CHARACTERIZATION OF A MAIZE CA2-DEPENDENT PROTEIN-KINASE PHOSPHORYLATING PHOSPHOENOLPYRUVATE CARBOXYLASE()

Phosphoenolpyruvate carboxylase (PEPC) [EC 4.1.1.31] of plants undergo es regulatory phosphorylation in response to light or nutritional cond itions. However, the nature of protein kinase(s) for this phosphorylat ion has not yet been fully elucidated. We separated a Ca2+-requiring p rotein kinase from Ca2+-independent one, both of which can phosphoryla te maize leaf PEPC and characterized the former kinase after partial p urification. Several lines of evidence indicated that the kinase is on e of the characteristic Ca2+-dependent but calmodulin-independent prot ein kinase (CDPK). Although the hi, of native CDPK was estimated to be about 100 kDa by gel permeation chromatography, in situ phosphorylati on 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 u sing PEPC as a substrate. Although the CDPK showed an ability of regul atory phosphorylation (Ser-15 in maize PEPC), no significant desensiti zation to feedback inhibitor, malate, could be observed presumably due to lour extent of phosphorylation. The kinase was not specific to PEP C but phosphorylated a variety of synthetic peptides, The possible phy siological role of this kinase was discussed.