Glycoproteins in Membranes

Summary 1. Although the classical models of biomembranes have emphasized the lipid and protein nature of these structures, a small quantity of carbohydrate is present as glycoprotein and glycolipid in animal cell membranes. In this article an attempt has been made to indicate that such carbohydrate materials should be considered in any complete model of animal cell membranes. 2. Various techniques that have demonstrated the presence of glycoproteins in animal cell membranes have been discussed here. In particular, cell electrophoresis, especially when the measurements are combined with specific enzyme treatments of the cells, has indicated that the net negative surface charge on intact viable cells is mainly due to sialic acid‐containing glycoproteins and not as previously thought to ionizable phosphate groups associated with a complex phospholipid system. 3. Membrane‐bound antigens are complex carbohydrate‐containing macro‐molecules whose antigenic activity is principally associated with the configuration at terminal positions on the carbohydrate chains. Enzymic degradation of the glycoproteins of the intact plasma membrane of cells, especially erythrocytes, causes profound changes in the immunological behaviour of the cell. 4. Histology and electron microscopy have indicated the presence of carbohydrate at the periphery of many mammalian cells. Some workers consider that such materials are present in a ‘cell coat’ covering the plasma membrane, rather than in the membrane itself. 5. Studies on the isolation and characterization of membrane glycoproteins have indicated that small oligosaccharide units linked to O ‐seryl or glutamyl residues in proteins are important structural units in plasma membranes. 6. The glycosylation of proteins takes place within the membranes of the endo‐plasmic reticulum. Studies of glycoprotein biosynthesis suggest that the cell is able to synthesize a diversity of cell‐surface structures from a relatively small number of monosaccharides. 7. Modification of the sialic acid‐containing glycoproteins of the plasma membrane affects the transport of materials in and out of cells. Glycoproteins are present in membranes other than the plasma membrane, and are therefore considered to be integral components of membranes; hence the designation ‘cell coat’, whilst useful as a descriptive term, should not be taken to indicate that glycoproteins are constituents of a functional entity separate from that of the membrane. 8. Evidence that membrane glycoproteins may act as sites of interaction between cells is discussed. The involvement of glycoproteins in such a role would explain why the cell had developed a biosynthetic process capable of producing varied surface oligosaccharide structures from monosaccharides.