A hypothetical model for interactions of the SNARE machinery in platelet secretion.

90-97,106,120 158-162 Platelets contain the SNARE molecules syntaxin 2 and 4, SNAP-23, and VAMP. PSP (platelet Sec1 protein) binds to syntaxin 4 and can inhibit SNARE complex formation. Platelet granule membranes contain Rabs that may interact with PSP through effector molecules to play a role in vesicle docking and targeting. PSP is phosphorylated in a protein kinase C-dependent manner when platelets are activated by thrombin. This relieves the inhibitory effects of PSP on SNARE complex formation by blocking its binding with syntaxin. Alternatively, conformational changes induced in PSP by other molecules (potentially homologues of Doc2, Munc13, and so forth) may release it from syntaxin, permitting SNARE complex formation and the initiation of fusion and exocytosis.