Protein kinase C isozymes ε and α in murine erythroleukemia cells

Protein kinase C (PKC) has a role in signal transduction during hexamethylene bisacetamide (HMBA)-induced differentiation of murine erythroleukemia cells (MELC). Separation of MELC PKC isozymes by hydroxylapatite chromatography yields a major peak (III) and a minor peak (II) of PKC activity, previously reported to contain the PKC α and β isozymes, respectively. In the present study, we confirm that peak III activity is PKCα but show that peak II contains PKCε and little or no PKCβ. Immunoblot analysis with isozyme-specific anti-α and anti-ε PKC antibodies detected PKCα in peak III and PKCε in peak II. Peak III activity was markedly enhanced (up to 20-fold) by phosphatidylserine, diolein, and Ca²⁺, whereas addition of these cofactors to the reaction mixture stimulated peak II activity only 2- to 4-fold. RNase protection analysis of MELC RNA showed that PKCα and PKCε RNAs were in a ratio of ≃2:1, but PKCβ RNA was barely detectable. Taken together, these data indicate that MELC contain PKCα and PKCε but little or no PKCβ.