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

Abstract

The crucial enzyme in diacylglycerol-mediated signaling is protein kinase C (PKC). In this paper we provide evidence for the existence and role 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 requirement of OAG. [³H]PMA binding to the 70-kDa protein was competed by unlabeled PMA and OAG but not by 4α-PMA, an inactive analog. The kinase phosphorylates histone H1 at serine residue(s), and this activity was inhibited by H-7 and staurosporine. These properties suggest that the 70-kDa protein is a conventional serine/threonine protein kinase C (cPKC). Polyclonal antibodies raised against the polypeptide precipitate the enzyme activity and immunostained the protein on Western blots. The antibodies also cross-reacted with a protein of expected size from sorghum, rice, and tobacco. A rapid increase in the protein level was observed in maize following PMA treatments. In order to assign a possible role of PKC in gene regulation, the nitrate reductase transcript level was investigated. The transcript level increased by PMA, not by 4α-PMA treatments, and the increase was inhibited by H-7 but not by okadaic acid. The data show the existence and possible function of PKC in higher plants.