title: Phosphorylation of serine-15 of maize leaf sucrose synthase. Occurrence in vivo and possible regulatory significance.
abstract: Experiments were conducted to determine whether sucrose synthase (SuSy) was phosphorylated in the elongation zone of maize (Zea mays L.) leaves. The approximately 90-kD subunit of SuSy was 32P-labeled on seryl residue(s) when excised shoots were fed [32P]orthophosphate. Both isoforms of SuSy (the SS1 and SS2 proteins) were phosphorylated in vivo, and tryptic peptide-mapping analysis suggested a single, similar phosphorylation site in both proteins. A combination of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and automated Edman sequencing analysis unequivocally identified the phosphorylation site in the maize SS2 protein as serine-15. This site was phosphorylated in vitro by endogenous protein kinase(s) in a strictly Ca(2+)-dependent manner. A synthetic peptide, based on the phosphorylation site sequence, was used to identify and partially purify an endogenous Ca(2+)-dependent protein kinase(s) from the maize leaf elongation zone and expanding spinach leaves. Phosphorylation of SuSy in vitro selectively activates the cleavage reaction by increasing the apparent affinity of the enzyme for sucrose and UDP, suggesting that phosphorylation may be of regulatory significance. Conservation of the phosphorylation site, and the sequences surrounding it, among plant species suggests that phosphorylation of SuSy may be widespread, if not universal, in plants.
authors.affiliation: United States Department of Agriculture, Agricultural Research Service, North Carolina State University, Raleigh 27695-7609, USA.
authors.forename: S C
authors.initials: SC
authors.lastname: Huber
authors.forename: J L
authors.initials: JL
authors.lastname: Huber
authors.forename: P C
authors.initials: PC
authors.lastname: Liao
authors.forename: D A
authors.initials: DA
authors.lastname: Gage
authors.forename: R W
authors.initials: RW
authors.lastname: McMichael
authors.forename: P S
authors.initials: PS
authors.lastname: Chourey
authors.forename: L C
authors.initials: LC
authors.lastname: Hannah
authors.forename: K
authors.initials: K
authors.lastname: Koch
mesh_terms: D000595:Amino Acid Sequence; D017124:Conserved Sequence; D015971:Gene Expression Regulation, Enzymologic; D005964:Glucosyltransferases / Q000378:metabolism*; D007527:Isoenzymes / Q000378:metabolism*; D008969:Molecular Sequence Data; D009842:Oligopeptides / Q000378:metabolism; D010449:Peptide Mapping; D010748:Phosphopeptides / Q000737:chemistry; D010766:Phosphorylation; D018515:Plant Leaves / Q000201:enzymology / Q000254:growth & development / Q000378:metabolism*; D010940:Plant Proteins; D011494:Protein Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000302:isolation & purification; D017421:Sequence Analysis; D012694:Serine / Q000378:metabolism*; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D003313:Zea mays / Q000201:enzymology / Q000254:growth & development / Q000378:metabolism*; D017871:Calcium-Calmodulin-Dependent Protein Kinases
chemical_list: D005964:Glucosyltransferases; D007527:Isoenzymes; D009842:Oligopeptides; D010748:Phosphopeptides; D010940:Plant Proteins; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases; D012694:Serine; D017871:Calcium-Calmodulin-Dependent Protein Kinases; C022699:sucrose synthase; C102599:sucrose synthase kinase