Diagrams illustrating the major structural changes associated with activation gating in Kv compared with changes proposed for Slo2.1 channels. (A and B) Opening of Kv channels requires a voltage-activated outward displacement of the S4 segments and splaying of the inner S6 segments (activation gate) away from the central cavity. Each panel depicts the S4-S6 segments of two diagonally positioned channel subunits. These segments, the pore helices (PH), and the selectivity filter (SF) are labeled in B. In the S4 segments, “+” indicates a basic residue. (C and D) Model of Slo2.1 channel activation. The S6 segments do not form a bundle crossing, and the S4 segments, each with an equal number of basic (+) and acidic (−) residues, do not move in response to voltage. The selectivity filter serves as the activation gate to control the transmembrane flux of K⁺. Mutation of Phe240 (yellow circles, located at the base of the pore helix) to Cys induces constitutive channel opening, which suggests a key, but mechanistically undefined role for this residue in selectivity filter-mediated gating. The kink in the S6 segment of Slo2.1 represents the approximate position of Pro271 that is located four residues above Glu275 (red circles). Together these residues are proposed to prevent formation of an S6 bundle crossing in Slo2.1. Finally, it is proposed that the selectivity filter gate is allosterically activated by a twisting of S6 segments (green arrows) in response to binding of cytoplasmic Na⁺ to the C termini of Slo2.1 subunits.