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The molecular chaperone Hsp90 cooperates with multiple cochaperone proteins as it promotes the folding and activation of diverse client proteins. Some cochaperones regulate the ATPase activity of Hsp90, while others appear to promote Hsp90 interaction with specific types of client proteins. Through its interaction with the adenylate cyclase Cyr1, the Sgt1 cochaperone modulates the activity of the cAMP pathway in Saccharomyces cerevisiae. A specific mutation in yeast Hsp90, hsc82-W296A, or a mutation in Sgt1, sgt1-K360E, resulted in altered transcription patterns genetically linked to the cAMP pathway. Hsp90 interacted with Cyr1 in vivo and the hsc82-W296A mutation resulted in reduced accumulation of Cyr1. Hsp90-Sgt1 interaction was altered by either the hsc82-W296A or sgt1-K360E mutation, suggesting defective Hsp90-Sgt1 cooperation leads to reduced Cyr1 activity. Microarray analysis of hsc82-W296A cells indicated that over 80 % of all transcriptional changes in this strain may be attributed to altered cAMP signaling. This suggests that a majority of the cellular defects observed in hsc82-W296A cells are due to altered interaction with one specific essential cochaperone, Sgt1 and one essential client, Cyr1. Together our results indicate that specific interaction of Hsp90 and Sgt1 with Cyr1 plays a key role in regulating gene expression, including genes involved in polarized morphogenesis.

Ydj1 of Saccharomyces cerevisiae is an abundant cytosolic Hsp40, or J-type, molecular chaperone. Ydj1 cooperates with Hsp70 of the Ssa family in the translocation of preproteins to the ER and mitochondria and in the maturation of Hsp90 client proteins. The substrate-binding domain of Ydj1 directly interacts with steroid receptors and is required for the activity of diverse Hsp90-dependent client proteins. However, the effect of Ydj1 alteration on client interaction was unknown. We analyzed the in vivo interaction of Ydj1 with the protein kinase Ste11 and the glucocorticoid receptor. Amino acid alterations in the proposed client-binding domain or zinc-binding domain had minor effects on the physical interaction of Ydj1 with both clients. However, alteration of the carboxy-terminal farnesylation signal disrupted the functional and physical interaction of Ydj1 and Hsp90 with both clients. Similar effects were observed upon deletion of RAM1, which encodes one of the subunits of yeast farnesyltransferase. Our results indicate that farnesylation is a major factor contributing to the specific requirement for Ydj1 in promoting proper regulation and activation of diverse Hsp90 clients.

The molecular chaperone Hsp (heat-shock protein) 90 is critical for the activity of diverse cellular client proteins. In a current model, client proteins are transferred from Hsp70 to Hsp90 in a process mediated by the co-chaperone Sti1/Hop, which may simultaneously interact with Hsp70 and Hsp90 via separate TPR (tetratricopeptide repeat) domains, but the mechanism and in vivo importance of this function is unclear. In the present study, we used truncated forms of Sti1 to determine the minimal regions required for the Hsp70 and Hsp90 interaction, as well as Sti1 dimerization. We found that both TPR1 and TPR2B contribute to the Hsp70 interaction in vivo and that mutations in both TPR1 and TPR2B were required to disrupt the in vitro interaction of Sti1 with the C-terminus of the Hsp70 Ssa1. The TPR2A domain was required for the Hsp90 interaction in vivo, but the isolated TPR2A domain was not sufficient for the Hsp90 interaction unless combined with the TPR2B domain. However, isolated TPR2A was both necessary and sufficient for purified Sti1 to migrate as a dimer in solution. The DP2 domain, which is essential for in vivo function, was dispensable for the Hsp70 and Hsp90 interaction, as well as Sti1 dimerization. As evidence for the role of Sti1 in mediating the interaction between Hsp70 and Hsp90 in vivo, we identified Sti1 mutants that result in reduced recovery of Hsp70 in Hsp90 complexes. We also identified two Hsp90 mutants that exhibit a reduced Hsp70 interaction, which may help clarify the mechanism of client transfer between the two molecular chaperones.

Through simultaneous interactions with Hsp70 and Hsp90 via separate tetratricopeptide repeat (TPR) domains, the cochaperone protein Hop/Sti1 has been proposed to play a critical role in the transfer of client proteins from Hsp70 to Hsp90. However, no prior mutational analysis demonstrating a critical in vivo role for the TPR domains of Sti1 has been reported. We used site-directed mutagenesis of the TPR domains combined with a genetic screen to isolate mutations that disrupt Sti1 function. A single amino acid alteration in TPR2A disrupted Hsp90 interaction in vivo but did not significantly affect function. However, deletion of a conserved residue in TPR2A or mutations in the carboxy-terminal DP2 domain completely disrupted Sti1 function. Surprisingly, mutations in TPR1, previously shown to interact with Hsp70, were not sufficient to disrupt in vivo functions unless combined with mutations in TPR2B, suggesting that TPR1 and TPR2B have redundant or overlapping in vivo functions. We further examined the genetic and physical interaction of Sti1 with a mutant form of Hsp90, providing insight into the importance of the TPR2A domain of Sti1 in regulating Hsp90 function.

The ATP-dependent molecular chaperone Hsp90 and partner cochaperone proteins are required for the folding and activity of diverse cellular client proteins, including steroid hormone receptors and multiple oncogenic kinases. Hsp90 undergoes nucleotide-dependent conformational changes, but little is known about how these changes are coupled to client protein activation. In order to clarify how nucleotides affect Hsp90 interactions with cochaperone proteins, we monitored assembly of wild-type and mutant Hsp90 with Sti1, Sba1, and Cpr6 in Saccharomyces cerevisiae cell extracts. Wild-type Hsp90 bound Sti1 in a nucleotide-independent manner, while Sba1 and Cpr6 specifically and independently interacted with Hsp90 in the presence of the nonhydrolyzable analog of ATP, AMP-PNP. Alterations in Hsp90 residues that contribute to ATP binding or hydrolysis prevented or altered Sba1 and Cpr6 interaction; additional alterations affected the specificity of Cpr6 interaction. Some mutant forms of Hsp90 also displayed reduced Sti1 interaction in the presence of a nucleotide. These studies indicate that cycling of Hsp90 between the nucleotide-free, open conformation and the ATP-bound, closed conformation is influenced by residues both within and outside the N-terminal ATPase domain and that these conformational changes have dramatic effects on interaction with cochaperone proteins.