Evidence that galactosyltransferase is a surface receptor for poly(N)-acetyllactosamine glycoconjugates on embryonal carcinoma cells.

Cell surface glycosyltransferases are thought to participate in a variety of cellular interactions, but their specific glycoside acceptors have received little attention. In this paper, poly(N)-acetyllactosamine glycoconjugates are shown to be the endogenous substrates for embryonal carcinoma (EC) cell surface galactosyltransferases. All controls have been performed to ensure a surface localization for the galactosyltransferase activity. The galactosylated product(s) is relatively insoluble in organic solvents, is larger than conventional glycopeptides following pronase digestion, and is highly sensitive to endo-beta-galactosidase degradation. Solubilized polylactosaminyl glycoconjugates serve as competitive exogenous acceptors for the surface galactosyltransferase. In addition, the endogenous galactosyl acceptor(s) reacts with antiserum directed against EC cell poly(N)-acetyllactosamines. Anti-EC antiserum inhibits galactosylation of endogenous acceptors, simultaneously stimulates galactosylation of an exogenous acceptor, and immunoprecipitates 74% of the reaction product. Differentiated EC cells no longer react with anti-EC antiserum and no longer show anti-EC antiserum effects on surface galactosyltransferase activity. Interestingly, forced galactosylation with UDPGal releases polylactosaminyl substrates from the cell surface. In the absence of UDPGal, glycoconjugate release is dramatically reduced. GDPMan cannot substitute for UDPGal, and a galactosyltransferase inhibitor prevents glycoside release from the cell surface. Thus, surface galactosyltransferase preferentially binds poly(N)-acetyllactosamine glycoconjugates and serves as at least one of their surface receptors on EC cells.