Dynamics of SNARE Assembly and Disassembly during Sperm Acrosomal Exocytosis

BLAS, GERARDO A. DE; ROGGERO, CARLOS M; TOMES, CLAUDIA N; MAYORGA, LUIS S

PLoS Biology

Public Library of Science

Lugar: San Francisco; Año: 2005 vol. 3

The dynamics of SNARE assembly and disassembly during membrane recognition and fusion is a central issue inintracellular trafficking and regulated secretion. Exocytosis of sperm?s single vesicle?the acrosome?is asynchronized, all-or-nothing process that happens only once in the life of the cell and depends on activation ofboth the GTP-binding protein Rab3 and of neurotoxin-sensitive SNAREs. These characteristics make acrosomalexocytosis a unique mammalian model for the study of the different phases of the membrane fusion cascade. By usinga functional assay and immunofluorescence techniques in combination with neurotoxins and a photosensitive Ca2þchelator we show that, in unactivated sperm, SNAREs are locked in heterotrimeric cis complexes. Upon Ca2þ entry intothe cytoplasm, Rab3 is activated and triggers NSF/a-SNAP-dependent disassembly of cis SNARE complexes. MonomericSNAREs in the plasma membrane and the outer acrosomal membrane are then free to reassemble in loose transcomplexes that are resistant to NSF/a-SNAP and differentially sensitive to cleavage by two vesicle-associatedmembrane protein (VAMP)?specific neurotoxins. Ca2þ must be released from inside the acrosome to trigger the finalsteps of membrane fusion that require fully assembled trans SNARE complexes and synaptotagmin. Our resultsindicate that the unidirectional and sequential disassembly and assembly of SNARE complexes drive acrosomalexocytosis.