THE YEAST FKS1 GENE ENCODES A NOVEL MEMBRANE-PROTEIN, MUTATIONS IN WHICH CONFER FK506 AND CYCLOSPORINE-A HYPERSENSITIVITY AND CALCINEURIN-DEPENDENT GROWTH

FK506 and cyclosporin A (CsA) are potent immunosuppressive agents that display antifungal activity. They act by blocking a Ca2+-dependent si gnal transduction pathway leading to interleukin-2 transcription. Each drug forms a complex with its cognate cytosolic immunophilin receptor (i.e., FKBP12-FK506 and cyclophilin-CsA) which acts to inhibit the Ca 2+/calmodulin-dependent protein phosphatase 2B, or calcineurin (CN). W e and others have defined the Saccharomyces cerevisiae FKS1 gene by re cessive mutations resulting in 100-1000-fold hypersensitivity to FK506 and CsA (as compared to wild type), but which do not affect sensitivi ty to a variety of other antifungal drugs. The fks1 mutant also exhibi ts a slow-growth phenotype that can be partially alleviated by exogeno usly added Ca2+ [Parent et al., J. Gen, Microbiol, 139 (1993) 2973-298 4] We have cloned FKS1 by complementation of the drug-hypersensitive p henotype. It contains a long open reading frame encoding a novel 1876- amino-acid (215 kDa) protein which shows no similarity to CN or to oth er protein phosphatases. The FKS1 protein is predicted to contain 10 t o 12 transmembrane domains with a structure resembling integral membra ne transporter proteins, Genomic disruption experiments indicate that FKS1 encodes a nonessential function; fks1::LEU2 cells exhibit the sam e growth and recessive drug-hypersensitive phenotypes observed in the original fsk1 mutants. Furthermore, the fsk1::LEU2 allele is synthetic ally lethal in combination with disruptions of both of the nonessentia l genes encoding the alternative forms of the catalytic A subunit of C N (CNA1 and CNA2). These data suggest that FKS1 provides a unique cell ular function which, when absent, increases FK506 and CsA sensitivity by making the CNs (or a CN-dependent function) essential.