Understanding hERG Channels Gating using Voltage-Clamp Fluorimetry

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hERG channels encode the alpha-subunit that underlies the rapidly activating delayed rectifier K+ current (IKr) in the heart. These proteins play a central role in cardiac action potential repolarization. hERG channel is a tetramer of four subunits, each containing six segments, S1-S6, and a re-entrant P-loop with a pore helix. The importance of hERG channels resides in their unusual gating, that are slow activation and deactivation, and much faster inactivation/recovery from inactivation.One debated and undetermined issue is whether S4 movement in hERG channels is slow, or if its coupling to pore opening is. Previous studies have suggested that slow activation may be due to slow movement of the S4 segments, however, we recently showed that such movement is actually fast, but that a secondary conformational rearrangement, related to the pore, is directly connected to the slow kinetics of activation/deactivation of hERG channels1.To get an overall picture of the conformational changes that occur in the whole channel throughout its activation/deactivation mechanisms, we performed a fluorescence scan of the S4 segment, the S1-S2 and S3-S4 linkers using voltage-clamp fluorimetry technique. Results show different profiles of fluorescence depending on the location where the fluorescent dye is attached. But interestingly, two profiles seem to be highlighted: 1) minor fast fluorescence signals with voltage-dependence that correlates well with S4 segment movement, and 2) major slow fluorescence signals that consistently relate to pore opening/G-V in terms of time course and voltage-dependence, leading to the conclusion that a concerted channel rearrangement is responsible for or resultant of slow opening in hERG channels.Our data suggest that re-assessment of mechanisms underlying hERG activation gating is required, particularly with regard to the coupling mechanisms between voltage sensor movement and channel pore opening.1 Es-Salah-Lamoureux et al., 2010, PLoS One 28;5(5):e10876