(A) Probable closed (Protein Data Bank accession no. 1R3J) and open (PDB no. 3F5W) conformations of the 2TM channel KcsA. The four subunits are colored differently, and those amino acid residues likely to form the activation gate are shown using spheres. (B) Probable open conformation of the voltage-gated K⁺ channel Kv1.2/2.1 (PDB no. 2R9R). Only the pore segments of two of the four subunits are shown. The residues in the selectivity filter (top) and T402 in S6 postulated to be equivalent to M314 in Slo1 are illustrated using spheres, and the purple spheres are K⁺ ions. This figure was prepared using MacPyMol.