Figure 1. Blockers as probes for Ca²⁺-dependent gate location in K⁺ channels.

(a) Comparison of BK (left) and MthK (right) channel architectures. The gating rings (red ribbons) with bound Ca²⁺ (green spheres) are structurally conserved between BK (pdb 3U6N) and MthK (pdb ILNQ). The unknown transmembrane structure for BK is illustrated with voltage-sensor domains (VSDs, brown) and pore domain (grey) with selectivity filter (blue). MthK lacks VSDs and the full-length protein structure is available, except for linkers between pore and gating ring (dashed lines). Locations of two putative gates are indicated by arrows. Only two subunits for the transmembrane domains are illustrated for clarity. (b) The gated access model: the pore domain has a bundle-crossing gate that prevents QA blocker binding to the aqueous vestibule. Rapid activation by Ca²⁺ results in immediate channel activity (purple circles and arrow) followed by blocker binding just below the selectivity filter. (c) The selectivity filter gate model: blocker has access to the binding site in both closed and open channels and steady-state block is reached before MthK is activated by Ca²⁺.