Metallo-GTPase HypB from Helicobacter pylori and Its Interaction with Nickel Chaperone Protein HypA*

The maturation of [NiFe]-hydrogenase is highly dependent on a battery of chaperone proteins. Among these, HypA and HypB were proposed to exert nickel delivery functions in the metallocenter assembly process, although the detailed mechanism remains unclear. Herein, we have overexpressed and purified wild-type HypB as well as two mutants, K168A and M186L/F190V, from Helicobacter pylori. We demonstrated that all proteins bind Ni²⁺ at a stoichiometry of one Ni²⁺ per monomer of the proteins with dissociation constants at micromolar levels. Ni²⁺ elevated GTPase activity of WT HypB, which is attributable to a lower affinity of the protein toward GDP as well as Ni²⁺-induced dimerization. The disruption of GTP-dependent dimerization has led to GTPase activities of both mutants in apo-forms almost completely abolished, compared with the wild-type protein. The GTPase activity is partially restored for HypB(M186L/F190V) mutant but not for HypB(K168A) mutant upon Ni²⁺ binding. HypB forms a complex with its partner protein HypA with a low affinity (K_d of 52.2 ± 8.8 μM). Such interactions were also observed in vivo both in the absence and presence of nickel using a GFP-fragment reassembly technique. The putative protein-protein interfaces on H. pylori HypA and HypB proteins were identified by NMR chemical shift perturbation and mutagenesis studies, respectively. Intriguingly, the unique N terminus of H. pylori HypB was identified to participate in the interaction with H. pylori HypA. These structural and functional studies provide insight into the molecular mechanism of Ni²⁺ delivery during maturation of [NiFe]-hydrogenase.