KcsA Crystal Structure as Framework for a Molecular Model of the Na+ Channel Pore

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Abstract
The crystal structure of the pore-forming part of the KcsA bacterial K+-selective channel suggests a possible motif for related voltage-gated channels. We examined the hypothesis that the spacial orientation of the KcsA M1 and M2 α-helices also predicts the backbone location of S5 and S6 helices of the voltage-gated Na+ channel. That channel's P region structure is expected to be different because selectivity is determined by side-chain interactions rather than by main-chain carbonyls, and its outer vestibule accommodates relatively large toxin molecules, tetrodotoxin (TTX) and saxitoxin (STX), which interact with selectivity ring residues. The Na+ channel P loop was well-modeled by the α-helix-turn-β-strand motif, which preserves the relationships for toxin interaction with the Na+ channel found experimentally. This outer vestibule was docked into the extracellular part of the inverted teepee structure formed by the S5 and S6 helices that were spacially located by coordinates of the KcsA M1 and M2 helix main chains [Doyle et al. (1998) Science 280, 69−74], but populated with side chains of the respective S5 and S6 structures. van der Waals contacts were optimized with minimal adjustment of the S5, S6, and P loop structures, forming a densely packed pore structure. Nonregular external S5−P and P−S6 segments were not modeled here, except the P−S6 segment of domain II. The resulting selectivity region structure is consistent with Na+ channel permeation properties, offering suggestions for the molecular processes involved in selectivity. The ability to construct a Na+ channel pore model consistent with most of the available biophysical and mutational information suggests that the KcsA structural framework may be conserved in voltage-gated channels.