Hsp90 promotes kinase evolution Authors: Jennifer Lachowiec, Department of Genome Sciences, Molecular and Cellular Biology Program, University of Washington, Seattle

Heat-shock protein 90 (Hsp90) promotes the maturation and stability of its client proteins, including many kinases. In doing so, Hsp90 may allow its clients to accumulate mutations as previously proposed by the capacitor hypothesis. If true, Hsp90 clients should show increased evolutionary rate compared to non-clients; however, other factors, such as gene expression and protein connectivity, may confound or obscure the chaperone’s putative contribution. Here, we compared the evolutionary rates of many Hsp90 clients and non-clients in the human protein kinase superfamily. We show that Hsp90 client status promotes evolutionary rate independently of, but in a small magnitude similar to that of gene expression and protein connectivity. Hsp90’s effect on kinase evolutionary rate was detected across mammals, specifically relaxing purifying selection. Hsp90 clients also showed increased nucleotide diversity and harbored more damaging variation than non-client kinases across humans. These results are consistent with the central argument of the capacitor hypothesis that interaction with the chaperone allows its clients to harbor genetic variation. Hsp90 client status is thought to be highly dynamic with as few as one amino acid change rendering a protein dependent on the chaperone. Contrary to this expectation, we found that across protein kinase phylogeny Hsp90 client status tends to be gained, maintained, and shared among closely related kinases. We also infer that the ancestral protein kinase was not an Hsp90 client. Taken together, our results suggest that Hsp90 played an important role in shaping the kinase superfamily. Introduction The conserved heat shock protein Hsp90 facilitates the proper folding and stability of its substrates (clients) (Taipale, et al. 2010), many of which are kinases with important roles in growth and development. Hsp90 perturbation increases the penetrance of expressed genetic variants and reveals cryptic genetic variation in genetically divergent populations of plant, fly, yeast, and fish (Rutherford and Lindquist 1998; Queitsch, et al. 2002; Yeyati, et al. 2007; Jarosz and Lindquist 2010). In worms, naturally varying Hsp90 levels predict mutation penetrance with lower Hsp90 levels resulting in greater penetrance (Burga, et al. 2011; Casanueva, et al. 2012). These observations with traditional model organisms prompted the controversial hypothesis that Hsp90 plays an important evolutionary role, allowing genetic variation to remain phenotypically silent and releasing it in environments that perturb Hsp90 function (Rutherford and Lindquist . CC-BY-NC-ND 4.0 International license peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/006411 doi: bioRxiv preprint first posted online Jun. 19, 2014;