Biosynthesis of yeast mannan. Independent formation of two carbohydrate moieties in the mannoprotein molecule.

The formation of two distinct types of carbohydrates moieties, beta-elminable saccharides attached to serine and/or threonine and of the polysaccharide portion of yeast mannan-protein was investigated using the particulate mannan synthetase from Saccharomcyes cerevisiae and GDP-[U-14C]mannose as mannosyl donor. The accumulated evidence obtained by following the kinetics of mannose incorporation into the different carbohydrate portions of mannoprotein, kinetics of thermal denaturation of enzymes responsible for their synthesis and by "pulse-chase" experiment where the change in the distribution of incorporated radioactivity was followed between the two carbohydrate moieties strongly suggests that the two carbohydrate portions in yeast mannoprotein are being synthesized independently, most probably by different sets of enzymes. At the same time, the obtained data show the beta-eliminable saccharides attached to serine and threonine in the peptide do not serve as precursors in the formation of polysaccharide part of mannoprotein.