Fig. 6: Proposed Mg²⁺ autoregulation gating mechanism of Mrs2.

a,b, Schematic model as viewed from the mitochondrial inner membrane (a; with two monomers for simplicity) and from the matrix (b). In the closed state, Mg²⁺ ions bind between the NTDs of separate monomers (positions M1 and M4 or M1–M4 in CtMrs2), thereby assisting in stabilizing the symmetrical shape, and to distinct positions of the pore (U, S, P1 and P2). In the open configuration, Mg²⁺ is only present at some of the pore sites (S, P1 and P2) and the NTD is instead maintained in an alternative symmetrical assembly by the RDLR motif and residues that previously formed the M1 and M2 sites (blue). At elevated Mg²⁺ levels in the mitochondrial matrix, Mrs2 is closed. Acidic rings formed by the loops in the intermembrane space (shown in red) attract fully hydrated Mg²⁺ (large green circles), which can be transferred as partially hydrated Mg²⁺ (small green circles) to the asparagine ring of the GMN motif selectivity filter (pink) and then to the P1 and P2 sites (purple) of the pore in the TM domain. However, flux is not permitted as the pore-gating methionine and arginine rings (brown polygons), located approximately at the membrane interface to the matrix, are closed. At low Mg²⁺ concentrations in the matrix, the open structure is present. Removal of the ions from M1 and M4 (and from M1–M2 and M3–M4 in CtMrs2) of the closed structure triggers a shift and rotation of the stalk helix (light brown), resulting in opening of the pore gate, which permits Mg²⁺ influx into the matrix. It is possible that site S also prevents backflow to the intermembrane space. The closed state is reobtained through Mg²⁺ destabilization of the RDLR motif interaction and through Mg²⁺ bridging of the separated residues of the M3–M4 site.