NONCOVALENT ASSOCIATIONS OF T-LYMPHOCYTE SURFACE-PROTEINS

A number of T cell surface transmembrane molecules such as CD2, CD4, C D8, lymphocyte functional antigen (LFA)-1 and CD45 are known to intera ct functionally with the T cell receptor (TCR) complex during T cell a ctivation. Several previous communications have also reported physical associations between some of these molecules. On the other hand, ther e are indications that signaling through T cell surface molecules anch ored via glycosylphosphatidylinositol (GPI), such as Thy-1, Ly-6 or CD 59, is dependent on the TCR. Therefore, it was of interest to determin e in a systematic way which T cell surface molecules are noncovalently associated with the TCR/CD3 complex and with the major intracellular signaling molecules, the protein tyrosine kinases of the Src family. T o this aim, membrane proteins of human thymoma HPB-ALL cells were solu bilized in a solution of the mild detergent Brij-58 and subjected to i mmunoprecipitation followed by in vitro kinase assays. Two types of la rge complexes containing protein tyrosine kinases were observed: the f irst one contained CD3 and the transmembrane proteins CD2, CD4, CD5, C D6, CD7, CD8, CD11a, CD38, CD43, CD45, CD71, CD98 and CD99 and the oth er contained mainly the GPI-anchored proteins CD48, CD55, CD59 and CDw 108 as well as a fraction of CD4 and CD8. The GPI-anchored protein com plexes were of larger size and lower buoyant density than the CD3 comp lexes. In agreement with these biochemical data, co-cross-linking of C D3 with most of the relevant transmembrane proteins on the surface of another T cell line, Jurkat, markedly enhanced tyrosine phosphorylatio n of several intracellular proteins. These data indicate the existence of at least two types of membrane microdomains of very different comp osition in the membranes of T cells which may play a role in signaling through different types of receptors and in functional cooperation be tween TCR/CD3 and various accessory molecules.