Activation Gating of hERG Potassium Channels

Abstract

The opening of ion channels is proposed to arise from bending of the pore inner helices that enables them to pivot away from the central axis creating a cytosolic opening for ion diffusion. The flexibility of the inner helices is suggested to occur either at a conserved glycine located adjacent to the selectivity filter (glycine gating hinge) and/or at a second site occupied by glycine or proline containing motifs. Sequence alignment with other K ؉ channels shows that hERG possesses glycine residues (Gly 648 and Gly 657 ) at each of these putative hinge sites. In apparent contrast to the hinge hypotheses, substitution of both glycine residues for alanine causes little effect on either the volt- channels. Our findings indicate that the hERG inner helix glycine residues are required for the tight packing of the channel helices and that the flexibility afforded by glycine or proline residues is not universally required for activation gating.Potassium (K ϩ ) channels are integral membrane proteins that form a pore for conduction of K ϩ ions through the lipid bilayer membrane. They have evolved to perform a wide range of physiological processes and share a similar overall structure consisting of four subunits, which co-assemble to form a central pore coupled to additional regulatory domains that detect a huge variety of different signals. hERG (human ether-à-go-gorelated gene) belongs to the voltage gated family of potassium (K v ) 2 channels. It is widely expressed in the heart and nervous system. It may also be ectopically expressed in certain types of cancer (reviewed in Ref. 1). The physiological importance of hERG is illustrated by the discovery that block of the pore by medications or loss of channel function due to inherited mutations carries an increased risk of sudden cardiac death (2). Therefore, there is considerable interest in gaining further insight into the structural basis of hERG gating and drug binding (3, 4).In recent years, crystal structures have provided tremendous insight into how K ϩ channels function. The KcsA and KirBac1.1 structures correspond to channels in the closed conformation (5, 6). The inner helices of the pore that extend across the membrane and line the inner cavity are straight and come together to form a right-handed helical bundle constricting the channel at the intracellular entrance to the inner cavity, thus presenting a barrier to K ϩ movement. In contrast, MthK (7, 8), K v AP (9), and K v 1.2 (10) have all been crystallized in the open state. All these channels display a bend in the inner helices, which splays the C-terminal (intracellular) end of the inner helix away from the central axis of the pore to create a large aperture at the intracellular mouth of the channel.The inner helices appear to bend at a position, which is highly conserved as a glycine in K ϩ channels (Fig. 1). This led to the proposal that the glycine residue forms a kink or gating hinge that is required for the opening of the activation gate (7). An important feature of glycine is its ability to intro...

Structure of the voltage-gated K + channel Eag1 reveals an alternative voltage sensing mechanism

“…Furthermore, the S6 hinge proline is replaced by G469 in Eag1 (Fig. S6), which mutational experiments suggest does not function as a hinge (41). In agreement, S6 of Eag1 is entirely helical with no noticeable kinks before the S6 bundle crossing (Fig.…”

Linking structure to function: Recent lessons from inositol 1,4,5-trisphosphate receptor mutagenesis

“…Nevertheless, the deliberate introduction of a bend in the helix with a G2586P substitution does produce a constitutively open channel [76]. It is possible that the TM 6 helix is inherently flexible and movement at a hinge may not be involved in the gating process, as suggested for the hERG K + channel [112]. …”

Timothy Mutation Disrupts the Link between Activation and Inactivation in CaV1.2 Protein

“…1), which we call the G/A/G/A motif. A corresponding Gly-657 in hERG (Human Ether-a-go-go related Gene) potassium channels was found to be important for close helix packing (10), and in KcsA, replacement of the corresponding small and hydrophobic Ala-108 by a polar serine or threonine (A108S/T) dramatically increased channel open probability (11, 12). Small gating-sensitive residues at this position thus seem to be essential for helix-helix interactions in a number of ion channels.…”