Allosteric gating mechanism underlies the flexible gating of KCNQ1 potassium channels

Abstract

KCNQ1 (Kv7.1) is a unique member of the superfamily of voltagegated K + channels in that it displays a remarkable range of gating behaviors tuned by coassembly with different β subunits of the KCNE family of proteins. To better understand the basis for the biophysical diversity of KCNQ1 channels, we here investigate the basis of KCNQ1 gating in the absence of β subunits using voltage-clamp fluorometry (VCF). In our previous study, we found the kinetics and voltage dependence of voltage-sensor movements are very similar to those of the channel gate, as if multiple voltage-sensor movements are not required to precede gate opening. Here, we have tested two different hypotheses to explain KCNQ1 gating: (i) KCNQ1 voltage sensors undergo a single concerted movement that leads to channel opening, or (ii) individual voltage-sensor movements lead to channel opening before all voltage sensors have moved. Here, we find that KCNQ1 voltage sensors move relatively independently, but that the channel can conduct before all voltage sensors have activated. We explore a KCNQ1 point mutation that causes some channels to transition to the open state even in the absence of voltage-sensor movement. To interpret these results, we adopt an allosteric gating scheme wherein KCNQ1 is able to transition to the open state after zero to four voltage-sensor movements. This model allows for widely varying gating behavior, depending on the relative strength of the opening transition, and suggests how KCNQ1 could be controlled by coassembly with different KCNE family members.) is a member of the superfamily of voltagegated potassium channels (K V ), which contain six transmembrane helices and form functional tetramers with four peripheral voltage-sensing domains surrounding a single potassiumselective pore domain (1). Much study spanning recent decades has focused on the detailed gating mechanisms of these molecular machines, establishing general principles of K V channel gating, as well as unique structural and functional properties that underlie the diverse physiological functions of different family members (2). Within the K V family, KCNQ1 displays a unique flexibility in its gating properties, depending on the tissue where it is expressed and the corresponding β subunit with which it coassembles: in the intestine KCNQ1/KCNE3 channels display voltage-independent current that supports chloride secretion (3), whereas in the heart, KCNQ1/KCNE1 channels display slowly activating voltage-dependent current that is critical to cardiac action potential repolarization (4-6). Remarkably, neither of these physiologically essential phenotypes resembles that of the KCNQ1 channel expressed alone, which activates rapidly over a hyperpolarized range of voltages (4, 5). Still other KCNE proteins coassemble with KCNQ1 to form heteromeric channels with distinct biophysical characteristics (7,8). This diverse array of gating schema allows this protein to play unique important roles in a vast number of systems in the body, including the heart, brain, inner ear, ...

Steric hindrance between S4 and S5 of the KCNQ1/KCNE1 channel hampers pore opening

“…In WT KCNQ1 without KCNE1, the ionic current and the fluorescence intensity almost completely overlapped (Fig. 6a first row and 6b upper left), as previously reported 9,34 . In WT KCNQ1 þ KCNE1, on the other hand, the fluorescence signal always preceded the ionic current.…”

“…4). This is in sharp contrast to the F-V relationship in the absence of KCNE1, which completely overlaps with G-V relationship 9,34 . In terms of kinetics, according to our present data, the KCNQ1 channel showed similar time constants of the VCF signals in the absence and the presence of KCNE1 (Fig.…”

“…KCNQ1 is widely expressed in various tissues including the heart, where a genetic abnormality in KCNQ1 is known to cause long QT syndrome, a type of cardiac arrhythmia 5,6 . Although the KCNQ1 tetramer forms a functional ion channel like other voltage-gated potassium channels, the gating of KCNQ1 may not require all VSDs to be in the upstate for opening, unlike most of the other voltage-gated potassium channels [7][8][9] . On the other hand, KCNQ1 can have KCNE1 as an auxiliary subunit 10,11 .…”

Single-channel basis for the slow activation of the repolarizing cardiac potassium current, I Ks

“…To place our data in the context of gating schemes, we used a well-recognized model of macroscopic I Ks complex currents (3) as the basis for modeling singlechannel activity. Other, recent, allosteric models have been proposed for the KCNQ1 alpha-subunit of the I Ks complex alone (33,34), and these have been used mainly to model activation curve position and slope in WT and mutant channels. However, our preliminary investigations demonstrated that such models cannot easily reproduce the long latencies to opening seen in the I Ks experimental data and also simulate the rapid transitions between channel substates and closed states during bursts of opening activity.…”

Grafting voltage and pharmacological sensitivity in potassium channels

“…The gating charge residues in S4 are the principal components to sense the voltage changes [34]. Neutralizing the first arginine (R228) of Kv7.1 S4 dramatically affects the voltage dependence of activation, whereas neutralizing the second arginine (R231) leads to loss of voltage dependence, characterized by a linear G-V relationship [35][36][37]. The two corresponding positive residues in KT channels were mutated to alanine (A).…”