Characterization of Activators and Inhibitors of Protein Kinase Cμ

Abstract

In order to investigate regulatory mechanisms and to identify potential substrates of a novel member of the protein kinase C (PKC) family, PKCμ, specific antibodies have been raised against unique amino- and carboxy-terminal regions. PKCμ kinase activity was studied upon immunoprecipitation from stably transfected cell lines as well as from the A549 carcinoma cell line expressing the endogeneous PKCμ gene. Cell fractionation revealed that PKCμ is predominantly found in the particulate fraction, suggesting an association with the membrane or membrane-bound structures. In vitro kinase assays with immunoprecipitated PKCμ demonstrated a Ca²⁺ independent enhancement of constitutive autophosphorylation activity by phosphatidylserine. Despite a limited in vitro phorbol ester response, an apparent phorbol ester activation of PKCμ was observed when cell cultures, instead of immunoprecipitated enzyme, were treated with either phorbol 12-myristate 13-acetate or 1,2 dioleoyl-sn-glycerol. Both in vitro autophosphorylation and substrate phosphorylation of myelin basic protein and histone III were enhanced under these conditions. However, long-term treatment with the phorbol ester did not result in downregulation of PKCμ protein levels and kinase activity. Studies with several protein kinase inhibitors revealed a novel sensitivity profile of PKCμ, with no inhibition by calphostin C, reduced sensitivity to staurosporine but, compared to other PKCs, an approximately 60-fold higher sensitivity to the selective PKA inhibitor H89. Together, the data presented here show that localization of PKCμ and regulation of its kinase activity differ from that of other PKCs suggesting a novel function of PKCμ in intracellular signal pathways.