Heat-shock protein 90 (Hsp90) chaperones a key subset of signaling proteins and is necessary for malignant transformation. Hsp90 is subject to an array of posttranslational modifications that affect its function, including acetylation. Histone deacetylase (HDAC) inhibitors and knockdown of HDAC6 induce Hsp90 acetylation and inhibit its activity. However, direct determination of the functional consequences of Hsp90 acetylation has awaited mapping of specific sites. We now demonstrate that Hsp90 K294 is acetylated. Mutational analysis of K294 shows that its acetylation status is a strong determinant of client protein and cochaperone binding. In yeast, Hsp90 mutants that cannot be acetylated at K294 have reduced viability and chaperone function compared to WT or to mutants that mimic constitutive acetylation. These data suggest that acetylation/deacetylation of K294 plays an important role in regulating the Hsp90 chaperone cycle.

TRAP1 (TNF receptor-associated protein), a member of the HSP90 chaperone family, is found predominantly in mitochondria. TRAP1 is broadly considered to be an anticancer molecular target. However, current inhibitors cannot distinguish between HSP90 and TRAP1, making their utility as probes of TRAP1-specific function questionable. Some cancers express less TRAP1 than do their normal tissue counterparts, suggesting that TRAP1 function in mitochondria of normal and transformed cells is more complex than previously appreciated. We have used TRAP1-null cells and transient TRAP1 silencing/overexpression to show that TRAP1 regulates a metabolic switch between oxidative phosphorylation and aerobic glycolysis in immortalized mouse fibroblasts and in human tumor cells. TRAP1-deficiency promotes an increase in mitochondrial respiration and fatty acid oxidation, and in cellular accumulation of tricarboxylic acid cycle intermediates, ATP and reactive oxygen species. At the same time, glucose metabolism is suppressed. TRAP1-deficient cells also display strikingly enhanced invasiveness. TRAP1 interaction with and regulation of mitochondrial c-Src provide a mechanistic basis for these phenotypes. Taken together with the observation that TRAP1 expression is inversely correlated with tumor grade in several cancers, these data suggest that, in some settings, this mitochondrial molecular chaperone may act as a tumor suppressor.M olecular chaperones help to maintain cellular homeostasis.The heat-shock protein 90 (HSP90) family of molecular chaperones is highly conserved from bacteria to mammals. HSP90 itself is an essential molecular chaperone found in the cytoplasm and nucleus of all eukaryotic cells (1, 2). In multicellular eukaryotes, the HSP90 family includes the mitochondrial chaperone TRAP1 (TNF receptor-associated protein), which shares 50% sequence similarity with HSP90. Although TRAP1 binds and hydrolyzes ATP in an analogous manner to HSP90 (3), its cellular function is less well understood. Thus, although many HSP90-dependent proteins ("clients") and interacting cochaperones have been described (www.picard.ch/downloads/Hsp90interactors.pdf), the validated list of TRAP1-dependent clients is quite small and TRAP1-interacting cochaperones, if they exist, have yet to be identified (4).Several studies have suggested that TRAP1 plays a cytoprotective role by buffering reactive oxygen species (ROS)-mediated oxidative stress (5, 6), and others have reported that TRAP1 overexpression attenuates ROS production (7). The antioxidant properties of TRAP1, together with its reported ability to regulate opening of the mitochondrial permeability transition pore (8, 9), may contribute to its antiapoptotic activity (4). For these reasons, TRAP1 has been proposed as an anticancer molecular target, and first-generation inhibitors have shown some anticancer activity in preclinical models (10). However, these inhibitors do not distinguish between HSP90 and TRAP1 (11), and TRAP1 expression in cancer is variable but HSP90 comprises as much as 5% of...

Summary Swe1 (Saccharomyces WEE1), the only “true” tyrosine kinase in budding yeast, is an Hsp90 client protein. Here we show that Swe1Wee1 phosphorylates a conserved tyrosine residue (Y24 in yeast Hsp90 and Y38 in human Hsp90α) in the N-domain of Hsp90. Phosphorylation is cell cycle-associated and modulates the ability of Hsp90 to chaperone a selected clientele, including v-Src and several other kinases. Non-phosphorylatable mutants have normal ATPase activity, support yeast viability, and productively chaperone the Hsp90 client glucocorticoid receptor. Deletion of SWE1 in yeast increases Hsp90 binding to its inhibitor geldanamycin, and pharmacologic inhibition/silencing of Wee1 sensitizes cancer cells to Hsp90 inhibitor-induced apoptosis. These findings demonstrate that Hsp90 chaperoning of distinct client proteins is differentially regulated by specific post-translational modification of a unique subcellular pool of the chaperone, and they provide a novel strategy to increase the cellular potency of Hsp90 inhibitors.

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