FKBP12 is a ubiquitous and a highly conserved prolyl isomerase that binds the immunosuppressive drugs FK506 and rapamycin. Members of the FKBP12 family have been implicated in many processes that include intracellular protein folding, transport, and assembly. In the budding yeast Saccharomyces cerevisiae and in human T cells, rapamycin forms a complex with FKBP12 that inhibits cell cycle progression by inhibition of the TOR kinases. We reported previously that rapamycin does not inhibit the vegetative growth of the fission yeast Schizosaccharomyces pombe; however, it specifically inhibits its sexual development. Here we show that disruption of the S. pombe FKBP12 homolog, fkh1+, at its chromosomal locus results in a mating-deficient phenotype that is highly similar to that obtained by treatment of wild type cells with rapamycin. A screen for fkh1 mutants that can confer rapamycin resistance identified five amino acids in Fkh1 that are critical for the effect of rapamycin in S. pombe. All five amino acids are located in the putative rapamycin binding pocket. Together, our findings indicate that Fkh1 has an important role in sexual development and serves as the target for rapamycin action in S. pombe.
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