THE SACCHAROMYCES-CEREVISIAE FKS1 (ETG1) GENE ENCODES AN INTEGRAL MEMBRANE-PROTEIN WHICH IS A SUBUNIT OF 1,3-BETA-D-GLUCAN SYNTHASE

In Saccharomyces cerevisiae, mutations in FKS1 confer hypersensitivity to the immunosuppressants FK506 and cyclosporin A, while mutations in ETG1 confer resistance to the cell-wall-active echinocandins (inhibit ors of 1,3-beta-D-glucan synthase) and, in some cases, concomitant hyp ersensitivity to the chitin synthase inhibitor nikkomycin Z. The FKS1 and ETG1 genes were cloned by complementation of these phenotypes and were found to be identical. Disruption of the gene results in (i) a pr onounced slow-growth phenotype, (ii) hypersensitivity to FK506 and cyc losporin A, (iii) a slight increase in sensitivity to echinocandin, an d (iv) a significant reduction in 1,3-beta-D-glucan synthase activity in vitro. The nucleotide sequence encodes a 215-kDa polypeptide predic ted to be an integral membrane protein with 16 transmembrane helices, consistent with previous observations that the etg1-1 mutation results in echinocandin-resistant glucan synthase activity associated with th e nonextractable membrane fraction of the enzyme. These results sugges t that FKS1 encodes a subunit of 1,3-beta-D-glucan synthase. The resid ual activity present in the disruption mutant, the nonessential nature of the gene, and results of Southern blot hybridization analysis poin t to the existence of a glucan synthase isozyme.