Stomatin is a major lipid-raft component of platelet α granules

Lipid rafts are detergent-resistant, cholesterol- and sphingolipid-rich membrane domains that are involved in important cellular processes such as signal transduction and intracellular trafficking. Stomatin, a major lipid-raft component of erythrocytes and epithelial cells, is also an abundant platelet protein. Microscopical methods and subcellular fractionation showed that stomatin is located mainly at the alpha-granular membrane. The lipid-raft marker proteins flotillin-1 and flotillin-2 were also present in platelets but excluded from a granules. Stomatin and the flotillins were associated with Triton X-100-insoluble lipid rafts. Whereas stomatin was partly soluble in Triton X-100, it was insoluble in the detergents Lubrol and 3-[(3-cholamidopropyl)dimethylamonio]-1-propyl sulfonate (CHAPS). Flotation experiments after CHAPS lysis of platelets revealed a distinct set of lipidraft-associated proteins, which were identified by matrix-assisted laser desorption/ ionization mass spectrometry as stomatin, flotillin-11, flotillin-2, CD36, CD9, integrin alpha(IIb)beta(3), and the glucose transporter GLUT-3. Stomatin, the flotillins, and CD36 were exclusively present in this lipid-raft fraction. Activation of platelets by calcium ionophore A23187 or thrombin led to translocation of stomatin to the plasma membrane, cleavage by calpain, and specific sorting into released microvesicles. In conclusion, this study demonstrated the existence of alpha-granular lipid rafts and suggests an important role for stomatin in the organization and function of alpha granules. (C) 2002 by The American Society of Hematology.