Isolation and properties of gamma protein,the major transmembrane sialoglycoprotein of the HeLa cell

The surface of the HeLa cell is composed of a heterogeneous population of sialogly coproteins which undergo lectin-mediated endocytosis (Kramer and Canellakis, Biochim Biophys Acta 551:328, 1979). One such sialoglyco-protein, gamma protein, is the major periodate-Schiff-reactive and ³H]-glucosamine-labeled component of the plasma membrane; it has an apparent molecular weight of 165,000. Gamma protein is also the major ¹²⁵I]-wheat germ agglutinin-binding component in sodium dodecyl sulfate gels. Neuraminidase digestion of HeLa cells abolishes binding of ¹²⁵I]-wheat germ agglutinin to gamma protein, and pretreatment of cells with wheat germ agglutinin protects gamma protein from desialation by neuraminidase. suggesting that wheat germ agglutinin binds to the sialic acid residues of gamma protein at the cell surface. Gamma protein can be extracted with various detergents but not with high-salt, chelating, or chaotropic agents. Intact inside-out plasma membrane vesicles have been prepared from HeLa cells that had phagocytosed latex particles. Treatment of these isolated vesicles with trypsin reduces the molecular weight of gamma protein. These results suggest that gamma protein is an integral membrane protein that spans the plasma membrane. Gamma protein can be purified to homogeneity by sequential lithium diiodosalicylate-phenol extraction, wheat germ agglutinin-agarose affinity chromatography, and preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis.