title: Crystal structure of the heteromolecular chaperone, AscE-AscG, from the type III secretion system in Aeromonas hydrophila.
abstract: BACKGROUND
  The putative needle complex subunit AscF forms a ternary complex with the chaperones AscE and AscG in the type III secretion system of Aeromonas hydrophila so as to avoid premature assembly. Previously, we demonstrated that the C-terminal region of AscG (residues 62-116) in the hetero-molecular chaperone, AscE-AscG, is disordered and susceptible to limited protease digestion.
  
  
  RESULTS
  Here, we report the crystal structure of the ordered AscG(1-61) region in complex with AscE at 2.4 Å resolution. Helices α2 and α3 of AscE in the AscE-AscG(1-61) complex assumes a helix-turn-helix conformation in an anti-parallel fashion similar to that in apo AscE. However, in the presence of AscG, an additional N-terminal helix α1 in AscE (residues 4-12) is observed. PscG or YscG in the crystal structures of PscE-PscF-PscG or YscE-YscF-YscG, respectively, assumes a typical tetratricopeptide repeat (TPR) fold with three TPR repeats and one C-terminal capping helix. By comparison, AscG in AscE-AscG(1-61) comprises three anti-parallel helices that resembles the N-terminal TPR repeats in the corresponding region of PscG or YscG in PscE-PscF-PscG or YscE-YscF-YscG. Thermal denaturation of AscE-AscG and AscE-AscG(1-61) complexes demonstrates that the C-terminal disordered region does not contribute to the thermal stability of the overall complex.
  
  
  CONCLUSIONS
  The N-terminal region of the AscG in the AscE-AscG complex is ordered and assumes a structure similar to those in the corresponding regions of PscE-PscG-PscF or YscE-YscF-YscG complexes. While the C-terminal region of AscG in the AscE-AscG complex is disordered and will assume its structure only in the presence of the substrate AscF. We hypothesize that AscE act as a chaperone of the chaperone to keep AscG in a stable but partially disordered state for interaction with AscF.
authors.affiliation: Department of Biological Sciences, National University of Singapore, Singapore.
authors.forename: Chiradip
authors.initials: C
authors.lastname: Chatterjee
authors.forename: Sundramurthy
authors.initials: S
authors.lastname: Kumar
authors.forename: Smarajit
authors.initials: S
authors.lastname: Chakraborty
authors.forename: Yih Wan
authors.initials: YW
authors.lastname: Tan
authors.forename: Ka Yin
authors.initials: KY
authors.lastname: Leung
authors.forename: J
authors.initials: J
authors.lastname: Sivaraman
authors.forename: Yu-Keung
authors.initials: YK
authors.lastname: Mok
mesh_terms: D016980:Aeromonas hydrophila / Q000378:metabolism*; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry*; D003001:Cloning, Molecular; D018360:Crystallography, X-Ray / Q000379:methods; D004926:Escherichia coli / Q000378:metabolism; D006358:Hot Temperature; D018832:Molecular Chaperones / Q000737:chemistry*; D008969:Molecular Sequence Data; D011485:Protein Binding; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013696:Temperature
chemical_list: D001426:Bacterial Proteins; D018832:Molecular Chaperones