIXATION AND PHOTOCHEMICAL FEATURES}, volume={30}, ISSN={["1471-9053"]}, DOI={10.1093/oxfordjournals.pcp.a077846}, abstractNote={Photosynthetic characteristics of mature maize leaves that may be determinants of plant growth. Nitrogen supply was varied to manipulate growth. Specific objectives were to determine whether biomass production was related to carbon exchange rate and/or export rate and whether carbon exchange rate was related to photosynthetic enzyme levels, triose phosphate utilization and/or photochemical capacity as expressed in the functional organization of the photosynthetic unit}, number={8}, journal={PLANT AND CELL PHYSIOLOGY}, author={HUBER, SC and SUGIYAMA, T and ALBERTE, RS}, year={1989}, month={Dec}, pages={1063–1072} }
 @inbook{huber_1985, title={Role of potassium in photosynthesis and respiration}, ISBN={9780891180869}, DOI={10.2134/1985.potassium.c15}, abstractNote={This chapter reviews the effects of potassium (K) deficiency on leaf photosynthesis are discussed to identify possible components affected. It discusses of the role of K in certain cellular and subcellular systems in an attempt to identify those that may be related to K deficiency symptoms observed in the intact plant. The mitochondrion is the site of aerobic respiration in plants. A moderate K deficiency results in increased dark respiration, which certainly contributes to the net carbon balance of K-deficient plants. In addition to possible effects on chloroplast metabolism, K-deficient C4 plants may have impaired intercellular transport that is essential for C4 photosynthesis. An important chloroplast enzyme that is affected by K+ is adenosine 5'-diphosphate glucose-starch synthase. Stomatal opening results when active uptake of K+ into guard cells increases cell turgor and, thus, opens the stomatal pore.}, booktitle={Potassium in Agriculture}, publisher={Madison, Wisconsin: American Society of Agronomy}, author={Huber, S. C.}, year={1985}, pages={369} }
 @article{huber_bickett_1984, title={EVIDENCE FOR CONTROL OF CARBON PARTITIONING BY FRUCTOSE 2,6-BISPHOSPHATE IN SPINACH LEAVES}, volume={74}, ISSN={["0032-0889"]}, DOI={10.1104/pp.74.2.445}, abstractNote={Excision of spinach (Spinacia oleracea L.) leaves had no effect on photosynthetic rates, but altered normal carbon partitioning to favor increased formation of starch and decreased formation of sucrose. The changes were evident within 2 hours after excision. Concurrently, leaf fructose-2,6-bisphosphate content increased about 5-fold (from 0.1 to 0.5 nanomoles per gram fresh weight). The activities of sucrose-P synthase and cytoplasmic fructose 1,6-bisphosphatase in leaf extracts remained constant during the time period tested. It is postulated that the rise in fructose 2,6-bisphosphate was responsible for the change in carbon partitioning.}, number={2}, journal={PLANT PHYSIOLOGY}, author={HUBER, SC and BICKETT, DM}, year={1984}, pages={445–447} }
 @article{huber_1983, title={ROLE OF SUCROSE-PHOSPHATE SYNTHASE IN PARTITIONING OF CARBON IN LEAVES}, volume={71}, ISSN={["0032-0889"]}, DOI={10.1104/pp.71.4.818}, abstractNote={Variations in leaf starch accumulation were observed among four species (wheat [Triticum aestivum L.], soybean [Glycine max L. Merr.], tobacco [Nicotiana tabacum L.], and red beet [Beta vulgaris L.]), nine peanut (Arachis hypogea L.) cultivars, and two specific peanut genotypes grown under different nutritional regimes. Among the genotypes tested, the activity of sucrose phosphate synthase was correlated negatively with leaf sucrose content in seven of the nine peanut cultivars as well as the two peanut cultivars grown with different mineral nutrition. The peanut cultivars differed in the effect of 10 millimolar sucrose on sucrose phosphate synthase activity in leaf extracts. Enzyme activity in crude leaf extracts was inhibited by sucrose (10-42%) in four of the cultivars tested whereas five cultivars were not. Overall, the results suggest that a correlation exists between the activity of sucrose phosphate synthase and starch/sucrose levels in leaves.}, number={4}, journal={PLANT PHYSIOLOGY}, author={HUBER, SC}, year={1983}, pages={818–821} }