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)
This family belongs to the Transporter-Opsin-G protein-coupled receptor (TOG) Superfamily.
References:
Adhikary, A., S. Biswal, D. Chatterjee, and A.S. Ghosh. (2022). A NiCoT family metal transporter of (Rv2856/NicT) behaves as a drug efflux pump that facilitates cross-resistance to antibiotics. Microbiology (Reading) 168:.
Degen, O. and T. Eitinger. (2002). Substrate specificity of nickel/cobalt permeases: insights from mutants altered in transmembrane domains I and II. J. Bacteriol. 184: 3569-3577.
Degen, O., M. Kobayashi, S. Shimizu, and T. Eitinger. (1999). Selective transport of divalent cations by transition metal permeases: the Alcaligenes eutrophus HoxN and the Rhodococcus rhodochrous NhlF. Arch. Microbiol. 171: 139-145.
Deng, X., J. He, and N. He. (2013). Comparative study on Ni²⁺-affinity transport of nickel/cobalt permeases (NiCoTs) and the potential of recombinant Escherichia coli for Ni²⁺ bioaccumulation. Bioresour Technol 130: 69-74.
Eitinger, T. and B. Friedrich. (1991). Cloning, nucleotide sequence, and heterologous expression of a high-affinity nickel transport gene from Alcaligenes eutrophus. J. Biol. Chem. 266: 3222-3227.
Eitinger, T. and B. Friedrich. (1994). A topological model for the high affinity nickel transporter of Alcaligenes eutrophus. Mol. Microbiol. 12: 1025-1032.
Eitinger, T. and M.-A. Mandrand-Berthelot. (2000). Nickel transport systems in microorganisms. Arch. Microbiol. 173: 1-9.
Eitinger, T., L. Wolfram, O. Degen, and C. Anthon. (1997). A Ni²⁺ binding motif is the basis of high affinity transport of the Alcaligenes eutrophus nickel permease. J. Biol. Chem. 272: 17139-17144.
Eitinger, T., O. Degen, U. Böhnke, and M. Müller. (2000). Nic1p, a relative of bacterial transition metal permeases in Schizosaccharomyces pombe, provides nickel ion for urease biosynthesis. J. Biol. Chem. 275: 18029-18033.
Eitinger, T., O. Degen, U. Bohnke, and M. Muller. (2000). Nic1p, a relative of bacterial transition metal permeases in schizosaccharomyces pombe, provides nickel ion for urease biosynthesis. J. Biol. Chem. 275: 33184.
Fu, C., S. Javedan, F. Moshiri, and R.J. Maier. (1994). Bacterial genes involved in incorporation of nickel into hydrogenase enzyme. Proc. Natl. Acad. Sci. USA 91: 5099-5103.
Fulkerson, J.F., Jr. and H.L.T. Mobley. (2000). Membrane topology of the NixA nickel transporter of Helicobacter pylori: two nickel transport-specific motifs within transmembrane helices II and III. J. Bacteriol. 182: 1722-1730.
Hebbeln, P. and T. Eitinger. (2004). Heterologous production and characterization of bacterial nickel/cobalt permeases. FEMS Microbiol. Lett. 230: 129-135.
Hernandez, J.A., P.S. Micus, S.A.L. Sunga, L. Mazzei, S. Ciurli, and G. Meloni. (2024). Metal selectivity and translocation mechanism characterization in proteoliposomes of the transmembrane NiCoT transporter NixA from. Chem Sci 15: 651-665.
Iwig, J.S., J.L. Rowe, and P.T. Chivers. (2006). Nickel homeostasis in Escherichia coli - the rcnR-rcnA efflux pathway and its linkage to NikR function. Mol. Microbiol. 62: 252-262.
Komeda, H., M. Kobayashi, and S. Shimizu. (1997). A novel transporter involved in cobalt uptake. Proc. Natl. Acad. Sci. USA 94: 36-41.
Rodrigue, A., G. Effantin, and M.A. Mandrand-Berthelot. (2005). Identification of rcnA (yohM), a nickel and cobalt resistance gene in Escherichia coli. J. Bacteriol. 187: 2912-2916.
Saier, M.H., Jr., B.H. Eng, S. Fard, J. Garg, D.A. Haggerty, W.J. Hutchinson, D.L. Jack, E.C. Lai, H.J. Liu, D.P. Nusinew, A.M. Omar, S.S. Pao, I.T. Paulsen, J.A. Quan, M. Sliwinski, T.-T. Tseng, S. Wachi, and G.B. Young. (1999). Phylogenetic characterization of novel transport protein families revealed by genome analyses. Biochim. Biophys. Acta 1422: 1-56.
Wolfram, L. and P. Bauerfeind. (2002). Conserved low-affinity nickel-binding amino acids are essential for the function of the nickel permease NixA of Helicobacter pylori. J. Bacteriol. 184: 1438-1443.
Wolfram, L., B. Friedrich, and T. Eitinger. (1995). The Alcaligenes eutrophus protein HoxN mediates nickel transport in Escherichia coli. J. Bacteriol. 177: 1840-1843.
Wolfram, L., E. Haas, and P. Bauerfeind. (2006). Nickel represses the synthesis of the nickel permease NixA of Helicobacter pylori. J. Bacteriol. 188: 1245-1250.
Examples:
TC#	Name	Organismal Type	Example
2.A.52.1.1	High affinity Ni²⁺-specific transporter, HoxN (Hebbeln and Eitinger 2004).	Bacteria	HoxN of Ralstonia eutropha

2.A.52.1.2	Co²⁺ transporter, NhlF (also transports Ni²⁺ with low affinity) (Hebbeln and Eitinger 2004).	Proteobacteria	NhlF of Rhodococcus rhodochrous

2.A.52.1.3	*Ni²⁺ -specific transporter, Nic1p (Eitinger et al.* 2000).**	Yeast	Nic1p of Schizosaccharomyces pombe

2.A.52.1.4	High affinity Ni²⁺ uptake porter, NixA. Transcription is regulated by NixR (Wolfram et al. 2006). The metal selectivity and translocation mechanism in proteoliposomes of the transmembrane NiCoT transporter NixA from Helicobacter pylori have been reported (Hernandez et al. 2024).	Bacteria	NixA of Helicobacter pylori

2.A.52.1.5	*NisA of 350 aas and 8 TMSs (Hebbeln and Eitinger 2004; Deng et al.* 2013).**	Firmicutes	NisA of Staphylococcus aureus

2.A.52.1.6	*HupN of 381 aas and 8 TMSs (Hebbeln and Eitinger 2004; Deng et al.* 2013).**	Proteobacteria	HupN of Brandyrhizobium japonicum

2.A.52.1.7	NicT is a heavy metal transporter of 372 aas with 8 TMSs in a 4 + 4 TMS arrangement suggesting of a 4 TMS repeat unit. Actually it looks like a 2 + 2 + 2 + 2 arrangement. It is a NiCoT family transporter of Mycobacterium tuberculosis (Rv2856/NicT). It also behaves as a drug efflux pump that facilitates cross-resistance to antibiotics (Adhikary et al. 2022).	Actinobacteria	NicT of Mycobacterium tuberculosis

2.A.52.1.8	Putative high affinity nickel ion uptake transporter of 413 aas and 8 TMSs	Fungi	Nickel transporter of Neurospora crassa

2.A.52.1.9	NicO homologue of 363 aas and 6 TMSs	Haptophyceae	NicO of Emailiania huxleyi

Examples:
TC#	Name	Organismal Type	Example
2.A.52.2.1	Putative Ni²⁺/Co²⁺ uptake transporter of 277 aas and 7 TMSs.	Nitrospirae	Nickel/cobalt transporter of Leptospirillum ferroxidans

2.A.52.2.2	Putative Ni²⁺/Co²⁺ transporter of 269 aas and 7 TMSs.	Proteobacteria	Nickel/cobalt transporter of Dechloromonas aromatica

2.A.52.2.3	Putative Ni²⁺/Co²⁺ uptake porter of 220 aas and 6 TMSs.	*Deinococcus/Thermus*	Nickel/cobalt porter of Thermus thermophilus

2.A.52.2.4	Putative nickel/cobalt porter of 227 aas and 6 TMSs	*Deinococcus/Thermus*	Ni²⁺ porter of Deinococcus deserti

2.A.52.2.5	Putative transporter of 274 aas and 6 TMSs.	Bacteroidetes	Transporter of Chitinophaga pinensis

2.A.52.2.6	NiCoT homologue of 224 aas and 6 TMSs.	Cyanobacteria	NiCoT family member of Gloeobacter violaceus

Examples:
TC#	Name	Organismal Type	Example
Examples:
TC#	Name	Organismal Type	Example