ANALYSIS OF THE TETRASPANIN CD9-INTEGRIN ALPHA(IIB)BETA(3) (GPIIB-IIIA) COMPLEX IN PLATELET MEMBRANES AND TRANSFECTED CELLS

The platelet integrin, alpha(IIb)beta(3) (GPIIb-IIIa), and the tetrasp anin, CD9, are integral membrane proteins that are abundant in platele t membranes. We have identified several proteins, including CD9, which were co-precipitated by anti-alpha(IIb)beta(3) antibody from untreate d, resting platelets that were solubilized with the poly(oxyethylene) non-ionic detergent, Brij-35. Immunoblot and quantitative immunoprecip itation showed that the association of alpha(IIb)beta(3) with CD9 is s pecific and stoichiometric. The interaction between CD9 and alpha(IIb) beta(3) is probably hydrophobic, as Triton X-100 and hydrophobic deter gents of the Brij series completely dissociated the CD9-alpha(IIb)beta (3) complex. Recombinant CD9 and alpha(IIb)beta(3) can associate after transfection into Chinese hamster ovary cells, as seen by co-immunopr ecipitation and co-localization in the periphery of spreading cells an d in the lamellipodia of cells plated on fibrinogen. This co-localizat ion is absent from focal adhesions. Furthermore, anti-CD9-coated latex beads co-clustered alpha(IIb)beta(3) with CD9. This work indicates th at the tetraspanin, CD9, is associated with beta(3) integrins in resti ng platelets and transfected cells.