SUBCELLULAR SITE OF STRUCTURAL GLYCOPROTEIN SYNTHESIS IN EHRLICH ASCITES TUMOR

Experiments were performed to determine the location in the cell at which incorporation of carbohydrate into membrane glycoproteins occurs. Pulse-label experiments were carried out both in vivo and in vitro, and fragments of cell membranes (PM), smooth (SER), and rough (RER) endoplasmic reticular membranes were then isolated. The time course of labeling of the protein-bound glucosamine residues was similar for all fractions. Incorporation of hexosamine-14C, mannose-14C, and glacatose-3H, from corresponding sugar nucleotides into endogenous acceptors was catalyzed by each fraction. The hexosamine and galactose transfer reactions were carried out much better by the PM fraction than by the ER fragments. Mannose, on the other hand, was incorporated equally well by all fractions. The transfer of galactose to a fetuin preparation from which sialic acid and galactose had been removed was some eight-fold more efficient in each fraction examined than transfer to endogenous acceptor. The results are consistent with the suggestion that carbohydrate incorporation into the membrane proteins studied occurs at many sites rather than at a particular site. © 1969.