HSP90AA1

HSP90AA1 (Hsp90α) is an ATP-dependent homodimeric molecular chaperone that operates as a central node in proteostasis by loading, folding, stabilizing, and activating a broad clientele of signaling proteins through an ATPase-driven conformational cycle (PMID:34937942, PMID:30193096). Cryo-EM of the glucocorticoid receptor system established the structural logic of the chaperone cycle: two Hsp70 molecules together with Hop load partially unfolded client onto the open Hsp90 dimer through an extended composite binding pocket (PMID:34937942), after which the client is threaded through the Hsp90 lumen and restored to a folded, ligand-competent state stabilized by the co-chaperone p23 (PMID:34937936). Kinase clients are delivered via the Hsp90-Cdc37 system, which recognizes determinants such as the G-box motif and helix αC/αE of the client kinase (PMID:16285732), and a PP5-containing complex is scaffolded by Hsp90 so that the phosphatase dephosphorylates sites adjacent to a bound CRaf kinase domain, with the order of dephosphorylation and kinase release sterically controlled by the chaperone (PMID:37069154). Through these cycles Hsp90 maintains and activates client kinases and transcription factors including Raf-1, Pim-1, Cdk2, and STAT3 (PMID:11751906, PMID:11237709, PMID:16285732, PMID:22271514), and TPR-domain immunophilins couple client-loaded Hsp90 complexes to dynein for retrograde transport and nuclear import of steroid receptors and p53 (PMID:15157665); Sgt1 bridges Hsp90 to SCF E3 ubiquitin ligase assemblies through a distinct N-terminal interaction site (PMID:18818696). The chaperone's activity is gated by post-translational control: Wee1 phosphorylates a conserved tyrosine that modulates ATPase activity, inhibitor binding, and G2/M checkpoint behavior (PMID:20519952); CDK5 phosphorylation at Ser595 disrupts Hsp90α binding to TFEB and thereby autophagy induction (PMID:35941759); and USP14-mediated deubiquitination removes K48-linked chains to stabilize the protein (PMID:37633951). Beyond client folding, an evolutionarily conserved amphipathic helix confers a membrane-deforming activity that drives multivesicular body fusion and exosome release in the open conformation, structurally separable from chaperone function (PMID:30193096). Hsp90α is also subject to a toxic conversion: nitration of a single tyrosine (position 33 or 56) confers a pro-apoptotic gain-of-function that kills motor neurons via P2X7/Fas signaling (PMID:23487751). In disease contexts, elevated HSP90AA1 promotes autophagy-dependent chemoresistance and apoptosis suppression in cancer and supports CYP2E1-driven oxidative stress in fatty liver disease (PMID:37633951, PMID:30153855).

1. 2001 Medium

   Establishing which signaling kinases depend on Hsp90 defined its role as an activator of oncogenic kinases; Raf-1 and Pim-1 were shown to be Hsp90 clients whose stability and activity require the chaperone.

   Evidence Co-immunoprecipitation, kinase assays, and geldanamycin-induced degradation in cultured cells; a membrane-targeted Hsp90N variant binding Raf-1 independent of Cdc37

   PMID:11237709 PMID:11751906

   Open questions at the time

   • No structural detail of the kinase-Hsp90 interface
   • Client recognition determinants on the kinase not mapped
2. 2004 Medium

   How Hsp90-client complexes reach the nucleus was addressed by showing TPR-immunophilins link the complex to dynein for retrograde microtubule transport and importin-dependent nuclear entry.

   Evidence Biochemical fractionation, co-IP, genetic disruption, and live-cell imaging of steroid receptor/p53 trafficking

   PMID:15157665

   Open questions at the time

   • Mechanism integrates data across multiple clients rather than a single reconstituted system
   • Stoichiometry of immunophilin-dynein coupling unresolved
3. 2005 Medium

   Dissecting kinase client loading, the Hsp90-Cdc37 complex was shown to engage Cdk2 through separable binding determinants, distinguishing Cdc37-binding from Hsp90-binding surfaces on the kinase.

   Evidence Geldanamycin treatment, pull-down with deletion/point mutants, and molybdate stabilization in K562 cells

   PMID:16285732

   Open questions at the time

   • No structural model of the loaded kinase complex at this stage
   • Conformational state of the kinase during loading not defined
4. 2008 High

   Identifying a distinct N-terminal interaction site for Sgt1 explained how Hsp90 is bridged to SCF E3 ubiquitin ligase assemblies, expanding its role beyond folding into ubiquitin-pathway scaffolding.

   Evidence X-ray crystallography of the core Hsp90-Sgt1 complex with structure-guided mutagenesis validated in yeast and plants

   PMID:18818696

   Open questions at the time

   • Human SCF client repertoire dependent on this interface not enumerated
   • Functional coupling to the ATPase cycle unclear
5. 2010 Medium

   Linking Hsp90 to cell-cycle control, Wee1-mediated tyrosine phosphorylation was shown to modulate ATPase activity, inhibitor binding, and kinase-client chaperoning, with non-phosphorylatable Hsp90 escaping the G2/M checkpoint.

   Evidence Phosphorylation and ATPase assays, Y24F mutagenesis, and yeast cell-cycle analysis with co-IP

   PMID:20519952

   Open questions at the time

   • Human residue and physiological stoichiometry of phosphorylation not fully defined
   • Which client subset is selectively affected is incompletely characterized
6. 2012 Medium

   Direct binding measurements established STAT3 as an Hsp90 partner requiring an intact DNA-binding domain, extending the client repertoire to transcription factors.

   Evidence Surface plasmon resonance, mutagenesis of STAT3 arginines 414/417, co-IP, and confocal colocalization in MCF7 cells

   PMID:22271514

   Open questions at the time

   • Functional consequence for STAT3 transcriptional output not measured
   • Isoform specificity (α vs β) for the interaction not resolved
7. 2013 Medium

   A toxic gain-of-function was uncovered: nitration of a single Hsp90 tyrosine converts the chaperone into a pro-death signal in motor neurons through P2X7/Fas signaling, relevant to ALS.

   Evidence Site-specific nitration, cell death assays, pathway inhibition, and immunohistochemistry in ALS patient tissue and animal models

   PMID:23487751

   Open questions at the time

   • Structural basis of how nitration triggers extracellular P2X7/Fas signaling unclear
   • Generality beyond motor neurons not established
8. 2018 High

   Separating a non-canonical activity from folding, a conserved amphipathic helix was shown to deform membranes and drive MVB fusion for exosome release, gated by the open Hsp90 conformation.

   Evidence Cell-free membrane deformation assay, in vivo exosome release, helix mutagenesis, and drug-locked conformational states

   PMID:30193096

   Open questions at the time

   • How conformational gating is regulated in cells not defined
   • Whether ATPase cycle and membrane activity compete in vivo unresolved
9. 2021 High

   Cryo-EM of the GR loading and maturation complexes resolved the central mechanistic question of how clients enter and exit the chaperone, defining Hsp70/Hop-mediated loading of partially unfolded client and p23-stabilized refolding within the Hsp90 lumen.

   Evidence Cryo-EM of the Hsp90-Hsp70-Hop-GR loading complex and the GR-Hsp90-p23 maturation complex with functional validation

   PMID:34937936 PMID:34937942

   Open questions at the time

   • Whether the same loading architecture applies to kinase clients not shown here
   • Energetics and timing of the loading-to-maturation transition not captured
10. 2022 Medium

    Hsp90α was placed in autophagy regulation through a CDK5-Ser595 phosphorylation switch that controls its binding to TFEB and TFEB nuclear translocation.

    Evidence Co-IP, phosphorylation mapping, CDK5 manipulation, TFEB localization imaging, and C. elegans lifespan assays

    PMID:35941759

    Open questions at the time

    • Whether Hsp90α chaperones TFEB folding or merely escorts it is unclear
    • Direct CDK5 phosphorylation site verification across systems limited
11. 2023 High

    A PP5-containing kinase complex structure revealed Hsp90 both activates PP5 and scaffolds its action on bound CRaf, with steric control ordering kinase release before Cdc37 dephosphorylation.

    Evidence Cryo-EM of the Hsp90:Cdc37:CRaf:PP5 complex with biochemical dephosphorylation assays

    PMID:37069154

    Open questions at the time

    • Generality across other kinase clients not established
    • How PP5 recruitment is timed within the chaperone cycle unresolved
12. 2023 Medium

    Protein-level control of HSP90AA1 was shown via USP14 deubiquitination removing K48 chains, with stabilized Hsp90 promoting CYP2E1 accumulation and oxidative stress in fatty liver disease.

    Evidence Co-IP, ubiquitination analysis, and USP14 manipulation in vitro and in a mouse diet model

    PMID:37633951

    Open questions at the time

    • E3 ligase opposing USP14 not identified
    • Whether CYP2E1 is a direct Hsp90 client not shown

• It remains unresolved how the distinct post-translational switches (Wee1 and CDK5 phosphorylation, nitration, ubiquitination) are integrated to select which clients and non-chaperone activities Hsp90α executes in a given cellular context.
• No unified model linking modification state to client/activity selection
• Structural basis for kinase-client loading (vs GR) not directly determined
• In vivo regulation of conformational gating between chaperone and membrane functions unknown