Long QT Syndrome-associated Mutations in the S4-S5 Linker of KvLQT1 Potassium Channels Modify Gating and Interaction with minK Subunits

Abstract

Long QT syndrome is an inherited disorder of cardiac repolarization caused by mutations in cardiac ion channel genes, including KVLQT1. In this study, the functional consequences of three long QT-associated missense mutations in KvLQT1 (R243C, W248R, E261K) were characterized using the Xenopus oocyte heterologous expression system and two-microelectrode voltage clamp techniques. These mutations are located in or near the intracellular linker between the S4 and S5 transmembrane domains, a region implicated in activation gating of potassium channels. The E261K mutation caused loss of function and did not interact with wildtype KvLQT1 subunits. R243C or W248R KvLQT1 subunits formed functional channels, but compared with wild-type KvLQT1 current, the rate of activation was slower, and the voltage dependence of activation and inactivation was shifted to more positive potentials. Co expression of minK and KvLQT1 channel subunits induces a slow delayed rectifier K ؉ current, I Ks , characterized by slow activation and a markedly increased magnitude compared with current induced by KvLQT1 subunits alone. Coexpression of minK with R243C or W248R KvLQT1 subunits suppressed current, suggesting that coassembly of mutant subunits with minK prevented normal channel gating. The decrease in I Ks caused by loss of function or altered gating properties explains the prolonged QT interval and increased risk of arrhythmia and sudden death associated with these mutations in KVLQT1.The slow delayed rectifier K ϩ current, I Ks , 1 is one of several outward currents that modulate repolarization of cardiac action potentials (1, 2). The channel mediating I Ks is formed by coassembly of two subunits, minK and KvLQT1 (3, 4). Mutations in KVLQT1 (also called KCNQ1) cause long QT syndrome (LQT) (5), an inherited disorder of ventricular repolarization that predisposes affected individuals to cardiac arrhythmias and sudden death (6). The physiologic consequences of some mutations have been studied by heterologous expression of KvLQT1 in Xenopus oocytes or mammalian cells (7-10). Most missense mutations in KVLQT1 cause loss of channel function when mutant subunits are expressed alone and a dominantnegative effect when coexpressed with wild-type (WT) KvLQT1 subunits. However, some mutations alter the function of Kv-LQT1 channels, and a study of their biophysical properties has led to novel insights into the structural basis of channel function. For example, the LQT-associated mutation V254M caused loss of function but accelerated activation of KvLQT1 channel current when mutant subunits were coexpressed with WT Kv-LQT1 subunits (8). V254M is located in the cytosolic loop connecting the S4 and S5 transmembrane domains of KvLQT1. Missense mutations in the S4-S5 linker have also been reported to accelerate activation and deactivation of human ether-a-go-go-related gene (HERG) channels (11). Furthermore, the S4-S5 linker determines the rate of deactivation of chimeric channels constructed from Kv2.1 and Kv3.1 (12). Together, these studies indicate th...

Identification by mass spectrometry and functional characterization of two phosphorylation sites of KCNQ2/KCNQ3 channels

“…6, which is published as supporting information on the PNAS web site). Similar effects of S4-S5 loop mutations on voltage dependence have been reported for other voltage-gated potassium channels, such as for Shaker (28) and KCNQ1 (29).…”

“…1). Because the S4-S5 loop has been shown to influence channel conductance and gating (28,29), we hypothesized that the addition of a phosphate group here, with its negative charge, could modulate the activity of the channel.…”

Role of an S4-S5 Linker Lysine in the Trafficking of the Ca2+-activated K+ Channels IK1 and SK3

“…Sanguinetti and co-workers (50,51) have similarly demonstrated a significant role for the S4-S5 linker region in coupling channel voltage sensing with activation in the HCN pacemaker channels. Finally, in the KvLQT channel family, mutations within the S4-S5 linker region cause a decrease in current and modify channel interactions with the minK ␤-subunit, increasing the likelihood of long QT syndrome (52). We have demonstrated here for the first time that this region is also crucial for the trafficking of a class of non-voltage-gated K ϩ channels, the Ca 2ϩ -activated IK and SK channels.…”

A Novel Mutation Associated With Jervell and Lange-Nielsen Syndrome in a Japanese Family

“…Franqueza et al reported a W248R-KCNQ1 mutation; the same position as our mutation but a different amino acid change. 19 Without KCNE1, the W248R-KCNQ1 channel produced reduced current densities compared to WT-KCNQ1 and displayed no dominant negative effect. And the co-expression of mutant KCNQ1 with KCNE1 caused a marked suppression of IKs current.…”

“…Recent studies demonstrated that KCNQ1 mutations in the intracellular S4-S5 linker affected the voltage sensitivity of the reconstituted channels. 19 Fig 3C represents the voltage-dependence of current activation. Tail current densities after each test potential were fitted to Eqn 1 (see methods).…”