PIP2 is Required to Couple the Voltage-Sensing Domain to the Pore for the Activation of KCNQ1 by Voltage

Abstract

Voltage-gated ion channels are vital for the generation of action potentials that orchestrate various physiological processes. Phosphatidylinositol 4,5-bisphophate (PIP2) is a membrane lipid that modulates several voltage-gated channels (TRP, HCN, CaV, Kv, KCNQ). KCNQ channels are unique in that they cannot be activated by voltage in the absence of PIP2 and the loss of KCNQ channel activity due to PIP2 depletion is known to be physiologically relevant. Although residues governing PIP2 sensitivity have been identified for several KCNQ channels, it is unclear where PIP2 binds and what it does to channel gating. Voltage-dependent gating involves coupled conformational changes in the voltage-sensing domain (VSD) and the pore domain (PD) providing three possible mechanisms of PIP2 action: VSD activation, PD opening, or coupling between VSD activation and PD opening. We combine voltage-clamp fluorometry wih expression of a voltage-sensitive lipid-phosphatase in order to measure VSD activation and PD opening simultaneously while manipulating PIP2 abundance. We find that PIP2 is not necessary for VSD activation; however, PIP2 is required for VSD-PD coupling. Deriving a simple mathematical model, we find that PIP2 regulation of coupling is sufficient to recapitulate the experimental results without any direct effects on PD opening. These findings suggest PIP2 may bind at the VSD-PD interface that, in KCNQ1, is densely populated with basic residues near the intracellular face of the membrane. By mutating these residues and measuring the effects on channel function, PIP2 sensitivity and VSD-PD coupling, we propose a PIP2 binding site. In our resulting model, the negatively charged PIP2 headgroup resolves the electrostatic repulsion between basic residues on the VSD and the PD, thus holding the two domains together and allowing the transfer of conformational energy between them.