Proteins of the exocytotic core complex mediate platelet alpha-granule secretion - Roles of vesicle-associated membrane protein, SNAP-23, and syntaxin 4

To understand the molecular basis of granule release from platelets, we exa mined the role of vesicle-associated membrane protein, SNAP-23, and syntaxi n 4 in alpha-granule secretion. A vesicle-associated membrane protein, SNAP -23, and syntaxin 4 were detected in platelet lysate. These proteins form a SDS-resistant complex that disassembles upon platelet activation, To deter mine whether these proteins are involved in alpha-granule secretion, we dev eloped a streptolysin O-permeabilized platelet model of alpha-granule secre tion Streptolysin O-permeabilized platelets released alpha-granules, as mea sured by surface expression of beta-selectin, in response to Ca2+ up to 120 min after permeabilization. Incubation of streptolysin O-permeabilized pla telets with an antibody directed against vesicle-associated membrane protei n completely inhibited Ca2+-induced alpha-granule release. Tetanus toxin cl eaved platelet vesicle-associated membrane protein and inhibited Ca2+-induc ed alpha-granule secretion from streptolysin O-perneabilized platelets. An antibody to syntaxin 4 also inhibited Ca2+-induced alpha-granule release by approximately 75% in this system. These results show that vesicle-associat ed membrane protein, SNAP-23, and syntaxin 4 form a heterotrimeric complex in platelets that disassembles with activation and demonstrate that alpha-g ranule release is dependent on vesicle SNAP receptor-target SNAP receptor ( vSNARE-tSNARE) interactions.