Molecular Template for a Voltage Sensor in a Novel K+ Channel. I. Identification and Functional Characterization of KvLm, a Voltage-gated K+ Channel from Listeria monocytogenes

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Abstract
The fundamental principles underlying voltage sensing, a hallmark feature of electrically excitable cells, are still enigmatic and the subject of intense scrutiny and controversy. Here we show that a novel prokaryotic voltage-gated K+ (Kv) channel from Listeria monocytogenes (KvLm) embodies a rudimentary, yet robust, sensor sufficient to endow it with voltage-dependent features comparable to those of eukaryotic Kv channels. The most conspicuous feature of the KvLm sequence is the nature of the sensor components: the motif is recognizable; it appears, however, to contain only three out of eight charged residues known to be conserved in eukaryotic Kv channels and accepted to be deterministic for folding and sensing. Despite the atypical sensor sequence, flux assays of KvLm reconstituted in liposomes disclosed a channel pore that is highly selective for K+ and is blocked by conventional Kv channel blockers. Single-channel currents recorded in symmetric K+ solutions from patches of enlarged Escherichia coli (spheroplasts) expressing KvLm showed that channel open probability sharply increases with depolarization, a hallmark feature of Kv channels. The identification of a voltage sensor module in KvLm with a voltage dependence comparable to that of other eukaryotic Kv channels yet encoded by a sequence that departs significantly from the consensus sequence of a eukaryotic voltage sensor establishes a molecular blueprint of a minimal sequence for a voltage sensor.