KCNE1 enhances phosphatidylinositol 4,5-bisphosphate (PIP ₂ ) sensitivity of I _Ks to modulate channel activity

Abstract

Phosphatidylinositol 4,5-bisphosphate (PIP 2 ) is necessary for the function of various ion channels. The potassium channel, I Ks , is important for cardiac repolarization and requires PIP 2 to activate. Here we show that the auxiliary subunit of I Ks , KCNE1, increases PIP 2 sensitivity 100-fold over channels formed by the pore-forming KCNQ1 subunits alone, which effectively amplifies current because native PIP 2 levels in the membrane are insufficient to activate all KCNQ1 channels. A juxtamembranous site in the KCNE1 C terminus is a key structural determinant of PIP 2 sensitivity. Long QT syndrome associated mutations of this site lower PIP 2 affinity, resulting in reduced current. Application of exogenous PIP 2 to these mutants restores wild-type channel activity. These results reveal a vital role of PIP 2 for KCNE1 modulation of I Ks channels that may represent a common mechanism of auxiliary subunit modulation of many ion channels.K CNQ1 α-subunits coassemble with KCNE1 β-subunits to form the cardiac slow-delayed rectifier channel, I Ks , which conducts a potassium current that is important for the termination of the cardiac action potential. Although exogenous expression of KCNQ1 alone is sufficient to produce a voltage-gated channel, coexpression of KCNE1 with KCNQ1 leads to changes in current properties to resemble the physiologically important cardiac I Ks current ( Fig. 1A) (1, 2). These changes include increased current amplitude, shift of the voltage dependence of activation toward more positive potentials, slowed activation and deactivation kinetics, and suppressed inactivation, all of which are essential for the physiological role of I Ks (3). After many years of investigation, it is still poorly understood how the KCNE1 peptide exerts such a dramatic effect on the I Ks current. Loss-offunction mutations in I Ks lead to delayed cardiac cell repolarization that manifests clinically as long QT (LQT) syndrome. Patients with LQT syndrome are predisposed to ventricular arrhythmia and suffer from syncope and a high risk of sudden cardiac death. At present, the molecular mechanisms of disease pathogenesis are still unknown for many LQT-associated mutations in I Ks .Phosphatidylinositol 4,5-bisphosphate (PIP 2 ) is a minor acidic membrane lipid found primarily in the inner leaflet of the plasma membrane. PIP 2 has been shown to be a necessary cofactor for a wide variety of ion channels, e.g., voltage-gated K þ and Ca 2þ channels, transient receptor potential channels, inward rectifying K þ channels, and epithelial Na þ channels (4). Recently, I Ks has been shown to require PIP 2 for channel activity, and several LQTassociated mutations located in KCNQ1 have been suggested to decrease the PIP 2 affinity of I Ks (5, 6). Here we provide evidence that KCNE1, as a β-subunit, alters the function of I Ks by modulating the interaction between PIP 2 and the heteromeric ion channel complex. Knowledge of the molecular mechanisms of channel modulation by KCNE1 is of great importance as at least 36 LQT-associated muta...

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

“…If dormant states reflect an early closed state in the activation pathway, two slow transitions are needed to explain this behavior, one to account for the seconds needed for the channel to activate ordinarily and another, even slower, transition to account for the sometimes long dormant periods that can last many minutes. Alternatively, the long dormant state we observe may not be part of the linear activation pathway but instead may be an alternative gating mode of the channel, similar to that seen for sodium (35) and calcium (36) channels, and may be influenced by turnover of molecules that modulate I Ks activity, such as Phosphatidylinositol 4,5-bisphosphate (PIP2), ATP, PKA, or calmodulin (37)(38)(39)(40). It is likely that channels can be recruited from dormant to active states in the face of sympathetic activation when I Ks function becomes most prominent, but this possibility remains to be proven (38,41).…”

Kv7.1 ion channels require a lipid to couple voltage sensing to pore opening

“…However, our data demonstrate that PIP 2 is required to link the activation of the VSD to the PD; therefore, if fewer than four PIP 2 binding sites are occupied, the PD will not be functionally coupled to all four VSDs. Indeed, the activation of Kv7.1 is far from saturated by the endogenous level of PIP 2 available in the oocyte membrane (32) such that most of the channel population may have only one PIP 2 bound and one VSD coupled. In addition, a recent study suggested that the voltage-dependent activation of Kv7.1 can be explained by an allosteric model where activation of only one VSD can open the channel (27).…”

KCNE3 acts by promoting voltage sensor activation in KCNQ1

“…The steady-state fluorescence data are fit with a single (or double) Boltzmann and normalized between the minimum and maximum fluorescence for each experiment. Thermodynamic mutant cycle analysis was conducted as described previously (48). The amount of free energy required to shift the S4 from the resting to the activated state was calculated as ΔG 0 C→A = -zFV 1/2 , where z and V 1/2 were measured from the F(V) and F is Faraday's number.…”