Cyclosporin A (CsA) and FK506 are antimicrobial, immunosuppressive natural products that inhibit signal transduction. In T cells and Saccharomyces cerevisiae, CsA and FK506 bind to the immunophilins cyclophilin A and FKBP12 and the resulting complexes inhibit the Ca2+‐regulated protein phosphatase calcineurin. We find that growth of the opportunistic fungal pathogen Cryptococcus neoformans is sensitive to CsA and FK506 at 37°C but not at 24°C, suggesting that CsA and FK506 inhibit a protein required for C.neoformans growth at elevated temperature. Genetic evidence supports a model in which immunophilin–drug complexes inhibit calcineurin to prevent growth at 37°C. The gene encoding the C.neoformans calcineurin A catalytic subunit was cloned and disrupted by homologous recombination. Calcineurin mutant strains are viable but do not survive in vitro conditions that mimic the host environment (elevated temperature, 5% CO2 or alkaline pH) and are no longer pathogenic in an animal model of cryptococcal meningitis. Introduction of the wild‐type calcineurin A gene complemented these growth defects and restored virulence. Our findings demonstrate that calcineurin is required for C.neoformans virulence and may define signal transduction elements required for fungal pathogenesis that could be targets for therapeutic intervention.

The cyclophilins and FK506 binding proteins (FKBPs) bind to cyclosporin A, FK506, and rapamycin and mediate their immunosuppressive and toxic effects, but the physiological functions of these proteins are largely unknown. Cyclophilins and FKBPs are ubiquitous and highly conserved enzymes that catalyze peptidyl-prolyl isomerization, a rate-limiting step during in vitro protein folding. We have addressed their functions by a genetic approach in the yeast Saccharomyces cerevisiae. Five cyclophilins and three FKBPs previously were identified in yeast. We identified four additional enzymes: Cpr6 and Cpr7, which are homologs of mammalian cyclophilin 40 that have also recently been independently isolated by others, Cpr8, a homolog of the secretory pathway cyclophilin Cpr4, and Fpr4, a homolog of the nucleolar FKBP, Fpr3. None of the eight cyclophilins or four FKBPs were essential. Surprisingly, yeast mutants lacking all 12 immunophilins were viable, and the phenotype of the dodecuplet mutant resulted from simple addition of the subtle phenotypes of each individual mutation. We conclude that cyclophilins and FKBPs do not play an essential general role in protein folding and find little evidence of functional overlap between the different enzymes. We propose that each cyclophilin and FKBP instead regulates a restricted number of unique partner proteins that remain to be identified.Cyclophilin A and FKBP12 originally were isolated as cyclosporin A (CsA) and FK506 binding proteins (FKBPs) and later shown to inhibit the calcium͞calmodulin-dependent serine͞threonine phosphatase, calcineurin, as protein-drug complexes (reviewed in refs.