Voltage-dependent gating of HERG potassium channels

Abstract

The mechanisms by which voltage-gated channels sense changes in membrane voltage and energetically couple this with opening of the ion conducting pore has been the source of significant interest. In voltage-gated potassium (Kv) channels, much of our knowledge in this area comes from Shaker-type channels, for which voltage-dependent gating is quite rapid. In these channels, activation and deactivation are associated with rapid reconfiguration of the voltage-sensing domain unit that is electromechanically coupled, via the S4-S5 linker helix, to the rate-limiting opening of an intracellular pore gate. However, fast voltage-dependent gating kinetics are not typical of all Kv channels, such as Kv11.1 (human ether-à-go-go related gene, hERG), which activates and deactivates very slowly. Compared to Shaker channels, our understanding of the mechanisms underlying slow hERG gating is much poorer. Here, we present a comparative review of the structure-function relationships underlying activation and deactivation gating in Shaker and hERG channels, with a focus on the roles of the voltage-sensing domain and the S4-S5 linker that couples voltage sensor movements to the pore. Measurements of gating current kinetics and fluorimetric analysis of voltage sensor movement are consistent with models suggesting that the hERG activation pathway contains a voltage independent step, which limits voltage sensor transitions. Constraints upon hERG voltage sensor movement may result from loose packing of the S4 helices and additional intra-voltage sensor counter-charge interactions. More recent data suggest that key amino acid differences in the hERG voltage-sensing unit and S4-S5 linker, relative to fast activating Shaker-type Kv channels, may also contribute to the increased stability of the resting state of the voltage sensor.

Keywords: S4-S5 linker; Shaker; gating; gating charge transfer center; hERG; potassium channel; voltage sensor.

Figure 1

Kv channel structure. (A) Cartoon representation of the membrane topology of a Kv channel α-subunit. Key charged residues within the voltage-sensing domain are highlighted. Negative charges highlighted in black are highly conserved, whereas those in gray are conserved only in the eag channel family. (B) Ribbon representation of the Kv channel tetrameric assembly based on the Kv1.2 crystal structure (Long et al., 2005a). Each subunit has been highlighted a different color for clarity. The model shows a view from the top of the channel looking down along the permeation pathway (at center). (C) Alignments of primary S1–S4 sequences in Shaker, Kv1.2, and hERG channels. Key charged residues are highlighted with the same color scheme as in panel A.

Figure 2

Comparison of gating and kinetics in Shaker and hERG channels. Cartoon representation of ionic current, gating current and fluorescence reports for Shaker (fast-inactivation removed) and hERG channels in response to a depolarizing step from -80 to 0 mV, followed by a repolarizing step to -60 mV. Fluorescence traces represent reports from fluorophores attached at the outer end of S4, to Shaker A359C or hERG L520C. Note the different timescales and step durations for the fast-activating Shaker and slow-activating hERG channels. Schematics were generated based on data from primary sources cited in the text (see Does Slow S4 Voltage Sensor Movement Underlie Slow hERG Gating Kinetics?”).

Figure 3

Proposed schemes to describe hERG channel gating. (A) the three closed state model proposed by Wang et al. (1997). Kf and Kb represent voltage independent transitions that are rate limiting for opening at strongly depolarized potentials. (B) The modified gating scheme proposed by Piper et al. (2003) to account for the biphasic nature of hERG gating currents and multiple sequential voltage-dependent transitions of the voltage sensor.

Figure 4

Structural models of the voltage-sensing domain and pore domain. (A) ribbon representation of the voltage-sensing domain (S1, black; S2, red; S3, yellow; S4, blue) in an open conformation based on the Kv1.2–Kv2.1 chimera crystal structure (Long et al., 2007). Side chains found in the proposed gating charge transfer center (D316, E283, and F290) and along S4 (R1, R2, R3, R4, K5) are colored according to atom type: C, yellow; N, blue; O, red; phenylalanine, green. For clarity, the S1–S2 linker has been omitted. (B) Ribbon representation of the open pore domain (S5, gray; S6, blue) and S4–S5 linker (red), based on the Kv1.2–Kv2.1 chimera crystal structure, showing the close apposition of the S4–S5 linker and distal S6 helices. Proline residues that form the highly conserved PxP motif in the distal S6 helix and allow electromechanical coupling via the S4–S5 linker are highlighted in yellow. Only two subunits of the tetramer are shown (two subunits have been removed for clarity).