Extracytoplasmic Nickel‐Binding Proteins

International audiencen bacteria, nickel is required for the activation of at least nine enzymes. Among several nickel import systems dedicated to Ni(II) acquisition, canonical ABC‐type importers possess an extracytoplasmic nickel‐binding protein (or Ni‐BP) that serves as the initial receptor and the major determinant of the transporter specificity. Ni‐BPs evolved from peptide‐BPs from at least four independent evolutionary origins. Their evolutionary history is dominated by horizontal gene transfers among bacteria and Archaea, among Gram‐positive and Gram‐negative bacteria, and among organisms having different lifestyles. Phylogeny of Ni‐BPs explains the promiscuous behavior of this class of Ni(II) carriers. In the Ni‐BPs family, a diversity of nickel‐binding motifs was identified, precluding the determination of a common consensus nickel‐binding mode. Different strategies to bind Ni(II) in the protein cavity have been described. Ni‐BPs binds either free nickel ions or chelated Ni(II). Naked nickel binds to the protein via three histidine residues. In several Ni‐BPs, only one histidine residue is involved in direct nickel–protein contact and a chelator that binds to the protein via supramolecular interactions is required to complete the coordination sphere of the metal. In Staphylococcus aureus, Ni‐BPs do not possess any histidine in their nickel‐binding motifs and the nickel complex is stabilized in the binding pockets via salt bridges with arginines and interactions with aromatic residues. Depending on the growth conditions and nickel availability in the environment, the chelators are either available molecules in the environment such as L‐histidine or by‐products of the metabolism or putative endogenous biosynthetic nickelophores