The binding mode of Ni-(L-His)2 in NikA revealed by X-ray crystallography

Abstract

The ABC-type importer NikABCDE mediates nickel acquisition in Escherichia coli. The periplasmic nickel-binding component NikA has a folding similar to that of the peptide transporter OppA and does not bind free nickel. Instead, we showed that the metal is tetra-coordinated by an organic tri-dentate molecule and His416. Conversely, it has been recently reported that NikA binds Ni-(L-His)2 and that addition of histidine increases the rate of nickel uptake in vivo. Here, we report the structure of NikA/Ni-(L-His)2 and show that histidine binding differs from peptide binding in OppA. The structure also confirms the central role of His416 in nickel binding.