Fig. 30.5 Comparison of the tetrameric ion filter of the highly selective potassium channel KcsA from Streptomyces lividans (a) and the sodium and potassium-permeable channel from Bacillus cereus (b). The selective channel forms a tetramer from a TVGYG motif; a TVGDG is found at the same place in the Na+/K+ channel. Both channels have the same geometry in the lower part formed by threonine and valine residues. The backbone carbonyl groups of the following amino acids Gly–Tyr are rotated toward the interior in the potassium-selective channel and contribute to the filter, whereas the C=O groups from the four Gly–Asp motifs are rotated away in the unselective channel. It opens to a chamber that can accommodate an ion, but does not achieve selectivity filtering.