Here we describe the isolation and characterization of a Triton X-100-insoluble fraction isolated from lysates of platelets by flotation in sucrose gradients. Transmission electron microscopy of the insoluble material revealed a heterogeneous population of vesicles ranging in size from 20 to 1000 nm, and Western blot analyses of platelet lysates for the caveolae structural coat protein, caveolin/VIP21, were negative. Biochemical characterization of the Triton X-100-insoluble fraction showed it to be cholesterol-rich, greatly and specifically enriched in the plasma membrane glycoprotein CD36, and also to contain Src and the Src-related kinase, Lyn. CD36 within this fraction is shown to be palmitoylated, but the fraction itself is not generally enriched in palmitoylated platelet proteins. These results suggest that this fraction represents caveolin-negative, CD36-rich microdomains in the resting platelet membrane. CD36 can form associations with certain Src-related kinases and can signal to activate platelets. These results suggest the possibility that such microdomains are implicated in platelet activation.

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