Rab3A triggers the acrosome reaction in permeabilized human spermatozoa

The acrosome reaction is a regulated exocytotic process leading to a massiv e fusion between the outer acrosomal membrane and the cell membrane. In spi te of the great amount of information available related to the acrosome rea ction in several species, there is a remarkable paucity about the role of m onomeric guanosine triphosphatases (GTPases) of the Rab family-well-establi shed participants in exocytosis in other fell types-in the acrosome reactio n, Western blot and immunofluorescence analysis indicate that Rab3A is pres ent in human spermatozoa and localizes to the acrosomal region in the sperm head. One difficulty in studying the role of proteins in intact cells is t he fact that they are unable to cross the cell membrane. Therefore, we esta blished a working model of streptolysin O-permeabilized human spermatozoa. Permeabilized spermatozoa were able to respond in a regulated way to differ ent stimuli, such as G protein activators and calcium, An acrosomal reactio n was also triggered by a Rab3A peptide corresponding to the effector regio n. More important, recombinant Rab3A protein in the GTP-bound form caused a crosome exocytosis, The same protein loaded with GDP or Rab11 in the CTP-bo und form was inactive. Also, recombinant GDI (GDP dissociation inhibitor)-a protein that releases Rab proteins from membrane-inhibited a GTP gamma S-s timulated acrosome reaction. Our results indicate that 1) permeabilized spe rmatozoa can be used to study the role of macromolecules in the acrosome re action, 2) Rab3A is present in human spermatozoa, and 3) Rab3A or another R ab3 isoform is involved in the exocytosis of the acrosomal granule in human spermatozoa.