Localization of a syntaxin isoform, syntaxin 2, to the acrosomal region of rodent spermatozoa

The acrosome reaction includes a membrane fusion event that is a prerequisite for sperm penetration through the zona pellucida and subsequent fertilization. Since SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins have been shown to be key players in membrane fusion during regulated exocytosis in nerve terminals and secretory cells, and since the acrosome reaction has some features in common with regulated exocytosis, we hypothesized that SNARE proteins might also regulate acrosomal exocytosis. RT-PCR analysis demonstrated the expression of SNARE proteins, three isoforms of syntaxin 2 (2A, 2B, and 2C) and syntaxin 4A, in rat testes. Immunoblot analysis with anti-syntaxin 2 antibody showed that the protein was expressed in rodent spermatozoa, and that it was associated with membrane components of spermatozoa prepared by sucrose density gradient centrifugation. Confocal laser scanning microscopy with double immunolabeling revealed that syntaxin 2 was colocalized with acrin 1, a 90 kDa acrosomal protein, over the acrosomal region of spermatozoa but was not associated with the posterior half of head or tail. Localization of syntaxin 2 over the acrosomal region was supported by the finding that it was shed from sperm heads during an acrosome reaction induced by calcium ionophore A23187 in vitro. In view of the putative role of syntaxin proteins in other membrane fusion systems, these data suggest that syntaxin 2 may be involved in regulating the acrosomal reaction in rodent spermatozoa.