PLANT CALCIUM-DEPENDENT PROTEIN KINASE-RELATED KINASES (CRKS) DO NOT REQUIRE CALCIUM FOR THEIR ACTIVITIES

In plants, calcium-dependent protein kinases (CDPKs) make up a large f amily that is characterized by a C-terminal calmodulin(CaM)-like domai n. Recently, a novel carrot cDNA clone encoding an atypical CDPK, whic h has a significantly degenerate sequence in the CaM-like domain, was found and named CDPK-related protein kinase (CRK) [Lindzen, E. and Cho i, J.H. (1995) Plant Mol. Biol. 28, 785-797]. We obtained two differen t cDNA clones from maize which encode CRKs. For the first enzymatic ch aracterization of CRK, a maize cDNA clone was expressed in E. coli. Th e recombinant protein efficiently phosphorylated casein, a conventiona l protein substrate. Notably, in this in vitro phosphorylation assay, the kinase activity did not require calcium as an activator. Thus, CRK s were suggested to be novel calcium-independent protein kinases havin g a degenerate CaM domain, the function of which remains to be elucida ted.