A dual role for Rab3A in the acrosome reaction of human sperm

BUSTOS MATIAS A; BRANHAM MARIA T; MAYORGA LUIS S; TOMES CLAUDIA N

San Miguel de Tucumán, Tucumán, Argentina

Congreso; Sociedad Argentina de Investigación en Bioquímica y Biología Molecular; 2009

The sperm is endowed with a single secretory granule that undergoes a special type of regulated exocytosis (the acrosome reaction, AR). The AR relies on the same fusion machinery as typical exocytotic cells (i.e. SNAREs and Rabs). Rab3A is a GTPase that cycles between a GDP-bound (inactive) state that localizes at the cytosol and a GTP-bound (active) state that is attached to membranes. The role of Rab3A is controversial because in some models it stimulates exocytosis whereas in others behaves as an inhibitor. In human sperm, recombinant, prenylated Rab3A induces the AR in intact as well as SLO-permeabilized cells when is GTPγS-loaded. Different AR inducers like Ca2+ and 8-pCPT-2’-O-Me-cAMP (8pCPT, Epac-selective cAMP analogue) augment the levels of GTP-bound Rab3A. Here, we report that Rab3A distributed in the same proportion between cytosol and membranes in resting sperm. There was an enhanced binding of Rab3A to membranes after stimulating with A23187 or 8pCPT; but not with N6-Benzoyl-cAMP (a PKA-selective analogue), consistent with previous observations that Epac, but not PKA, exhibits a role in the AR. Interestingly, recombinant, prenylated and loaded with GTPγS, but not GTP, Rab3A exhibited an inhibitory role during the late stages of the AR. In summary, Rab3A seems to have a dual role depending on the stage it is added, this could explain the differences among models.