A CALCIUM AND FREE FATTY ACID-MODULATED PROTEIN-KINASE AS PUTATIVE EFFECTOR OF THE FUSICOCCIN 14-3-3 RECEPTOR

A protein kinase that is activated by calcium and cis-unsaturated fatt y acids has been characterized from oat (Avena sativa L.) root plasma membranes. The kinase phosphorylates a synthetic peptide with a motif (-R-T-L-S-) that can be phosphorylated by both protein kinase C (PKC) and calcium-dependent protein kinase (CDPK)-type kinases. Calphostin C and chelerythrine, two PKC inhibitors, completely inhibited the kinas e activity with values of inhibitor concentration for 50% inhibition o f 0.7 and 30 mu M, respectively. At low Ca2+ concentrations cis-unsatu rated fatty acids (linolenic acid, linoleic acid, arachidonic acid, an d oleic acid) stimulated the kinase activity almost 10-fold. The two i nhibitors of the kinase, calphostin C and chelerythrin, strongly reduc ed the fusicoccin (FC)-induced H+ extrusion, and the activators of the kinase, the cis-unsaturated fatty acids, prevented [H-3]FC binding to the FC 14-3-3 receptor. CDPK antibodies cross-reacted with a 43-kD ba nd in the plasma membrane and in a purified FC receptor fraction. A po lypeptide with the same apparent molecular mass was recognized by a sy nthetic peptide that had a sequence homologous to the annexin-like dom ain from barley 14-3-3. The possibility of the involvement of a kinase , with properties from both CDPK and PKC, and a phospholipase A(2) in the FC signal transduction pathway is discussed.