Gating and flickery block differentially affected by rubidium in homomericKCNQ1 and heteromeric KCNQ1/KCNE1 potassium channels

The voltage-gated potassium channel KCNQ1 associates with the small KCNE1 s ubunit to form the cardiac IKs delayed rectifier potassium current and muta tions in both genes can lead to the long QT syndrome. KCNQ1 can form functi onal homotetrameric channels, however with drastically different biophysica l properties compared to heteromeric KCNQ1/KCNE1 channels. We analyzed gati ng and conductance of these channels expressed in Xenopus oocytes using the two-electrode voltage-clamp and the patch-clamp technique and high extrace llular potassium (K) and rubidium (Rb) solutions. Inward tail currents of h omomeric KCNQ1 channels are increased about threefold upon substitution of 100 mM potassium with 100 mM rubidium despite a smaller rubidium permeabili ty, suggesting an effect of rubidium on gating. However, the kinetics of ta il currents and the steady-state activation curve are only slightly changed in rubidium, Single-channel amplitude at negative voltages was estimated b y nonstationary noise analysis, and it was found that rubidium has only a s mall effect on homomeric channels (1.2-fold increase) when measured at a 5- kHz bandwidth. The apparent single-channel conductance was decreased after filtering the data at lower cutoff frequencies indicative of a relatively f ast "flickery/block" process. The relative conductance in rubidium compared to potassium increased at lower cutoff frequencies (about twofold at 10 Hz ), suggesting that the main effect of rubidium is to decrease the probabili ty of channel blockage leading to an increase of inward currents without la rge changes in gating properties. Macroscopic inward tail currents of heter omeric KCNQ1/KCNE1 channels in rubidium are reduced by about twofold and sh ow a pronounced sigmoidal time course that develops with a delay similar to the inactivation process of homomeric KCNQ1, and is indicative of the pres ence of several open states. The single channel amplitude of heteromers is about twofold smaller in rubidium than in potassium at a bandwidth of 5 kHz . Filtering at lower cutoff frequencies reduces the apparent single-channel conductance, the ratio of the conductance in rubidium versus potassium is, however, independent of the cutoff frequency. Our results suggest the pres ence of a relatively rapid process (flicker) that can occur almost independ ently of the gating state. Occupancy by rubidium at negative voltages favor s the flicker-open state and slows the flickering rate in homomeric channel s, whereas rubidium does not affect the flickering in heteromeric channels. The effects of KCNE1 on the conduction properties are consistent with an i nteraction of KCNEI in the outer vestibule of the channel.