THE IMMUNOSUPPRESSANT FK506 INHIBITS AMINO-ACID IMPORT IN SACCHAROMYCES-CEREVISIAE

The immunosuppressants cyclosporin A, FK506, and rapamycin inhibit gro wth of unicellular eukaryotic microorganisms and also block activation of T lymphocytes from multicellular eukaryotes. in vitro, these compo unds bind and inhibit two different types of peptidyl-prolyl cis-trans isomerases. Cyclosporin A binds cyclophilins, whereas FK506 and rapam ycin bind FK506-binding proteins (FKBPs). Cyclophilins and FKBPs are u biquitous, abundant, and targeted to multiple cellular compartments, a nd they may fold proteins in vivo. Previously, a 12-kDa cytoplasmic FK BP was shown to be only one of at least two FK506-sensitive targets in the yeast Saccharomyces cerevisiae. We find that a second FK506-sensi tive target is required for amino acid import. Amino acid-auxotrophic yeast strains (trp1 his4 leu2) are FK506 sensitive, whereas prototroph ic strains (TRP1 his4 leu2, trp1 HIS4 leu2, and trp1 his4 LEU2) are FK 506 resistant. Amino acids added exogenously to the growth medium miti gate FK506 toxicity. FK506 induces GCN4 expression, which is normally induced by amino acid starvation. FK506 inhibits transport of tryptoph an, histidine, and leucine into yeast cells. lastly, several genes enc oding proteins involved in amino acid import or biosynthesis confer FK 506 resistance. These findings demonstrate that FK506 inhibits amino a cid import in yeast cells, most likely by inhibiting amino acid transp orters. Amino acid transporters are integral membrane proteins which i mport extracellular amino acids and constitute a protein family sharin g 30 to 35% identity, including eight invariant prolines. Thus, the se cond FK506-sensitive target in yeast cells may be a proline isomerase that plays a role in folding amino acid transporters during transit th rough the secretory pathway.