PIP2 regulation of KCNQ channels: biophysical and molecular mechanisms for lipid modulation of voltage-dependent gating

Abstract

Voltage-gated potassium (Kv) channels contain voltage-sensing (VSD) and pore-gate (PGD) structural domains. During voltage-dependent gating, conformational changes in the two domains are coupled giving rise to voltage-dependent opening of the channel. In addition to membrane voltage, KCNQ (Kv7) channel opening requires the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP2). Recent studies suggest that PIP2 serves as a cofactor to mediate VSD-PGD coupling in KCNQ1 channels. In this review, we put these findings in the context of the current understanding of voltage-dependent gating, lipid modulation of Kv channel activation, and PIP2-regulation of KCNQ channels. We suggest that lipid-mediated coupling of functional domains is a common mechanism among KCNQ channels that may be applicable to other Kv channels and membrane proteins.

Keywords: KCNQ; PIP2; ion channel; lipid modulations; voltage-gating.

Figure 1

Topology of single KCNQ subunit and the structure (right) of KCNQ channels formed by coassembly of four KCNQ subunits. The structure shown is the KCNQ1 homology model (Smith et al., 2007) based on the Kv1.2 crystal structure (Long et al., 2005).

Figure 2

Models of voltage-dependent gating. (A) The VSD and PGD are independent structural and functional domains; each domain on its own may undergo conformational transitions that can be described by independent energy landscapes. These hypothetical reaction coordinates illustrate the free energy associated with the different states of the domains and the transitions among them. In the voltage-gated ion channel, the conformational changes of these domains are coupled, and such coupling has been modeled in two different ways (B,C). Here, for simplicity, all models are shown considering only one VSD, and both VSD activation and PGD opening are modeled as two-state processes, for full models see references (Zagotta et al., ; Horrigan and Aldrich, 1999). (B) Linear gating scheme in which VSD activation obligatorily precedes PGD opening, and coupling is not explicitly defined. (C) Allosteric gating scheme in which the distinctive nature of the two domains is represented, and the coupling is explicitly defined. In this simple model, the coupling factor (θ) has been normalized to represent the net coupling that is due to interactions among all states of the two domains and does not represent a single physical interaction (Chowdhury and Chanda, 2012). The effects of this coupling can be represented in the energy landscapes (C) where the difference between the dotted-gray and solid-black lines represent the free energy changes due to the VSD-PGD coupling. For example, within the PGD plane, solid-black and dotted-gray indicate the free energy landscape for PGD opening when the VSD is resting or activated, respectively. R, resting VSD; A, activated VSD; Kv, equilibrium constant for VSD activation; C, closed PGD; O, open PGD; Kg, equilibrium constant for PGD opening; Θ, allosteric coupling factor; ΔG, free energy of gating; ΔΔG_θ, free energy of coupling. As illustrated (C), θ describes how much the open-close equilibrium is biased toward open when the VSD is activated. Equivalently, coupling can be quantified by measuring the effect of PGD opening on VSD activation, a measurement that is often more experimentally tractable (Arcisio-Miranda et al., ; Ryu and Yellen, ; Zaydman et al., 2013).

Figure 3

Location of three proposed PIP₂ interacting sites on KCNQ. The VSD-PGD interface site includes contributions from the S2-S3 linker, S4-S5 linker, S6 and proximal C-terminus. Other sites have been proposed at the helix A-B linker or the distal C-terminus. Blue residues indicate residues for which mutations have been reported to affect PIP₂ dependent activation. Black residues highlight the conservation of such residues among other members of the KCNQ family. Numbering indicates the positions of highlighted residues in the human KCNQ1 channel sequence.

Figure 4

The VSD-PGD interface site. Side view (left) and bottom view (right) of a single KCNQ1 subunit from a homology model (Smith et al., 2007), which was built of the template of the Kv1.2 crystal structure (Long et al., 2005). PIP₂ molecule (magenta, orange, and red) is positioned by aligning the Kir2.2-PIP₂ crystal structure (Hansen et al., 2011) as done previously (Zaydman et al., 2013). Important residues for PIP₂-dependent coupling are shown. Note that some additional residues reside in more C-terminal regions that are not represented in the homology model due to a lack of a structural template for the KCNQ C-terminus.

Figure 5

Coupling of modular sensor and effector domains as a general mechanism for the regulation of transmembrane proteins by PIP₂ or other anionic phospholipids.