Control and manipulation of the phosphodolichol pathway of proteinN-glycosylation

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Abstract
Although the function of the otigosaccharide chains of many glycoproteins and glycolipids is not yet established there can be little doubt that in several instances their presence has an important biological significance. It has been known for several years that the surface of eucaryotic cells is decorated with a complex range of oligosaccharide chains many of which are linked to protein by an N-glycosyl bond. The pattern of these chains changes during the biologically important transformations of cells associated with differentiation and oncogenesis. In several cases interference with the pattern appears to effect a change in the transformation. Some glycosidases destined for lysosomes depend upon the presence of N-Hnked oligosaccharides, subsequently phosphorylated, to reach their subcetlular site of action. The rate of elimination from the circulation of soluble gtycoproteins that leave the cell and enter the blood appears to be influenced markedly by the precise nature of the oligosaccharide component. Several instances of the hydrophilicity of the oligosaccharide having a crucial effect on the conformation and/or orientation of the glycoprotein in a membrane have been described. In some cases the hydrophilic mass confers protection against proteolysis. These examples highlight the need to understand how protein N-glycosylation may be controlled and manipulated. This review concentrates on the more recent developments relevant to this understanding. More detailed analyses of various aspects of protein N-glycosylation have appeared elsewhere (1-11). Assembly of the Oligosaccharide Chains Although some of the oligosaccharide structures are quite complex nearly all of them are variations on the basic structures shown in Fig. 1. Indeed current techniques are showing several of the variants to be much closer to the basic structures than older methods indicated (2~8). The core of three mannose and two N-acetylglucosamine residues appears to be a highly conserved portion of all of these structures. These oligosaccharides are assembled from donor nucleotide sugars. The current view of the intracellular location of this process for glycoproteins destined for the plasma membrane or for secretion is summarized in Fig. 2. (Interestingly the work of Strous and Lodish [12] suggests that in at least one system, plasmamembrane glycoproteins may be made at twice the speed of a secreted glycoprotein, implying that the two processes differ at some stage.) It can: be seen that the protein receives its oligomannosyl N-acetylchitobiosyl chain in rough endoplasmic reticulum~ in several cases the protein appearing still to be nascent at the time.

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