Characterization of α-1,6-mannosyltransferase responsible for the synthesis of branched side chains in Candida albicans mannan

A particulate insoluble fraction from Candida albicans NIH B-792 (serotype B) strain cells was obtained as the residue after extracting a 105,000 x g pellet of cell homogenate with 1% Triton X-100. Incubation of this fraction with a mannopentaose, Manα1 → 3Manα1 → 2Manα1 → Manα1 → 2Man, in the presence of GDP-mannose and Mn²⁺ at pH 6.0 gave a branched mannohexaose, Manα1 (Manα1/6) → 3Manα1 → 2Manα1 → 2Manα1 → 2Man, the structure of which was identified by means of sequential NMR assignment. However, the enzyme fraction obtained from Candida parapsilosis gave Manα1 → 2Manα1 → 3Manα1 → 2Manα1 → 2Manα1 → 2-Man under the same conditions. These results demonstrate the finding that the structural difference in the mannans of these two species is due to the presence of α-1,6-linked branching mannose units in the C. albicans mannan [Shibata, N., Ikuta, K., Imai, T., Satoh, Y., Satoh, R., Suzuki, A., Kojima, C., Kobayashi, H., Hisamichi, K. and Suzuki, S. (1995) J. Biol. Chem. 270, 1113-1122]. The substrate-specificity study of the enzyme indicated that the structural requirement of the α-1,6-mannosyltransferase is Manα1 → 3Manα1 →. The α-1,6-mannosyltransferase also transferred the α-1,6-linked branching mannose unit to the mannan of Saccharomyces cerevisiae. The transformation of the mannan was detected by the appearance of antigenic factor 4 using an enzyme-linked immunosorbent assay and two-dimensional homonuclear Hartmann-Hahn spectroscopy.