PHOSPHORYLATION OF EXTRACELLULAR DOMAINS OF T-LYMPHOCYTE SURFACE-PROTEINS - CONSTITUTIVE SERINE AND THREONINE PHOSPHORYLATION OF THE T-CELLANTIGEN RECEPTOR ECTODOMAINS

The extracellular accumulation of ATP after activation of T-lymphocyte s, as well as the presence of ectoprotein kinases in these cells, led us to propose that T cell surface receptors could be regulated through the reversible phosphorylation of their extracellular domains (ectodo mains). Here, in a model system, we used T cell transfectants which ex press T cell antigen receptor chains lacking intracellular and transme mbrane protein domains and P-32(i) metabolic labeling of cells to defi nitively demonstrate phosphorylation of ectodomains of T cell surface proteins, We show that alpha beta TCR ectodomains were phosphorylated intracellularly and constitutively on serine and threonine residues an d were then expressed on the T cell surface in phosphorylated form, TC R ectodomains also could be phosphorylated at the cell surface when ex tracellular [gamma-P-32]ATP or [gamma-P-32]GTP were used as phosphate donors with the same cells, Consensus phosphorylation sites for serine and threonine protein kinases were found to be strongly evolutionary conserved in both a and beta TCR chains constant regions, These result s are consistent with the hypothesis, where T cell surface proteins wh ich are phosphorylated intracellularly on their ectodomains, could sub sequently be expressed at the cell surface and then be reversibly modi fied by ectoprotein phosphatase(s) and by ectokinase(s), Such modifica tions may change T cells cognate interactions by, e.g. affecting TCR-m ultimolecular complex formation and antigen binding affinity. It is su ggested that alpha beta TCR ectodomain phosphorylation could serve as a potential mechanism for regulation of alpha beta TCR-mediated T-lymp hocytes response.