INVOLVEMENT OF RABPHILIN-3A IN CORTICAL GRANULE EXOCYTOSIS IN MOUSE EGGS

Rabphilin-3A is a putative target protein for Rab3A, a member of the s mall GTP-binding protein superfamily that has been suggested to play a role in regulated exocytosis in presynapses, In this study we determi ned the expression and the function of Rabphilin-3A in mouse eggs at f ertilization. Rabphilin-3A mRNA and protein were detected by reverse t ranscriptase-PCR and immunoblot analysis, respectively, in metaphase I I mouse eggs, Immunofluorescence analysis showed that Rabphilin-3A pro tein was distributed in the cortical region in eggs, Sperm induces cor tical granule (CG) exocytosis via an increase in cytosolic Ca2+ at fer tilization, We microinjected the NH2- or COOH-terminal fragment of rec ombinant Rabphilin-3A into metaphase II eggs, Neither treatments alter ed the sperm-induced cytosolic Ca2+ increase, but both inhibited CG ex ocytosis in a dose-dependent manner, The NH2-terminal fragment was mor e effective than the COOH-terminal fragment, Full-length Rabphilin-3A did not affect CG exocytosis, but it attenuated the inhibition of CG e xocytosis by the NH2-terminal fragment, These results show that Rabphi lin-3A is involved in Ca2+-dependent CG exocytosis at fertilization in mouse eggs.