FK506 and rapamycin are immunosuppressants that inhibit signalling cascades required for T-cell activation, yet both are natural products of Streptomyces that live in the soil. FK506 and rapamycin also have potent antimicrobial activity against yeast and pathogenic fungi, suggesting a natural role in inhibiting growth of competing micro-organisms. The immunosuppressive and antimicrobial activities of FK506 and rapamycin are mediated by binding to the FKBP12 prolyl isomerase and the resulting FKBP12/FK506 and FKBP12/rapamycin complexes inhibit conserved protein targets, either the phosphatase calcineurin or the TOR (target of rapamycin) kinases, respectively. Streptomyces sp., 'Streptomyces hygroscopicus subsp. ascomyceticus' and Streptomyces hygroscopicus, which produce FK506, FK520 (also known as ascomycin, a C21 ethyl derivative of FK506) and rapamycin, respectively, produced toxins that inhibited the growth of competing cells of the yeast Saccharomyces cerevisiae and the pathogenic fungus Cryptococcus neoformans. Yeast and fungal mutants lacking FKBP12 or expressing dominant drug-resistant calcineurin or TOR mutants were resistant to FK506 and rapamycin, and to the toxins produced by Streptomyces. Streptomyces strains with mutations in the FK506 or rapamycin biosynthetic enzymes were impaired in toxin production. Finally, the toxins secreted by 'S. hygroscopicus subsp. ascomyceticus' and S. hygroscopicus promoted formation of FKBP12/calcineurin and FKBP12/TOR complexes in a two-hybrid assay and mutations that rendered calcineurin or TOR drug-resistant prevented interaction. These observations support the hypothesis that Streptomyces evolved to secrete FK506, FK520 and rapamycin as toxins to inhibit the growth of competing yeast and fungi.
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