SUMMARY: The immunosuppressants FK506 and cyclosporin A (CsA) bound to their receptors, FKBP12 or cyclophilin, inhibit the Ca/calmodulin-dependent protein phosphatase, calcineurin, preventing T cell activation or, in yeast, recovery from α-mating factor arrest. Vegetative growth of yeast does not require calcineurin, and in strains sensitive to FK506 or CsA, growth is inhibited by concentrations of drug much higher than those required to inhibit T cell activation or recovery from mating factor arrest. We now describe the isolation of a mutant of which is 100-1000-fold more sensitive to the growth inhibitory properties of these drugs. The mutation also confers a slow growth phenotype which is partially suppressed by exogenously added Ca and exacerbated by EGTA. Simultaneous disruption of the two genes and encoding the alternative forms of the catalytic A subunit of calcineurin, or of the gene encoding the regulatory B subunit, is lethal in an mutant. Disruption of the gene encoding FKBP12 or the major, cytosolic cyclophilin in cells results in the loss of hypersensitivity to the relevant drug. Overexpression of or in conjunction with results in a significant decrease in hypersensitivity to FK506 and CsA. The results show that the hypersensitivity of the mutant is due to the inhibition of calcineurin phosphatase activity by the receptor-drug complexes. The growth dependence of the mutant on the Ca/calcineurin signal pathway provides an important tool for studying in yeast certain aspects of immune suppression by these drugs.


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