Identification of a protein kinase C activating factor from murine erythroleukemia cells: Characterization of the activation kinetics

A protein kinase C (PKC) activating factor (AF) has been identified in the extracellular medium of V3.17 [44] vincristine resistant murine erythroleukemia (MEL) cells clone. The factor is a protein that stimulates the activity of PKC α and β isozymes isolated from MEL cells, rat and mouse brain approximately 2 to 2.5 fold over the V_max, respectively. AF promotes an identical activation in the presence of all the effectors but also when the amount of Ca²⁺ is reduced to μM concentration and in the absence of diacylglycerol (DAG). The factor shows a greater activating efficiency with PKC β isozymes. AF binds to PKC presumably at the DAG binding site as suggested by the competition between phorbol dibutyrate and AF for binding to the kinase. Moreover, AF promotes the selective binding of PKC β to natural or artificial membranes in the presence of μM concentrations of Ca²⁺. Altogether these results suggest the presence in MEL cells of a protein factor that can promote association of PKC to the membranes together with activation of the kinase, without the requirement for DAG formation. This could be visualized as a new mechanism for prolonged and selective activation of PKC.