A calcium- 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 Ca 2+ - 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 Ca 2+ and phosphatidylserine. The apparent molecular mass of the Ca 2+ - and phosphatidylserine-dependent protein kinase (Ca 2+ /PS PK), determined by phosphorylation in SDS-polyacrylamide gel containing histone III-S, was 50 kDa. The protein kinase differed from Ca 2+ -dependent protein kinase (CDPK) in rice leaves in that Ca 2+ /PS PK showed phospholipid dependency and the molecular mass of Ca 2+ /PS PK exceeded that of CDPK. Investigations were carried out on changes in Ca 2+ /PS PK and CDPK activity in the cytosolic and membrane fractions during germination. The maximum activity of Ca 2+ /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.