Fusion of membranes during the acrosome reaction: A tale of two SNAREs

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Abstract
During spermiogenesis, hydrolytic enzymes are sorted from the Golgi apparatus to the acrosome, a supranuclear megavesicle. At fertilization, the enzymatic content of the acrosome is released by exocytosis when a portion of the plasma membrane enveloping the sperm head fuses with the outer membrane of the acrosome. Membrane fusion involves the interaction of a specific pair of proteins, called SNAREs (for soluble N‐ethylmaleimide sensitive factor attachment protein receptor). v‐SNARE is presumably associated with the membrane of the acrosomal vesicle. Target t‐SNARE is associated with the plasma membrane. The interaction of v‐SNARE and t‐SNARE requires two additional proteins: Rab proteins, members of a family of small GTPases related to the Ras proteins, and a complex of two proteins, NSF‐SNAP, recruited by the interacting v‐SNARE‐tSNARE pair. Syntaxin 2, a v‐SNARE member, and Rab3A, a member of the Rab GTPases, have been localized in the acrosome of rodent sperm. Mol. Reprod. Dev. 57:309–310, 2000.