2.A.113 The Nickel/cobalt Transporter (NicO) Family

Homologues of the NicO family have differing predicted topologies: 6, 7 and 8 TMSs. One such homologue, RcnA (YohM) of E. coli (274 aas) has 6 putative TMSs in a 3+3 arrangement with a large hydrophilic loop between putative TMSs 3 and 4. This protein is believed to catalyze Co²⁺ and Ni²⁺ efflux (Marrero et al. 2007). Several homologues of RcnA (e.g., from Ralstonia solanacearum; CAD17703) have 7 putative TMSs (4+3). Still another homologue, UreH of Methanocaldococcus janaschii (TC# 2.A.113.1.4) has 6 putative TMSs in a more characteristic 3 + 3 TMS arrangement . The repeat unit of this family (within the LysE superfamily) may have a common orign with that in the NiCoT family (TC# 2.A.52) (within the TOG superfamily), having lost TMSs 1 and 4 in the 8 TMS TOG superfamily topology to give rise to a 3 TMS repeat unit. Thus, these two superfamilies may be related but different at the repeat unit level (M. Saier, unpublished observations).

The overall reaction catalyzed by proteins of the NicO family is probably:

[Ni²⁺ or Co²⁺] (in) [Ni²⁺ or Co²⁺] (out)