In complex with the peptidyl-prolyl isomerase cyclophilin A, the immunosuppressive antifungal drug cyclosporin A (CsA) inhibits a Ca/calmodulin-dependent protein phosphatase, calcineurin, which regulates signal transduction. We isolated and characterized cyclophilin A mutations that confer CsA resistance in a Saccharomyces cerevisiae strain whose growth is CsA-sensitive. Three mutations (G70S, H90Y, and G102A) alter single amino acids conserved between yeast and human cyclophilin A, which structural analyses implicate in CsA binding to human cyclophilin A. By Western analysis, all three mutant proteins are expressed in yeast. In vitro, two purified mutant cyclophilins (G70S, G102A) retain prolyl isomerase activity and have moderately reduced affinity for CsA and calcineurin but, when bound to CsA, do bind and inhibit calcineurin phosphatase activity. In contrast, the purified H90Y mutant cyclophilin is dramatically decreased in prolyl isomerase activity, CsA affinity, and calcineurin binding and inhibition. These studies identify conserved cyclophilin A residues that participate in CsA binding and catalysis.

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