Distance measurements reveal a common topology of prokaryotic voltage-gated ion channels in the lipid bilayer

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Abstract
Voltage-dependent ion channels are fundamental to the physiology of excitable cells because they underlie the generation and propagation of the action potential and excitation–contraction coupling. To understand how ion channels work, it is important to determine their structures in different conformations in a membrane environment. The validity of the crystal structure for the prokaryotic K + channel, K V AP, has been questioned based on discrepancies with biophysical data from functional eukaryotic channels, underlining the need for independent structural data under native conditions. We investigated the structural organization of two prokaryotic voltage-gated channels, NaChBac and K V AP, in liposomes by using luminescence resonance energy transfer. We describe here a transmembrane packing representation of the voltage sensor and pore domains of the prokaryotic Na channel, NaChBac. We find that NaChBac and K V AP share a common arrangement in which the structures of the Na and K selective pores and voltage-sensor domains are conserved. The packing arrangement of the voltage-sensing region as determined by luminescence resonance energy transfer differs significantly from that of the K V AP crystal structure, but resembles that of the eukaryotic K V 1.2 crystal structure. However, the voltage-sensor domain in prokaryotic channels is closer to the pore domain than in the K V 1.2 structure. Our results indicate that prokaryotic and eukaryotic channels that share similar functional properties have similar helix arrangements, with differences arising likely from the later introduction of additional structural elements.