How membrane proteins sense voltage

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Abstract
Voltage sensors are structures within a protein that can sense the membrane potential. The movement of the sensing charges, combined with the configuration of the electric field, determines the extent of the conformational change that occurs. Gating currents, or more generally sensing currents, are the transient currents that are produced by the movement of the sensing charges. The K+ channel voltage sensor is described in detail because it has been studied using several biophysical techniques and its crystal structure is available. Biophysical techniques can delineate the details of the movement of the voltage sensor in response to changes in the membrane potential. The Na+ channel is responsible for the upstroke of the nerve impulse and differs from K+ channels; it is faster and has intrinsic cooperativity. Channels that close on depolarization contain the voltage sensor for segments 1–4 (S1–S4); however, the gate operates in reverse to the classic Na+, K+ and Ca2+ channels. The proton channel is another member of the S1–S4 voltage sensor family, but it lacks a pore region. Another membrane protein that contains the S1–S4 voltage sensor is voltage-dependent phosphatase; here the sensor regulates the activity of its built-in phosphatase. Some G-protein coupled receptors are voltage dependent. The membrane potential regulates affinity in the m1 and m2 muscarinic receptors and shows sensing currents. The Na–glucose co-transporter is voltage dependent and shows sensing currents; and the Na+−K+ pump is electrogenic and shows sensing currents.