Partial purification and characterization of a calcium-dependent protein kinase in rice leaves

Abstract A protein from rice leaves, which was partially purified by sequential chromatography on DE52, MONO-Q and Superose 12, presented calcium-dependent protein kinase (CDPK) activity. This protein kinase phosphorylated the substrate, histone III-S, in a Ca 2+ -dependent manner and the half-maximum concentration of Ca 2+ to protein kinasae activity (EC 50 ) was 1μM. This phosphorylation was independent of phosphatidylserine and a phorbol ester. The apparent M r of the protein kinase, as determined by phosphorylation in SDS-polyacrylamide gel containing histone III-S, was 45 k. This kinase was found to react differently from other protein kinases, such as protein kinase C from rat brain or CDPK from soybean leaves, owing to the absence of a phospholipid or phorbol ester dependency. CDPK phosphorylated three endogenous proteins as detected by in vitro phosphorylation on two-dimensional PAGE.