3.A.1.23.4
The NikM2 (230 aas; 5 TMSs)/NikN2 (110 aas; 2 TMSs) pair is part or all of a nickel transporter.  The crystal structure of NikM2 is known (PDB 4M5C; 4M58).  It possesses an additional TMS at its N-terminal region not present on other ECF transporter of known structure, resulting in an extracellular N-terminus. The highly conserved N-terminal loop inserts into the center of NikM2 and occludes a region corresponding to the substrate-binding sites of the vitamin-specific S component. Nickel binds to NikM2 by coordination to four nitrogen atoms in Met1, His2 and His67. These nitrogens form a square-planar geometry, similar to that of the metal ion-binding sites in the amino-terminal Cu²⁺- and Ni²⁺-binding (ATCUN) motif (Yu et al. 2013).  Constituents other than NikN2 and NikM2 are not known but may be required for activity (T. Eitinger, personal communication).