Solubility-Promoting Function of Hsp90 Contributes to Client Maturation and Robust Cell Growth

Abstract

The Hsp90 chaperone is required for the maturation of signal transduction clients, including many kinases and nuclear steroid hormone receptors. The binding and hydrolysis of ATP by Hsp90 drive conformational rearrangements in three structure domains. Two intrinsically disordered regions of Hsp90 located between these domains and at the C terminus have traditionally been considered to impart flexibility. We discovered that the charged nature of these acid-rich disordered regions imparts a solubility-promoting function to Hsp90 that is important for its cellular activity in yeast. Both the solubility-promoting function and ATPase activity must occur in the same Hsp90 molecule in order to support robust growth, suggesting that the solubility-promoting function is required during the ATP-driven client maturation process. Expression of model clients together with Hsp90 variants indicated interdependent solubilities mediated by the aggregation propensities of both the client and Hsp90. We propose a model whereby the charge-rich disordered regions of Hsp90 serve a solubility-promoting function important for complexes with aggregation-prone clients. These findings demonstrate a novel biological function of the intrinsically disordered regions in Hsp90 and provide a compelling rationale for why their charged properties are conserved throughout eukaryotic evolution.

Analyses of the Effects of All Ubiquitin Point Mutants on Yeast Growth Rate

“…Of note, the WT residues at the ultra tolerant positions are never aliphatic or aromatic. Based on these properties (polar and tolerant of individual mutations) we speculate that this region of ubiquitin may perform a solubility promoting role as similar properties have been observed in solubility promoting regions of the Hsp90 chaperone 50; 51 . These studies in Hsp90 support the idea that stringent natural selection for stability can result in highly optimized sequences that are so soluble that they are robust to individual mutations when measured with experimentally limited sensitivity.…”

Latent Effects of Hsp90 Mutants Revealed at Reduced Expression Levels

“…By sampling across the variety of different amino acid substitutions, EMPIRIC provides detailed information about the physical constraints on protein function. We previously reported a bimodal distribution of fitness effects (DFE) for an evolutionarily conserved region of the yeast Hsp90 gene[9], an essential chaperone required for the maturation of many kinases [10]–[12]. Bimodal DFEs, where most mutants have fitness effects close to either null or wild type (WT), appear common in nature as they have been observed in many other fitness studies [13]–[17].…”

Systematic Mutant Analyses Elucidate General and Client-Specific Aspects of Hsp90 Function

“…The “lid” region of the ATPase domain adopts distinct nucleotide-bound conformations (Ali et al, 2006; Prodromou et al, 1997) including transient dimerization when ATP and the co-chaperone p23 are bound that in turn mediate dramatic conformational changes of the full-length Hsp90 dimer (Southworth and Agard, 2008). Purified Hsp90 can act as an anti-aggregation chaperone (Pursell et al, 2012), but activation of signaling clients including kinases and nuclear steroid receptors in vitro requires multiple co-chaperones including p23, Hop, Cdc37, and Hsp70 (Arlander et al, 2006; Boczek et al, 2015; Guy et al, 2015). Exciting recent views of Hsp90 bound to clients and co-chaperones (Karagoz et al, 2014; Kirschke et al, 2014; Vaughan et al, 2006) indicate that different clients can bind to distinct surfaces of Hsp90.…”