Epac activates the small G proteins Rap1 and Rab3A to achieve exocytosis

BRANHAM MT; BUSTOS PALOMINO MA; DE BLAS GA; REHMANN H; ZARELLI VEP; TREVIÑO C; DARSZON A; MAYORGA LS; TOMES CN

JOURNAL OF BIOLOGICAL CHEMISTRY (ONLINE)

Cadmus Communications for the American Society for Biochemistry and Molecular Biology

Lugar: Columbia, Maryland, USA; Año: 2009 vol. 284 p. 24825 - 24839

1083-351X

Exocytosis of the acrosome (the acrosome reaction) relies on cAMP production, assembly of a proteinaceous fusion machinery, calcium influx from the extracellular medium and mobilization from IP3-sensitive intracellular stores. Addition of cAMP to human sperm suspensions bypasses some of these requirements and elicits exocytosis in an A-kinase- and extracellular calcium-independent manner.  The relevant cAMP target is Epac, a guanine nucleotide exchange factor for the small GTPase Rap.  We show here that a soluble adenylyl cyclase synthesizes the cAMP required for the acrosome reaction.  Epac stimulates the exchange of GDP for GTP on Rap1, upstream of a phospholipase C.  The Epac selective cAMP analogue 8-pCPT-2´-O-Me-cAMP induces a phospholipase C-dependent calcium mobilization in human sperm suspensions.  In addition, our studies identify a novel connection between cAMP and Rab3A, a secretory granule-associated protein, revealing that the latter functions downstream of sAC/cAMP/Epac but not of Rap1.  Challenging sperm with calcium or 8-pCPT-2´-O-Me-cAMP boosts the exchange of GDP for GTP on Rab3A. Recombinant Epac does not release GDP from Rab3A in vitro, suggesting that the Rab3A-GEF activation by cAMP/Epac in vivo is indirect.  We propose that Epac sits at a critical point during the exocytotic cascade after which the pathway splits into two limbs, one that assembles the fusion machinery into place, and another that elicits intracellular calcium release