Xylosyltransferases in Cryptococcus laurentii

A particle-bound enzyme preparation from the fungus imperfectus, Cryptococcus laurentii, contains at least two different xylosyltransferases. One of these catalyzes the transfer of xylosyl units from UDP-xylose to endogenous cell envelope glycoproteins. The enzyme requires the disaccharide, 3-O-α-d-mannosyl-d-mannose, or the same structure at the terminal nonreducing end of an oligosaccharide as an acceptor. The xylosyl residue is transferred to the pentultimate mannosyl at the nonreducing end of the acceptor with the formation of a β-1,2 linkage. The product, 3-O-α-d-mannosyl-(2-O-β-d-xylosyl)-d-[3-14C]mannose, was synthesized, reduced with sodium borohydride, oxidized with periodate, again reduced, and then hydrolyzed. This procedure led to the formation of [14C]erythritol confirming the structure of the product. The xylosyltransferase has a pH optimum of 6.0 to 6.5. The apparent Km values for UDP-xylose and mannobiose are 1.4 mm and 19 mm, respectively. These properties, as well as the nature of the product formed, stability of the enzyme at 38°, and inhibition by nucleotides, clearly differentiate between this enzyme and a second xylosyltransferase that has been described for C. laurentii (Cohen, A., and Feingold, D. S. (1967) Biochemistry 6, 2933).