CLONING OF THE CANDIDA-ALBICANS HOMOLOG OF SACCHAROMYCES-CEREVISIAE GSC1 FKS1 AND ITS INVOLVEMENT IN BETA-1,3-GLUCAN SYNTHESIS/

Saccharomyces cerevisiae GSC1 (also called FKS1) and GSC2 (also called FKS2) have been identified as the genes for putative catalytic subuni ts of beta-1,3-glucan synthase. We have cloned three Candida albicans genes, GSC1, GSL1, and GSL2, that have significant sequence homologies with S. cerevisiae GSC1/FKS1, GSC2/FKS2, and the recently identified FKSA of Aspergillus nidulans at both nucleotide and amino acid levels. Like S. cerevisiae Gsc/Fks proteins, none of the predicted products o f C. albicans GSC1, GSL1, or GSL2 displayed obvious signal sequences a t their N-terminal ends, but each product possessed 10 to 16 potential transmembrane helices with a relatively long cytoplasmic domain in th e middle of the protein. Northern blotting demonstrated that C. albica ns GSC1 and GSL1 but not GSL2 mRNAs were expressed in the growing yeas t-phase cells. Three copies of GSC1 were found in the diploid genome o f C. albicans CA14. Although we could not establish the null mutation of C. albicans GSC1, disruption of two of the three GSC1 alleles decre ased both GSC1 mRNA and cell wall beta-glucan levels by about 50%. The purified C. albicans beta-1,3-glucan synthase was a 210-kDa protein a s judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and all sequences determined with peptides obtained by lysyl endopept idase digestion of the 210-kDa protein were found in the deduced amino acid sequence of C. albicans Gsc1p. Furthermore, the monoclonal antib ody raised against the purified beta-1,3-glucan synthase specifically reacted with the 210-kDa protein and could immunoprecipitate beta-1,3- glucan synthase activity. These results demonstrate that C. albicans G SC1 is the gene for a subunit of beta-1,3-glucan synthase.