Structural insights into how metallochaperones UreE and UreG interact to deliver a toxic metal to urease

Abstract

Maturation of urease, a virulence factor for Helicobacter pylori infection, requires the delivery of nickel ions to the metalloenzyme. To avoid cytotoxicity, nickel ions are transferred within protein complexes of metallochaperones to ensure that the toxic metal do not escape into the cytoplasm. In the urease maturation pathway, metallochaperone UreG receives its nickel ions by forming a 2:2 complex with another metallochaperone UreE. Using C-terminal truncation variants of UreE [UreE(1-158) and UreE(1-148)], we determined the crystal structures of the UreE2G2 complex bound with nickel ions and GMPPNP, a non-hydrolysable GTP analogue. UreE-UreG interactions are asymmetric, where the UreG dimer mainly interacts with the proximal UreE. GTP binding induces conformational changes in the G2 and CPH motifs of distal UreG that enable its anchorage to the UreE2G2 complex. His68 of distal UreG moves towards and chelates a nickel ion at the UreE binding site. Formation of the UreE2G2 complex juxtaposes the nickel binding sites of UreE and UreG. Nickel transfer from UreE to UreG induces conformational changes that weaken UreE-UreG interactions, thereby facilitating UreG dissociation. We further demonstrated that the hydrogenase maturation factor HypA, providing the nickel source, could activate urease in vitro through protein-protein interaction with wild-type UreE but not with its truncation variants. UreG binding induces conformational changes in the C-terminal tail of UreE, promoting dissociation of HypA-UreE complex. Our work presents a paradigm on how GTP and nicking binding allosterically regulate the formation of a metallochaperone complex to facilitate nickel transfer. SignificanceOur work provides insights into how cells solve the problem of trafficking a toxic metal, nickel, to the active site of urease. Colonization of Helicobacter pylori in acidic human stomach requires the biosynthesis of active urease, which involves the delivery of the toxic nickel ions to the active site of the metalloenzyme. To avoid cytotoxicity, nickel ions are transported from one metallochaperone to another via the formation of protein complexes so that the toxic metal ions do not escape into the cytoplasm. Supported by structural and biochemical evidence, we present a paradigm on how GTP and nickel binding allosterically promote the formation of a complex between metallochaperones UreE and UreG to facilitate nickel transfer between the two proteins.