A histidine-rich and cysteine-rich metal-binding domain at the C terminus of heat shock protein A from helicobacter pylori: Implication for nickel homeostasis and bismuth susceptibility

Abstract

HspA, a member of the GroES chaperonin family, is a small protein found in Helicobacter pylori with a unique histidine-and cysteine-rich domain at the C terminus. In this work, we overexpressed, purified, and characterized this protein both in vitro and in vivo. The apo form of the protein binds 2.10 ± 0.07 Ni2+ or 1.98 ± 0.08 Bi3+ ions/monomer with a dissociation constant (Kd) of 1.1 or 5.9 × 10 -19 μM, respectively. Importantly, Ni2+ can reversibly bind to the protein, as the bound nickel can be released either in the presence of a chelating ligand, e.g. EDTA, or at an acidic pH (pH1?2 3.8 ± 0.2). In contrast, Bi3+ binds almost irreversibly to the protein. Both gel filtration chromatography and native electrophoresis demonstrated that apo-HspA exists as a heptamer in solution. Unexpectedly, binding of Bi3+ to the protein altered its quaternary structure from a heptamer to a dimer, indicating that bismuth may interfere with the biological functions of HspA. When cultured in Ni2+-supplemented M9 minimal medium, Escherichia coli BL21(DE3) cells expressing wild-type HspA or the C-terminal deletion mutant clearly indicated that the C terminus might protect cells from high concentrations of external Ni2+. However, an opposite phenomenon was observed when the same E. coli hosts were grown in Bi 3+-supplemented medium. HspA may therefore play a dual role: to facilitate nickel acquisition by donating Ni2+ to appropriate proteins in a nickel-deficient environment and to carry out detoxification via sequestration of excess nickel. Meanwhile, HspA can be a potential target of the bismuth antiulcer drug against H. pylori. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.