A CALCIUM-DEPENDENT AND PHOSPHOLIPID-DEPENDENT PROTEIN-KINASE FROM RICE (ORYZA-SATIVA) LEAVES

Protein kinases in plants have not been examined in detail, but protein phosphorylation has been shown to be essential for regulating plant growth via the signal transduction system. A Ca2+- and phospholipid-dependent protein kinase, possibly involved in the intracellular signal transduction system from rice leaves, was partially purified by sequential chromatography on DE52, Phenyl Superose and Superose 12. This protein kinase phosphorylated the substrate, histone III-S, in the presence of Ca2+ and phosphatidylserine. The apparent molecular mass of the Ca2+- and phosphatidylserine-dependent protein kinase (Ca2+/PS PK), determined by phosphorylation in SDS-polyacrylamide gel containing histone III-S, was 50 kDa. The protein kinase differed from Ca2+-dependent protein kinase (CDPK) in rice leaves in that Ca2+/PS PK showed phospholipid dependency and the molecular mass of Ca2+/PS PK exceeded that of CDPK. Investigations were carried out on changes in Ca2+/PS PK and CDPK activity in the cytosolic and membrane fractions during germination. The maximum activity of Ca2+/PS PK in the cytosolic fraction was observed before imbibition and that of CDPK in the membrane fraction was noted at 6 days following imbibition. Protein kinases are likely to regulate plant growth through protein phosphorylation.