Structural basis for K(V)7.1-KCNE(x) interactions in the I(Ks) channel complex

Abstract

The cardiac I(Ks) current is involved in action potential repolarization, where its primary function is to limit action potential prolongation during sympathetic stimulation. The I(Ks) channel is mainly composed of K(V)7.1 ion channels associated with KCNE1 auxiliary subunits. The availability of KCNE1 solution structure by nuclear magnetic resonance spectroscopy in conjunction with biochemical assays addressing K(V)7.1-KCNE1 residue interactions has provided new insights into the structural basis for K(V)7.1 modulation by KCNE1. Recent evidence further suggests that KCNE2 may associate with the K(V)7.1-KCNE1 channel complex and modulate its current amplitude. Here we review recent studies in this area and discuss potential roles for multiple KCNE(x) subunits in I(Ks) generation and modulation as well as the clinical relevance of the new information.

Figure 1

K_V7.1–KCNE1–KCNE2 current characteristics. A: Topology of K_V7.1 and KCNE subunits. K_V7.1 subunits encompass six transmembrane segments with intracellular N- and C- termini, where S1–S4 encode the voltage-sensing domain (VSD) and S5–S6 encode the pore domain (PD). The KCNE proteins contain a single transmembrane segment flanked by an extracellular N-terminus and a cytosolic C-terminus. B: Normalized amplitudes of currents elicited from Xenopus oocytes upon stimulation with the indicated voltage clamp protocol 48 hours after injection of K_V7.1 and KCNE1–KCNE2 cRNA as a function of clamp potential reveals that KCNE1 shifts the voltage-dependence of K_V7.1 activation in the depolarizing direction to a much greater extent than does KCNE2. C: Representative current traces from the experiments analyzed in B illustrate the diverse effects of KCNE proteins on K_V7.1 channel properties. Homomeric K_V7.1 channels activate relatively rapidly, exhibit sustained currents at maintained depolarization, and slowly deactivate upon repolarization. Co-expression with KCNE1 greatly increases K_V7.1 current amplitude and slows activation, in addition to its effect on channel voltage-dependency. Co-expression with KCNE2 reduces current amplitudes and induces a constitutively active current component. Activation of the time-dependent current component is slower than activation in the absence of KCNE2. Co-expression with KCNE1 and KCNE2 generate currents with a mixture of the features, where KCNE1 dictates kinetics, whereas both KCNE1 and KCNE2 contribute to determination of current amplitude.^,D: Effects exerted by KCNE subunits on K_V7.1 currents. Strong effects are highlighted in bold.

Figure 2

Model of K_V7.1–KCNE1 channel structure. K_V7.1–KCNE1 closed- and open-state models as generated by Kang et al and graphed using UCSF-Chimera. Top: Docking of the nuclear magnetic resonance (NMR) structure of one KCNE1 transmembrane segment (red) onto a homology model of K_V7.1 in the open (left) and closed (right) state, respectively, shows that KCNE1 resides in different clefts in the two states. The channel complex is visualized from the extracellular side. K_V7.1 subunits are colored gray, pink, blue, and purple, with S1–S6 and P helices labeled for one subunit. The voltage-sensing domain (VSD) and pore domain (PD) are also marked. Yellow spheres indicate the extracellular ends of S1 (I145), S4 (A226), and S6 (V324). In the open state, the extracellular end of the KCNE1 transmembrane segment forms an interface with S1, S5, and S6 from three different subunits; in the closed state, it forms contact with S3 of one subunit and S5 and S6 of another subunit. Bottom: Enlarged and rotated view of the boxed regions in the top panels, with addition of 10 NMR structures of KCNE1 residues 40–45 (color coded differently for individual NMR structures). This was achieved by aligning the 10 NMR KCNE1 structures (residues 40–70) to the KCNE1 transmembrane segment of the K_V7.1–KCNE1 model using the MatchMaker function of UCSF-Chimera (for clarity, only residues 40–45 are shown). Assuming (1) a wide swing of S1–S4 upon transition between closed and open states and (2) changing KCNE1 position and orientation relative to K_V7.1 in these states, the model suggests a wide range of motions in the extracellular end of the KCNE1 transmembrane segment (highlighted by the red circle) and possibilities of contacts with K_V7.1. This model is partially supported by the proximity of –S–S– partners as delineated in cysteine mutagenesis, although further experimental confirmation is needed.

Figure 3

Possible heterogeneity in I_Ks channel components. I_Ks density may be dynamically regulated by the composition of KCNE proteins in the channel complex. Expression of K_V7.1–KCNE1 channels gives rise to greater current amplitude than do K_V7.1–KCNE1–KCNE2 channels. As KCNE2 can modulate current mediated by K_V7.1–KCNE1 channels in the plasma membrane, it can be speculated that KCNE2 may function as a dynamical down-regulator of K_V7.1–KCNE1 currents, where KCNE2 may be able to exchange position with KCNE1 in the channel complex. Such regulation of I_Ks density by KCNE2 could have a temporal and/or a spatial component, but this remains speculative based on the current evidence.