Bimodal regulation of an Elk subfamily K(+) channel by phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) regulates Shaker K(+) channels and voltage-gated Ca(2+) channels in a bimodal fashion by inhibiting voltage activation while stabilizing open channels. Bimodal regulation is conserved in hyperpolarization-activated cyclic nucleotide–gated (HCN) channels, but voltage activation is enhanced while the open channel state is destabilized. The proposed sites of PIP(2) regulation in these channels include the voltage-sensor domain (VSD) and conserved regions of the proximal cytoplasmic C terminus. Relatively little is known about PIP(2) regulation of Ether-á-go-go (EAG) channels, a metazoan-specific family of K(+) channels that includes three gene subfamilies, Eag (Kv10), Erg (Kv11), and Elk (Kv12). We examined PIP(2) regulation of the Elk subfamily potassium channel human Elk1 to determine whether bimodal regulation is conserved within the EAG K(+) channel family. Open-state stabilization by PIP(2) has been observed in human Erg1, but the proposed site of regulation in the distal C terminus is not conserved among EAG family channels. We show that PIP(2) strongly inhibits voltage activation of Elk1 but also stabilizes the open state. This stabilization produces slow deactivation and a mode shift in voltage gating after activation. However, removal of PIP(2) has the net effect of enhancing Elk1 activation. R347 in the linker between the VSD and pore (S4–S5 linker) and R479 near the S6 activation gate are required for PIP(2) to inhibit voltage activation. The ability of PIP(2) to stabilize the open state also requires these residues, suggesting an overlap in sites central to the opposing effects of PIP(2) on channel gating. Open-state stabilization in Elk1 requires the N-terminal eag domain (PAS domain + Cap), and PIP(2)-dependent stabilization is enhanced by a conserved basic residue (K5) in the Cap. Our data shows that PIP(2) can bimodally regulate voltage gating in EAG family channels, as has been proposed for Shaker and HCN channels. PIP(2) regulation appears fundamentally different for Elk and KCNQ channels, suggesting that, although both channel types can regulate action potential threshold in neurons, they are not functionally redundant.