Functional domains of the T lymphocyte plasma membrane: Characterization of the polypeptide composition

Abstract

Highly purified plasma membranes from calf thymocytes were fractionated by affinitychromatography on Concanavalin A-Sepharose into two subfractions, one eluting freely from the affinity column (MF1) and a second being specifically retained (MF2). SDS-polyacryl-amide-gel-electrophoresis revealed different polypeptide patterns of the two plasma membrane subfractions. Polypeptides of apparent molecular weights of 170, 150, 110, 94, 39 and 30 kDa were several-fold enriched in the adherent fraction, MF2. In contrast, several proteins in the 55–65 kDa range were preferentially recovered in the non-adherent fraction. Five of the six polypeptides, preferentially recovered in MF2 proved to be glycoproteins, the 39 kDa peptide was non-glycosilated. The differences in the amounts of the polypeptides specifically enriched in the adherent fraction MF2 became even more clear-cut when plasma membranes solubilized with non-ionic detergents (lysolecithin, ET-18-2H, Triton-X-100) were separated by affinity chromatography on Concanavalin A-Sepharose. The non-glycosilated peptide of apparent molecular weight of 39 kDa was recovered together with several glycoproteins in the adherent fraction, MF2, suggesting that not single glycoproteins, but plasma membrane domains were separated by Concanavalin A-Sepharose. Although the glycoproteins of the non-adherent fraction MF1 bound significant amounts of Concanavalin A, the major Concanavalin A binding glycoproteins were recovered in the adherent fraction, MF2. The plasma membrane subfractions showed also different functional properties, the specific activities [Na ++K +]AT Pase, Ca 2+ATPase and lysolecithin acyltransferase were several-fold enriched in the adherent fraction, MF2, as compared to MF1. The data suggest the existence of plasma membrane domains in the plasma membranes of thymocytes consisting of a different set of proteins, among others the major Concanavalin A binding glycoproteins with some membrane bound enzymes, probably implicated in the initiation of lymphocyte activation.