PHOSPHORYLATION OF SERINE-15 OF MAIZE LEAF SUCROSE SYNTHASE - OCCURRENCE IN-VIVO AND POSSIBLE REGULATORY SIGNIFICANCE

Experiments were conducted to determine whether sucrose synthase (SuSy ) was phosphorylated in the elongation zone of maize (Zea mays L.) lea ves. The approximately 90-kD subunit of SuSy was P-32-labeled on seryl residue(s) when excised shoots were fed [P-32]orthophosphate. Both is oforms of SuSy (the SS1 and SS2 proteins) were phosphorylated in vivo, and tryptic peptide-mapping analysis suggested a single, similar phos phorylation site in both proteins. A combination of matrix-assisted la ser desorption/ionization time-of-flight mass spectrometry and automat ed Edman sequencing analysis unequivocally identified the phosphorylat ion site in the maize SS2 protein as serine-15. This site was phosphor ylated in vitro by endogenous protein kinase(s) in a strictly Ca2+-dep endent manner. A synthetic peptide, based on the phosphorylation site sequence, was used to identify and partially purify an endogenous Ca2-dependent protein kinase(s) from the maize leaf elongation zone and e xpanding 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 b e 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 .