DOWN-REGULATION OF PROTEIN-KINASE C-GAMMA IS INDEPENDENT OF A FUNCTIONAL KINASE DOMAIN

Prolonged activation of protein kinase C (PKC) types alpha and beta by tumor-promoting phorbol esters leads to desensitization of the phorbol ester response, downregulation of protein kinase C activity and depletion of the protein kinase C polypeptide. When the gamma isoenzyme of PKC is transiently expressed in COS-1 cells and exposed to phorbol esters, PKC-gamma is downregulated in COS cells although these cells do not normally express this subtype. A point mutation in the putative ATP-binding site (Lys-380 --> Met-380) of the protein kinase C gamma isoenzyme which results in a kinase-deficient enzyme does not interfere with this downregulation. Our results suggest that autophosphorylation or constitutive signalling through the protein kinase C-gamma kinase domain is not a prerequisite for downregulation of PKC activity.