CHARACTERIZATION OF NOVEL COMPLEXES ON THE CELL-SURFACE BETWEEN INTEGRINS AND PROTEINS WITH 4 TRANSMEMBRANE DOMAINS (TM4 PROTEINS)

Here we identified several new integrin/TM4 protein complexes on the c ell surface. By immunoprecipitation using nonstringent conditions, and by reciprocal immunoprecipitation, we found that alpha(3) beta(1) and alpha(6) beta(1) integrins but not alpha(2) beta(1), alpha(5) beta(1) or alpha(6) beta(4) integrins associated with CD9 and CD81 in alpha(3 ) beta(1)/CD81, alpha(3) beta(1)/CD9, alpha(6) beta(1)/CD81, and alpha (6) beta(1)/CD9 complexes. Also, cross-linking experiments established that alpha(3) beta(1)/CD81, alpha(3) beta(1)/CD9, and alpha(3) beta(1 )/CD63 associations occur on the surface of intact cells and suggested that a critical interaction site is located within extracellular doma ins. Cross-linking in conjunction with reimmunoprecipitation indicated that larger multi-component alpha(3) beta(1)/TM4/TM4 complexes (alpha (3) beta(1)/CD9/CD63, alpha(3) beta(1)/CD81/CD63, and alpha(3) beta(1) /CD9/CD81) also could be detected on the cell surface. Immunofluoresce nt staining showed redistribution of alpha(3) beta(1)/TM4 complexes to ward the periphery of cells plated on various extracellular matrix sub strates and also showed that these complexes were localized in cell fo otprints. Staining of human tissues yielded additional results consist ent with co-localization of alpha(3) beta(1) and CD9, CD63, and CD81 p roteins. In conclusion we suggest that the prevalence of integrin/TM4 complexes in diverse cellular environments is indicative of their gene ral physiological importance.