GATED ACCESS TO THE PORE OF A VOLTAGE-DEPENDENT K+ CHANNEL

Voltage-activated K+ channels are integral membrane proteins that open or close a K+-selective pore in response to changes in transmembrane voltage. Although the S4 region of these channels has been implicated as the voltage sensor, little is known about how opening and closing o f the pore is accomplished. We explored the gating process by introduc ing cysteines at various positions thought to lie in or near the pore of the Shaker K+ channel, and by testing their ability to be chemicall y modified. We found a series of positions in the S6 transmembrane reg ion that react rapidly with water-soluble thiol reagents in the open s tate but not the closed state. An open-channel blocker can protect sev eral of these cysteines, showing that they lie in the ion-conducting p ore. At two of these sites, Cd2+ ions bind to the cysteines without af fecting the energetics of gating; at a third site, Cd2+ binding holds the channel open. The results suggest that these channels open and clo se by the movement of an intracellular gate, distinct from the selecti vity filter, that regulates access to the pore.