Binding of ATP is Required for Opening of IKs Channels

Abstract

The human heart demands a constant energy supply to fulfill its function, hence the decrease in myocardial ATP production plays a key role in the pathogenesis and progression of ischemic heart diseases. Ischemia also alters electrophysiology of the heart, exemplified by an association between an adverse prognosis and QT interval prolongation in acute myocardial ischemia. KCNQ1 and KCNE1 form the IKs potassium channel important in terminating cardiac action potentials. Congenital mutations that compromise IKs function prolong the duration of the ventricular action potential, causing Long QT (LQT) syndrome, which is associated with a high risk of sudden death. Here we report that the IKs channel activity increases with ATP concentration ([ATP]) and the EC50 is close to the physiological [ATP] in cardiac myocytes, which indicates that [ATP] changes such as in ischemia affect IKs channel function. Consistent with this observation, an LQT-associated mutation in KCNQ1 alters IKs function by reducing ATP sensitivity. The effect of this mutation is eliminated by increasing [ATP]. We find that GTP and a non-hydrolyzable ATP analog AMP-PNP can substitute for ATP in activating the channel, and an ATP analog can be photo-cross-linked to KCNQ1 proteins expressed in the membrane of Xenopus oocytes, indicating that the nucleotide directly binds to KCNQ1 to modify channel function. Compared to ATP, ADP and AMP are less effective in activating the IKs channel, suggesting that phosphate groups are important innucleotide binding. Correspondingly, a mutational scan of all cytosolic basic residues shows that the ATP binding site may reside in the cytosolic C-terminal. These results demonstrate that IKs is a bona fide ATP activated potassium channel that modulates cardiac electrophysiology by sensing intracellular [ATP], thus connecting the cellular energy state to membrane excitability.