Differential localization of conventional protein kinase C isoforms during mouse oocyte development

Protein kinase C (PKC), the major cell target for tumor-promoting phorbol esters, plays a central role in signal transduction pathways. In many biological systems where Ca²⁺ serves as a second messenger, regulatory control is mediated by PKC. The activation of PKC depends on its binding to RACK1 receptor, which is an intracellular protein anchor for activated PKC. We demonstrate that the conventional PKC (cPKC) isoforms, PKC-α, PKC-βI, and PKC-βII, as well as RACK1, are expressed in mouse oocytes (germinal vesicle [GV]) and mature eggs (metaphase II [MII]). In GV oocytes, PKC-α, PKC-βII, and RACK1 were uniformly distributed in the cytoplasm, while PKC- βI was localized in the cytoplasm and in the plasma membrane as well. Treatment of GV oocytes with the biologically active phorbol ester, 12-o- tetradecanoyl phorbol-13-acetate (TPA), resulted in a rapid translocation of the cytosolic PKC-α, but not PKC-βI, PKC-βII, or RACK1, to the plasma membrane. This was associated with inhibition of GV breakdown. In MII eggs (17 h post-hCG), PKC-α was uniformly distributed in the cytoplasm while PKC- βI and -βII were distributed in the cytoplasm and in the plasma membrane as well. Treatment with TPA resulted in a rapid translocation of PKC-α from the cytoplasm to the plasma membrane and a significant decrease of PKC-βI throughout the cytoplasm, while it also remained in the cell periphery. No change in the distribution of PKC-βII or RACK1 was observed. TPA also induced pronucleus formation. Physiological activation of MII eggs by sperm induced cortical granule exocytosis associated with significant translocation of PKC-α and -βI, but not -βII, to the plasma membrane. Overall, these results suggest a possible involvement of cPKC isoforms in the mechanism of mouse oocyte maturation and egg activation.