2.A.52 The Ni2+-Co2+ Transporter (NiCoT) Family
Currently sequenced proteins of the NiCoT family are found in Gram-negative and Gram-positive bacteria as well as archaea and eukaryotes. The functionally best characterized members of the family catalyze uptake of Ni2+ and/or Co2+ in a proton motive force-dependent process. These proteins vary in size from 301 to 381 amino acyl residues and possess 7 or 8 TMSs. Topological analyses with the HoxN Ni2+ transporter of Ralstonia eutropha (Alcaligenes eutrophus) suggest that it possesses 8 TMSs with its N- and C-termini in the cytoplasm. The Co2+ (Ni2+) transporter of Rhodococcus rhodochrous, NhlF, exhibits eight putative TMSs, and eight apparent TMSs are revealed by a hydropathy analysis of the multiple alignment of the NiCoT family protein sequences. An HX4DH sequence in helix 2 of the HoxN protein has been implicated in Ni2+ binding, and both helix 1 and helix 2 which interact spatially, form the selectivity filter (Degan and Eitinger, 2002). In the H. pylori NixA homologue, several conserved motifs have been shown to be important for Ni2+ binding and transport (Wolfram and Bauerfeind, 2002).
The overall reaction catalyzed by the proteins of the NiCoT family is:
[Ni2+ or Co2+] (out) [Ni2+ or Co2+] (in)