KvAP-Based Model of the Pore Region of Shaker Potassium Channel is Consistent with Cadmium- and Ligand-Binding Experiments

¹ McMaster University

^* To whom correspondence should be addressed. E-mail: zhorov{at}mcmaster.ca.

Submitted on March 2, 2005
Revised on April 11, 2005
Accepted on 17 May 2005

	Abstract

Potassium channels play fundamental roles in excitable cells. X-ray structures of bacterial potassium channels show that the pore-lining inner helices obstruct the cytoplasmic entrance to the closed channel KcsA, but diverge in widely open channels MthK and KvAP, suggesting a gating-hinge role for a conserved Gly in the inner helix. A different location of the gating hinge and a narrower open pore were proposed for voltage-gated Shaker potassium channels that have the Pro473-Val-Pro (PVP) motif. Two major observations back the proposal: cadmium ions lock mutant Val476Cys in the open state by bridging Cys476 and His486 in adjacent helices, and cadmium blocks the locked-open double mutant Val474Cys/Val476Cys by binding to Cys474 residues. Here we used molecular modeling to show that the open Shaker should be as wide as KvAP to accommodate an open-channel blocker, correolide. We further built KvAP-, MthK-, and KcsA-based models of the Shaker mutants and Monte Carlo-minimized them with constraints Cys476---Cd2+---His486. The latter were consistent with the KvAP-based model causing a small bend N-terminal to the PVP motif. The constraints significantly distorted the MthK-based structure, making it similar to KvAP. The KcsA structure resisted the constraints. Two Cd2+ ions easily block the locked-open KvAP-based model at Cys474 residues, while constraining a single cadmium ion to four Cys474 caused large conformational changes and electrostatic imbalance. While mutual disposition of the voltage-sensor and pore domains in the KvAP X-ray structure is currently disputed, our results suggest that the pore-region domain retains a native-like conformation in the crystal.

Key Words: Cadmium, Channel block, Channel gating, Correolide, Energy minimization, Monte Carlo-Minimization

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