Mannosyl Transfer in Cryptococcus laurentii

A particle-bound enzyme from the fungus imperfectus Cryptococcus laurentii var. flavescens (NRRL Y-1401) mediates transfer of mannosyl units from GDP-mannose to endogenous primer. The enzyme requires divalent cations for activity. The pH optimum is 7.5 and the apparent Km for GDP-mannose is 0.14 mm. A polysaccharide fraction that is associated with the enzyme was found to be similar to a polysaccharide fraction that can be isolated from intact cells or from cell wall preparations by ethylenediamine extraction and fractionation with Cetavlon. Both polysaccharide fractions contain mannose, galactose, arabinose, xylose, and glucose; both are soluble in the presence of Cetavlon. In contrast, an acidic polysaccharide produced by the organism contains mannose, glucuronic acid, and xylose and is Cetavlon-insoluble. The radioactive reaction product that is obtained from GDP-mannose-14C remains particle-bound, but is solubilized with ethylenediamine. The solubilized product is Cetavlon-soluble and nondialyzable. Complete hydrolysis releases mannose as the only 14C-monomer. Structural analyses of acetolysis products of the enzymatic product reveal that mannosyl residues are newly linked to nonreducing ends of the enzyme-bound primer, resulting in α-1,2- and α-1,3-mannosyl-mannose bonds. Comparison of electron micrographs of whole cells or cell wall preparations before and after ethylenediamine extraction suggests that a polymer similar to the enzymatic product resides in the cell wall.