Regulation of the HscA ATPase reaction cycle by the co-chaperone HscB and the iron-sulfur cluster assembly protein IscU

J Biol Chem. 2004 Dec 24;279(52):53924-31. doi: 10.1074/jbc.M410117200. Epub 2004 Oct 13.


The ATPase activity of HscA, a specialized hsp70 molecular chaperone from Escherichia coli, is regulated by the iron-sulfur cluster assembly protein IscU and the J-type co-chaperone HscB. IscU behaves as a substrate for HscA, and HscB enhances the binding of IscU to HscA. To better understand the mechanism by which HscB and IscU regulate HscA, we examined binding of HscB to the different conformational states of HscA and the effects of HscB and IscU on the kinetics of the individual steps of the HscA ATPase reaction cycle. Affinity sensor studies revealed that whereas IscU binds both ADP (R-state) and ATP (T-state) HscA complexes, HscB interacts only with an ATP-bound state. Studies of ATPase activity under single-turnover and rapid mixing conditions showed that both IscU and HscB interact with the low peptide affinity T-state of HscA (HscA++.ATP) and that both modestly accelerate (3-10-fold) the rate-determining steps in the HscA reaction cycle, k(hyd) and k(T-->R). When present together, IscU and HscB synergistically stimulate both k(hyd) (approximately = 500-fold) and k(T-->R) (approximately = 60-fold), leading to enhanced formation of the HscA.ADP-IscU complex (substrate capture). Following ADP/ATP exchange, IscU also stimulates k(R-->T) (approximately = 50-fold) and thereby accelerates the rate at which the low peptide affinity HscA++.ATP T-state is regenerated. Because HscA nucleotide exchange is fast, the overall rate of the chaperone cycle in vivo will be determined by the availability of the IscU-HscB substrate-co-chaperone complex.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adenosine Diphosphate / metabolism
  • Adenosine Triphosphatases / metabolism*
  • Adenosine Triphosphate / metabolism
  • Drug Synergism
  • Escherichia coli / metabolism
  • Escherichia coli Proteins / chemistry
  • Escherichia coli Proteins / metabolism*
  • Escherichia coli Proteins / pharmacology*
  • HSP70 Heat-Shock Proteins / chemistry
  • HSP70 Heat-Shock Proteins / metabolism*
  • Heat-Shock Proteins / metabolism
  • Heat-Shock Proteins / pharmacology*
  • Homeostasis* / drug effects*
  • Hydrolysis
  • Iron-Sulfur Proteins / metabolism
  • Iron-Sulfur Proteins / pharmacology*
  • Kinetics
  • Molecular Chaperones / metabolism
  • Molecular Chaperones / pharmacology*
  • Protein Conformation
  • Recombinant Proteins


  • Escherichia coli Proteins
  • HSP70 Heat-Shock Proteins
  • Heat-Shock Proteins
  • HscB protein, E coli
  • Iron-Sulfur Proteins
  • IscU protein, E coli
  • Molecular Chaperones
  • Recombinant Proteins
  • hscA protein, E coli
  • Adenosine Diphosphate
  • Adenosine Triphosphate
  • Adenosine Triphosphatases