Fig. 4: The Na⁺ passage.

In the Rnf structure the potential Na⁺ channel is found in a locked state. The constriction between helices I, III, VI, and VIII (magenta) of RnfD subdivided into two hydrophobic layers might be opened during turnover based on findings of architecturally similar transporters; however, alternative passages are possible. The extracellular half-channel is occluded by a loop (dark green) and the cytoplasmic half-channel presumably placed next to Asp248 is rather elusive. FMN_RnfD is in ET distance to FMN_RnfG and RBF. At the extracellular side, a small helix 230:235 (salmon) arranged nearly parallel to the membrane plane precedes helix VIII (hotpink). The latter contacts the isoalloxazine ring by Gly237. Helices VII, IX, X (green), and VIII (encircled by a dashed line) may move in a concerted fashion to open a passage for Na⁺ transport. The Thr153 and Thr164 hydroxy groups form a covalent bond with the phospho groups of FMN_RnfD and FMN_RnfG, respectively.