A MODEL OF PROTEIN TARGETING MEDIATED BY IMMUNOPHILINS AND OTHER PROTEINS THAT BIND TO HSP90 VIA TETRATRICOPEPTIDE REPEAT DOMAINS

We have shown recently that the CyP-40 and FKBP52/hsp56 bind to a hsp9 0 and that they exist in separate heterocomplexes with the glucocortic oid receptor (GR). FKBP52/hsp56 binds to hsp90 via its tetratricopepti de repeat (TPR) domains, it is not required for GR . hsp90 heterocompl ex assembly, and it is thought to play a role in targeted movement of the GR In this work we examine the hsp90 binding of four proteins (FKB P52/hsp56, CyP-40, p50, Mas70p) thought to be involved in targeted pro tein trafficking. FKBP52/hsp56 and CyP-40 (each with three TPR), local ize to the nucleus and nucleoli, respectively, and form relatively wea k complexes with hsp90 that are competed by a CyP-40 fragment containi ng its three TPRs. The p50 component of the Src . hsp90 and Raf . hsp9 0 heterocomplexes localizes to cytoskeletal fibers extending from the perinuclear region to the plasma membrane and forming a rim under the plasma membrane of endothelial cells, p50, Mas70p (seven TPRs), which is a receptor for mitochondrial import, and the p60 (six to eight TPRs ) component off the steroid receptor . hsp90 heterocomplex assembly sy stem bind very tightly to hsp90 in a manner that is not competed by th e CyP-40 fragment, However, bacterially expressed p60 blocks the bindi ng of p50, Mas70p, FKBP52/hsp56, and CyP-40 to purified hsp90. The dat a are consistent with binding of all of these proteins to a site on hs p90 that is a general TPR domain acceptor. Our localization and bindin g data are used to develop a model in which proteins that tare chapero ned by hsp90 move as dynamic complexes to their cellular sites of acti on, with the TPR-containing protein participating in targeting the mov ement of the complexes.