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Characterization of the carbohydrate fragments obtained from Saccharomyces cerevisiae mannan by alkaline degradation.

Abstract Mild alkaline degradation of Saccharomyces cerevisiae X2180 wild type mannan, under conditions that effect β elimination of the substituted serine and threonine residues, releases mannose, mannobiose, mannotriose, and mannotetraose. Methylation analysis demonstrated that these fragments have the same structures as those that are obtained by acetolysis of the polysaccharide component of the yeast mannan. Similar treatment of a mutant mannan that lacks the mannotetraose unit in the polysaccharide component produced only mannose, mannobiose, and mannotriose. Thus, the single mutation alters both the tetrasaccharide units in the polysaccharide component of the mannan and those that are attached to serine and threonine. Because these studies were carried out on the total cell wall mannan, rather than a single molecular species, we cannot say that both the polysaccharide chains and the shorter oligosaccharides linked to serine and threonine are present in the same molecule. Strong alkaline degradation in the presence of sodium borotritide of the mannan residue recovered from the β elimination reaction yielded a protein-free polysaccharide that gave radioactive N-acetylglucosaminitol on acetolysis. Thus, the mannose-containing polysaccharide of the mannan is attached to glucosamine, and it is probable that the linkage to protein is through a di-N-acetylchitobiose to asparagine as in many other glycoproteins. The mannan isolated from the wild type and mutant strains of S. cerevisiae by procedures that avoid exposure to alkali had a molecular weight of 133,000. On treatment with 0.1 n sodium hydroxide at room temperature, the molecular weight was reduced to about 40,000. These conditions could saponify acyl ester bonds but are unlikely to break phosphodiester linkages or any other linkages except those of the oligosaccharides attached to serine and threonine.