Manipulation of the biosynthesis of protein-modifying glycoconjugates by the use of specific inhibitors.

Since it is possible to interfere with different steps in the dolichol pathway of protein glycosylation and in the processing of N-linked oligosaccharides information can be deduced as to the role of protein-bound carbohydrate by comparing the biochemical fates and functions of glycosylated proteins with their non-glycosylated counterparts, or with proteins exhibiting differences in the type of oligosaccharide side-chains. Cells infected with enveloped viruses are excellent model systems for studying the inhibition of protein glycosylation, since they contain a restricted number of glycoproteins, in some cases with well-defined functions. Tunicamycin, an antibiotic, as well as several sugar analogues have been found to inhibit the glycosylation of proteins by virtue of their antiviral properties. They interfere with different steps in the dolichol pathway of N-glycosylation resulting in a lack of functional lipid-linked oligosaccharide precursors. Trimming inhibitors, a second generation of inhibitors of glycosylation, interfere with the processing of oligosaccharides by specific glucosidases and mannosidases, resulting in a block of the conversion of high-mannose to complex-type oligosaccharides. Depending upon the compound used, glycoproteins contain glucosylated-high-mannose, high-mannose, or hybrid oligosaccharide structures instead of complex ones. Various effects of glycosylation inhibitors have been described: susceptibility to proteases, improper protein processing and misfolding of polypeptide chains, loss of biological activity, and alteration of the site of virus-budding. A third generation of inhibitors of glycosylation is emerging which are designed to interfere with late steps of N-glycosylation occurring after trimming, to bring about changes in the makeup of complex carbohydrates.(ABSTRACT TRUNCATED AT 250 WORDS)