title: Apo-Hsp90 coexists in two open conformational states in solution.
abstract: BACKGROUND
  Hsp90 (90 kDa heat-shock protein) plays a key role in the folding and activation of many client proteins involved in signal transduction and cell cycle control. The cycle of Hsp90 has been intimately associated with large conformational rearrangements, which are nucleotide-binding-dependent. However, up to now, our understanding of Hsp90 conformational changes derives from structural information, which refers to the crystal states of either recombinant Hsp90 constructs or the prokaryotic homologue HtpG (Hsp90 prokaryotic homologue).
  
  
  CONCLUSIONS
  Here, we present the first nucleotide-free structures of the entire eukaryotic Hsp90 (apo-Hsp90) obtained by small-angle X-ray scattering and single-particle cryo-EM (cryo-electron microscopy). We show that, in solution, apo-Hsp90 is in a conformational equilibrium between two open states that have never been described previously. By comparing our cryo-EM maps with HtpG and known Hsp90 structures, we establish that the structural changes involved in switching between the two Hsp90 apo-forms require large movements of the NTD (N-terminal domain) and MD (middle domain) around two flexible hinge regions.
  
  
  CONCLUSIONS
  The present study shows, for the first time, the structure of the entire eukaryotic apo-Hsp90, along with its intrinsic flexibility. Although large structural rearrangements, leading to partial closure of the Hsp90 dimer, were previously attributed to the binding of nucleotides, our results reveal that they are in fact mainly due to the intrinsic flexibility of Hsp90 dimer. Taking into account the preponderant role of the dynamic nature of the structure of Hsp90, we reconsider the Hsp90 ATPase cycle.
authors.affiliation: UMR 6026 Université de Rennes 1-CNRS, Equipe SDM, Campus de Beaulieu, 35042 Rennes, France.
authors.forename: Patrick
authors.initials: P
authors.lastname: Bron
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Giudice
authors.forename: Jean-Paul
authors.initials: JP
authors.lastname: Rolland
authors.forename: Rubén M
authors.initials: RM
authors.lastname: Buey
authors.forename: Pascale
authors.initials: P
authors.lastname: Barbier
authors.forename: J Fernando
authors.initials: JF
authors.lastname: Díaz
authors.forename: Vincent
authors.initials: V
authors.lastname: Peyrot
authors.forename: Daniel
authors.initials: D
authors.lastname: Thomas
authors.forename: Cyrille
authors.initials: C
authors.lastname: Garnier
mesh_terms: D000251:Adenosine Triphosphatases / Q000737:chemistry / Q000648:ultrastructure; D000818:Animals; D018360:Crystallography, X-Ray; D018841:HSP90 Heat-Shock Proteins / Q000737:chemistry* / Q000648:ultrastructure; D008854:Microscopy, Electron; D008958:Models, Molecular; D011487:Protein Conformation; D017510:Protein Folding; D012996:Solutions; D013552:Swine
chemical_list: D018841:HSP90 Heat-Shock Proteins; D012996:Solutions; D000251:Adenosine Triphosphatases