Oligosaccharide transfer from lipid sugar intermediates to endogenous protein(s) of rat liver microsomal subfractions

The subcellular localization and characterization of some of the components involved in the glycosylation of asparagine type glycoproteins was attempted using dolichyl diphosphate [14C]mannose oligosaccharide as precursor of the glycosylation reaction in vitro. Isolated rough and smooth microsomal fractions were able to carry out the transfer of the carbohydrate moiety from lipid oligosaccharide to endogenous protein acceptors. The protein glycosylating activity remained practically the same after stripping the vesicles from their robosomes or partially releasing their vesicular content. Isolation of polysomes from rough microsomes after glycosylation has taken place, reveals that a large proportion of mannose labeled glycoproteins is in the membranous fraction. The remaining labeled glycoproteins co-sediment with the polysomal fraction. If the isolation is carried out before glycosylation only the membranous fraction shows enzyme activity, whereas the polysomes alone are not able to carry out glycosylation. All these results taken together indicate that the protein glycosylating enzyme is a structural component of the rough and smooth microsomes of rat liver.