IMMUNOSUPPRESSANT TARGET PROTEIN FKBP12 IS REQUIRED FOR P-GLYCOPROTEIN FUNCTION IN YEAST

The mammalian P-glycoprotein (Pgp) is a similar to 170-kDa membrane pr otein that mediates multidrug resistance in many chemotherapy-resistan t tumors by effluxing toxic compounds from the cell, Pgp homologs are expressed in many organisms, from bacteria to yeast and mammals, Previ ous studies established a model system to analyze the function of muri ne, human, and Plasmodium falciparum Pgp by heterologous expression in the yeast Saccharomyces cerevisiae, However, such studies have been h ampered by the inherent resistance of yeast cells to chemotherapeutic agents, We find that an erg6 mutation, which blocks the final syntheti c step of the membrane sterol ergosterol, renders yeast sensitive to a nthracyclines and dactinomycin, clinically relevant Pgp substrates, We demonstrate that expression of the murine mdr3 gene confers dactinomy cin resistance in both the erg6 mutant yeast strain and in an erg6 rad 52 DNA repair mutant yeast strain, Similarly, murine mdr3 expression c onfers resistance to the immunosuppressants cyclosporin A (CsA) and FK 506 in a CsA-FR506-sensitive vph6 mutant yeast strain, CsA and FK506 a re known to partially overcome Pgp-mediated drug resistance, suggestin g the targets of these drugs might regulate Pgp function, We find that both murine mdr3 and the yeast Pgp homolog STE6 function in yeast mut ants lacking the CsA target proteins cyclophilin A and calcineurin, In contrast, murine mdr3 function was severely compromised in yeast muta nts lacking the FK506/rapamycin target protein FKBP12. Both wild-type FKBP12 and an F43Y FKBP12 mutant with reduced prolyl isomerase activit y supported mdr3 function, Our results support the model that immunosu ppressants reverse multidrug resistance by competing with other Pgp su bstrates but reveal that inhibition of FKBP12-dependent Pgp function m ay also contribute to reversal of multidrug resistance by FK506 and ra pamycin.