= Solution
A typical <voltage-gated potassium channel> has four subunits, each with six membrane-spanning segments. S1–S4 form a <voltage-sensing domain>; S5–S6 and their intervening loop form the pore and <selectivity filter>. Conserved positively charged side chains along S4 move through part of the membrane electric field when the <voltage> changes. Countercharges and hydrated crevices in the surrounding sensor help make this movement possible. Coupling between the sensor and pore, including the S4–S5 region and S6 gate, changes the <probability> of opening. A <voltage-gated sodium channel> usually combines four related domains in one large <protein>, rather than four identical independent <proteins>.
The evidence separates charge movement, structural arrangement and functional coupling. A transient <ion-channel gating current> can persist when permeating ionic <electric current> is suppressed: some charged parts of the channel itself move. Comparing integrated <ion-channel gating charge> with channel opening tests their coupling. Charge-neutralization experiments change both total moved charge and <voltage> dependence, implicating S4 basic residues and countercharges elsewhere; they do not show that S4 alone supplies every charge. <Voltage-sensing residues in the S2 and S4 segments of the Shaker K+ channel by S. A. Seoh and others>[1996 primary abstract: neutralization identifies contributions from S2 and S4]{ref}
<Crystal structure of a mammalian voltage-dependent Shaker family K+ channel by Stephen B. Long and others> supplies structural evidence for the arrangement. Voltage-dependent accessibility and optical reports of sensor motion provide complementary functional tests. No one static structure is a movie of gating, and a perturbation that abolishes current can disrupt folding or pore function rather than selectively remove voltage sensing. Agreement between structural, electrical and motion-sensitive measurements supports the mechanism more strongly than any one observation alone.
Back to article page
