Roles of nickel binding proteins in Helicobacter species

Abstract

Proteins Hpn and Hpn-like of the gastric pathogen H. pylori were hypothesized to bind nickel since histidine residues make up 45% and 27% of their amino acid content, respectively. Characterization of an hpn, an hpn-like and an hpn, hpn-like double mutant revealed novel functions for these gene products in nickel detoxification and storage. Compared to the wild-type parent, mutant strains were more sensitive to elevated concentrations of nickel, cobalt, and cadmium, indicating roles for the two proteins in surviving metal toxicity. Under low nickel conditions, the mutants exhibited higher urease activities and had increased amount of Ni-associated with urease; but similar urease apo-protein levels to wild-type. The parent achieved mutant level urease activities under nickel supplementation and lower pH conditions while growth with a nickel chelator decreased mutant but not wild-type urease activities. These results strongly imply a role for these proteins as nickel reservoirs/storage proteins. H. hepaticus colonizes a non-acidic niche, hence nickel metabolism may be different than in H. pylori. A nikR mutant exhibited higher urease and hydrogenase activities under all supplemental nickel conditions, but there was no change in urease expression, and NikR did not bind pUreA or pHydA. Higher total nickel levels (detected by ICP-MS) in the nikR mutant implied possible higher nickel transporter (NikA) levels, which was later verified by qRT-PCR and binding of NikR to pNikA. Periplasmic nitrate reductase (NapA) was upregulated in the NikR strain. Wild-type H. hepaticus had increased H2 oxidation levels in the presence of nitrate compared to O2 provided as a terminal acceptor, suggesting the importance of anaerobic respiration for this bacterium. The NikR strain showed higher H2 oxidation levels than the parent with either O2/NO3- as electron acceptor. Also, the rate of nitrate disappearance was higher in a nikR mutant. Collectively, these results suggest NikR modulates the nickel-enzymes via regulating the nickel transporter and plays important roles other than in nickel metabolism. My studies aimed at characterizing nickel binding proteins by studying the mutant strains in two different organisms Helicobacter species. The data indicate that these proteins play vital functions to specifically suit the needs of each pathogen.