3.A.1.23.6
Cobalt (Co²⁺) porter (Rodionov et al., 2006).  CbiMN is a bipartite S-subunit with 8 TMSs (Siche et al. 2010). Dynamic interactions of CbiN and CbiM trigger activity of the transporter (Finkenwirth et al. 2019). Substrate binding (S) components rotate within the membrane to expose their binding pockets alternately to the exterior and the cytoplasm. In contrast to vitamin transporters, metal-specific systems rely on additional proteins with essential functions. CbiN, with two TMSs tethered by an extracytoplasmic loop of 37 amino-acid residues is the auxiliary component that temporarily interacts with the CbiMQO₂ Co²⁺ transporter. CbiN induces Co²⁺ transport activity in the absence of CbiQO₂ in cells producing the S component CbiM plus CbiN or a Cbi(MN) fusion. Finkenwirth et al. 2019 showed that any deletion in the CbiN loop abolished transport activity. Protein-protein contacts between segments of the CbiN loop and loops in CbiM were demonstrated, and an ordered structure of the CbiN loop was shown. The N-terminal loop of CbiM, containing three of four metal ligands was partially immobilized in wild-type Cbi(MN) but completely immobile in inactive variants with CbiN loop deletions. Thus, CbiM-CbiN loop-loop interactions facilitate metal insertion into the binding pocket (Finkenwirth et al. 2019).