CNS1 ENCODES AN ESSENTIAL P60 STI1 HOMOLOG IN SACCHAROMYCES-CEREVISIAE THAT SUPPRESSES CYCLOPHILIN-40 MUTATIONS AND INTERACTS WITH HSP90/

Cyclophilins are cis-trans-peptidyl-prolyl isomerases that bind to and are inhibited by the immunosuppressant cyclosporin A (CsA), The toxic effects of CsA are mediated by the 18-kDa cyclophilin a protein. A la rger cyclophilin of 40 kDa, cyclophilin 40, is a component of Hsp90-st eroid receptor complexes and contains two domains, an amino-terminal p rolyl isomerase domain and a carboxy-terminal tetratricopeptide repeat (TPR) domain, There are two cyclophilin 40 homologs in the yeast Sacc haromyces cerevisiae, encoded by the CPR6 and CPR7 genes. Yeast strain s lacking the Cpr7 enzyme are viable but exhibit a slow-growth phenoty pe, In addition, we show here that cpr7 mutant strains are hypersensit ive to the Hsp90 inhibitor geldanamycin. When overexpressed, the TPR d omain of Cpr7 alone complements both cpr7 mutant phenotypes, while ove rexpression of the cyclophilin domain of Cpr7, full-length Cpr6, or hu man cyclophilin 40 does not, The open reading frame YBR155w, which has moderate identity to the yeast p60 homolog ST11, was isolated as a hi gh-copy-number suppressor of the cpr7 slow-growth phenotype, We show t hat this Sti1 homolog Cns1 (cyclophilin seven suppressor) is constitut ively expressed, essential, and found in protein complexes with both y east Hsp90 and Cpr7 but not with Cpr6. Cyclosporin A inhibited Cpr7 in teractions with Cns1 but not with Hsp90, In summary, our findings iden tify a novel component of the Hsp90 chaperone complex that shares func tion,vith cyclophilin 40 and provide evidence that there are functiona l differences between two conserved sets of Hsp90 binding proteins in yeast.