Biosynthesis of glycoproteins in Candida albicans:: Solubilization and partial characterization of dolichol phosphate mannose synthase and protein mannosyl transferases

Incubation of a mixed membrane fraction isolated from C. albicans yeast cells with Nonidet P-40 at a detergent/protein ratio as low of 0.025 (0.016-0.019%, w/v) yielded a soluble fraction that catalyzed the transfer of mannose from GDP-[C-14] Man into dolichol phosphate mannose and from this intermediate into mannoproteins. Over 95% of the sugar in mannoproteins was O-linked as judged from its release after beta-elimination. Mannose was identified as the sole product after this treatment. Transfer activity did not depend on exogenous lipid acceptor indicating that the latter was solubilized along with the mannosyl transferases. Synthesis of mannolipid and mannoproteins occurred at optima temperatures of 20 degrees C and 37 degrees C, respectively, and at a pH in the range of 7.5-9.5. Mannosyl transfer into the mannolipid was stimulated by Mg2+ and inhibited by Ca2+ and Mn2+ whereas mannoprotein labeling was stimulated by Mn2+ and to a lower extent by Mg2+. When measured as a function of substrate concentration, the synthesis of the mannolipid was a nearly linear function of GDP-Man concentration in the range of 5 to 32 mu M whereas protein mannosylation exhibited hyperbolic kinetics with saturation reached at about 10 mu M. The solubilized preparation was able to utilize an exogenous source of mannolipid as sugar donor for protein mannosylation. Dinucleotides and, to a higher extent trinucleotides, inhibited mannosyl transfer into the mannolipid and hence into mannoproteins.