Ahnak protein activates protein kinase C (PKC) through dissociation of the PKC-protein phosphatase 2A complex

We have previously reported that central repeated units (CRUs) of Ahnak act as a scaffolding protein networking phospholipase Cγ and protein kinase C (PKC). Here, we demonstrate that an Ahnak derivative consisting of four central repeated units binds and activates PKC-α in a phosphatidylserine/1,2- dioleoyl-sn-glycerol-independent manner. Moreover, NIH3T3 cells expressing the 4 CRUs of Ahnak showed enhanced c-Raf, MEK, and Erk phosphorylation in response to phorbol 12-myristate 13-acetate (PMA) compared with parental cells. To evaluate the effect of loss-of-function of Ahnak in cell signaling, we investigated PKC activation and Raf phosphorylation in embryonic fibroblast cells (MEFs) of the Ahnak knock-out (Ahnak^-/-) mouse. Membrane translocation of PKC-α and phosphorylation of Raf in response to PMA or platelet-derived growth factor were decreased in Ahnak null MEF cells compared with wild type MEFs. Several lines of evidence suggest that PKC-α activity is regulated through association with protein phosphatase 2A (PP2A). A co-immunoprecipitation assay indicated that the association of PKC-α with PP2A was disrupted in NIH3T3 cells expressing 4 CRUs of Ahnak in response to PMA. Consistently, Ahnak null MEF cells stimulated by PMA showed enhanced PKC-PP2A complex formation, and add-back expression of Ahnak into Ahnak null MEF cells abolished the PKC-PP2A complex formation in response to PMA. These data indicate that Ahnak potentiates PKC activation through inhibiting the interaction of PKC with PP2A.