Persistently active Rab3A and a chimaeric protein prevent the late stages of sperm exocytosis

QUEVEDO, FLORENCIA; BUSTOS, MATÍAS; LUCCHESI, ORNELLA; TOMES, CLAUDIA

Mar del Plata, Buenos Aires

Congreso; LI Reunión anual de la Sociedad Argentina de de Investigación en Bioquímica y Biología Molecular; 2015

SAIB

The acrosome reaction is a type of regulated exocytosis that releases the acrosomal granule content in the vicinity of the egg during fertilization. Minutes after the arrival of the triggering signal, the acrosomal and plasma membranes dock at multiple sites and fusion pores open at the contact points. It was assumed that immediately afterward fusion pores dilated spontaneously, originating tubules and vesicles that were shed, together with the acrosomal contents in the vicinity of the egg. We have reported that full length, geranylgeranylated and active Rab3A elicits human sperm exocytosis per se. With Rab3A/Rab22A chimeric proteins we demonstrated that the carboxy-terminus domain of the Rab3A molecule is necessary and sufficient to promote exocytosis whereas its amino-terminus prevents calcium-triggered secretion. Interestingly, full length Rab3A halts secretion when added after docking of the acrosome to the plasma membrane. This effect depends on Rab3A?s inability to hydrolyze GTP. A fluorescent dye applied to the medium entered the acrosome of sperm with their plasma membrane permeabilized with streptolysin O and incubated with Rab3A/Rab22A followed by calcium as exocytosis trigger. These findings suggest that there was a connection between the intravesicular and extracellular compartments, most likely through fusion pores opened between the plasma and acrosomal membranes. Thus, we propose that inhibitory Rabs interfere with the vesiculation of membranes and release of the acrosomal contents after the opening of fusion pores.