ABSTRACT
The functional properties of the TWIK-1 (KCNK1) Two-Pore Domain (K2P) K+ channel remain poorly characterized due to the very low levels of functional activity it produces when heterologously expressed. Several underlying reasons have been proposed including retention in intracellular organelles, inhibition by post-translational sumoylation, a hydrophobic barrier within the pore, and a low intrinsic open-probability of the selectivity filter (SF) gate. By evaluating these different mechanisms, we found the latter to dominate this low intrinsic functional activity and investigated the underlying mechanism. The low activity of the SF gate appears to result from the inefficiency of K+ in stabilizing an active (i.e. conductive) SF conformation, while other permeant ion species such as Rb+, NH4+ and Cs+ strongly promote a pH-dependent activated conformation. Furthermore, while many K2P channels are activated by membrane depolarization via a SF-mediated gating mechanism, only very strong, non-physiological depolarization produces voltage-dependent activation and the channel displays unusual inactivation kinetics. Remarkably, we observed that TWIK-1 Rb+ currents were potently inhibited by intracellular K+ (IC50 = 2.8 mM). TWIK-1 therefore displays unique SF gating properties amongst the family of K2P channels. In particular, the apparent instability of the conductive conformation of the TWIK-1 SF in the presence of K+ appears to dominate the low levels of intrinsic functional activity observed when the channel is expressed at the cell surface.