ZMCPKC70, A PROTEIN-KINASE C-TYPE ENZYME FROM MAIZE - BIOCHEMICAL-CHARACTERIZATION, REGULATION BY PHORBOL 12-MYRISTATE 13-ACETATE AND ITS POSSIBLE INVOLVEMENT IN NITRATE REDUCTASE GENE-EXPRESSION

The crucial enzyme in diacylglycerol-mediated signaling is protein kin ase C (PKC). In this paper we provide evidence for the existence and r ole of PKC in maize. A protein of an apparent molecular mass of 70 kDa was purified. The protein showed kinase activity that was stimulated by phosphatidylserine and oleyl acetyl glycerol (OAG) in the presence of Ca2+. Phorbol 12-myristate 13-acetate (PMA) replaced the requiremen t of GAG. [H-3]PMA. binding to the 70-kDa protein was competed by unla beled PMA and OAG but not by 4 alpha-PMA, an inactive analog. The kina se phosphorylates histone H1 at serine residue(s), and this activity w as inhibited by H-7 and staurosporine. These properties suggest that t he 70-kDa protein is a conventional serine/threonine protein kinase C (cPKC). Polyclonal antibodies raised against the polypeptide precipita te the enzyme activity and immunostained the protein on Western blots. The antibodies also cross-reacted with a protein of expected size fro m sorghum, rice, and tobacco. A rapid increase in the protein level wa s observed in maize following PMA treatments. In order to assign a pos sible role of PKC in gene regulation, the nitrate reductase transcript level was investigated. The transcript level increased by PMA, not by 4 alpha-PMA treatments, and the increase wa.s inhibited by H-7 but no t by okadaic acid. The data sham the existence and possible function o f PKC in higher plants.