The formation of glycosidic bonds in yeast glycoproteins. Intracellular localisation of the reactions.

Membranes of Saccharomyces cerevisiae were separated on urografin gradients. The specific activity of the light membranes (endoplasmic reticulum), the Golgi-like vesicles and the plasma membrane in transferring mannosyl residues from GDP-mannose to mannoproteins and to dolichyl monophosphate has been determined. The first mannose of the O-glycosidically linked manno-oligosaccharides is incorporated with the highest specific activity by the endoplasmic reticulum. The incorporation of the second to fourth mannosyl groups is catalysed with increasing activity also by the Golgi-like vesicles and the plasma membrane. The incorporation of mannosyl groups into weak alkali-stable positions (N-glycosidically linked chains) is carried out with almost the same specific activity by all three membrane fractions, however, dolichol-dependent and -independent steps could not be distinguished as yet. The results are discussed in terms of a sequential addition of sugar residues along the route of export of the mannoprotiens. The dolichol-dependent steps seem to occur on the endoplasmic reticulum and thus very early in the event.