Immunochemical Studies of the Mannans of Saccharomyces

The mannans from Saccharomyces cerevisiae mutant strains X2180-1A-5 and 4484-24D-1, both of which were shown to contain small amounts of phosphate (less than 0.2%), were fractionated on a column of diethylaminoethyl-Sephadex into five subfractions designated as fractions I to V. These subfractions contain different amounts of phosphate, ranging from 0.03 to 0.09 (strain X2180-1A-5) and from 0.01 to 0.17% (strain 4484-24D-1). Fractions I to IV from strain X21801A-5 showed nearly identical precipitin activities against the homologous antiwhole cell serum, whereas fraction V, containing the largest amount of phosphate and protein among this mannan subfraction series, showed unexpectedly weaker precipitin activity than those ofthe other fractions. A synthetic mannan consisting of consecutive a-1---6-linked D-mannopyranosyl residues was found to be crossreactive with all the mannan subfractions of strain X2180-1A-5 against antiX2180-1A-5 serum. On the other hand, antibody-precipitating activities of the mannan subfractions of the latter strain were proportional to their phosphate content, although the increments of precipitated antibody nitrogen among the subfractions were quite small. However, fraction V of this mannan subfraction series, containing the largest amounts of phosphate and protein, showed lower precipitin activity than did the other four fractions. These findings indicate that mannans containing no phosphate or relatively small amounts of phosphate, such as those investigated in the present study, are less heterogeneous in the densities of the branching moieties than are highly phosphorylated mannans. These findings suggest that the transfer step of mannosyl-1-phosphate into the precursor(s) ofthe wild-type strain mannans during the biosynthetic process corresponds to the key reaction responsible for the anionic heterogeneity due to the density heterogeneity of the antigenic determinants.