p50(cdc37) is a nonexclusive hsp90 cohort which participates intimately inhsp90-mediated folding of immature kinase molecules

Hsp90 and p50(cdc37) provide a poorly understood biochemical function essen tial to certain protein kinases, and recent models describe p50(cdc37) as e xclusive hsp90 cohort which links hsp90 machinery to client kinases. We des cribe here the recovery of p50(cdc37) in immunoadsorptions directed against the hsp90 cohorts FKBP52, cyp40, p60HOP, hsp70, and p23. Additionally, mon oclonal antibodies against FKBP52 coadsorb maturation intermediates of the hsp90-dependent kinases p56(lck) and HRI, and the presence of these maturat ion intermediates significantly increases the representation of p50(cdc37) and hsp90 on FKPB52 machinery. Although the native heterocomplex between hs p90 and p50(cdc37) is salt-labile, their dynamic interactions with kinase s ubstrates produce kinase-chaperone heterocomplexes which are highly salt-re sistant. The hsp90 inhibitor geldanamycin does not directly disrupt the nat ive association of hsp90 with p50(cdc37) per se, but does result in the for mation of salt-labile hsp90-kinase heterocomplexes which lack the p50(cdc37 ) cohort. We conclude that p50(cdc37) does not simply serve as a passive st ructural bridge between hsp90 and its kinase substrates; instead, p50(cdc37 ) is a nonexclusive hsp90 cohort which responds to hsp90's nucleotide-regul ated conformational switching during the generation of high-affinity intera ctions within the hsp90-kinase-p50(cdc37) heterocomplex.