Several accessory proteins are required for the maturation of two nickel-containing enzymes in the gastric pathogen Helicobacter pylori. These two enzymes are hydrogenase and urease. Among the accessory/maturation proteins, the nickel-binding HypA protein has been previously shown to be required for the full activity of both the hydrogenase and the urease enzymes, while another nickel-binding protein, UreE, is known to be solely involved in the urease maturation process. In this study, UreE was shown to be required under all nickel levels for full activation of the apourease. By use of cross-linking studies, an interaction between purified HypA and UreE proteins was identified, leading to the formation of a 34 kDa heterodimer complex. The cross-linked adduct was detected by immunoblotting with either anti-HypA or anti-UreE antiserum. By using a two-plasmid system in Escherichia coli, the highest urease activity was achieved under low nickel conditions only when the UreE protein was expressed along with the wild-type HypA protein, but not with its nickel-binding-deficient variant HypA H2A. Addition of only 1 microM NiCl(2) into minimal medium abolished the need for HypA to activate the urease. Although various attempts to show direct nickel transfer from HypA to UreE failed, these results suggest that interactions between the nickel-binding accessory proteins HypA and UreE are required to allow nickel transfer from HypA eventually to the apourease in H. pylori.