Characterization of a maize Ca(2+)-dependent protein kinase phosphorylating phosphoenolpyruvate carboxylase

Phosphoenolpyruvate carboxylase (PEPC) [EC 4.1.1.31] of plants undergoes regulatory phosphorylation in response to light or nutritional conditions. However, the nature of protein kinase(s) for this phosphorylation has not yet been fully elucidated. We separated a Ca(2+)-requiring protein kinase from Ca(2+)-independent one, both of which can phosphorylate maize leaf PEPC and characterized the former kinase after partial purification. Several lines of evidence indicated that the kinase is one of the characteristic Ca(2+)-dependent but calmodulin-independent protein kinase (CDPK). Although the M(r) of native CDPK was estimated to be about 100 kDa by gel permeation chromatography, in situ phosphorylation assay of CDPK in a SDS-polyacrylamide gel revealed that the subunit has an M(r) of about 50 kDa suggesting dimer formation or association with other protein(s). Several kinetic parameters were also obtained using PEPC as a substrate. Although the CDPK showed an ability of regulatory phosphorylation (Ser-15 in maize PEPC), no significant desensitization to feedback inhibitor, malate, could be observed presumably due to low extent of phosphorylation. The kinase was not specific to PEPC but phosphorylated a variety of synthetic peptides. The possible physiological role of this kinase was discussed.