Intracellular ATP binding is required to activate the slowly activating K+ channel IKs

Gating of ion channels by ligands is fundamental to cellular function, and ATP serves as both an energy source and a signaling molecule that modulates ion channel and transporter functions. The slowly activating K⁺ channel I_Ks in cardiac myocytes is formed by KCNQ1 and KCNE1 subunits that conduct K⁺ to repolarize the action potential. Here we show that intracellular ATP activates heterologously coexpressed KCNQ1 and KCNE1 as well as I_Ks in cardiac myocytes by directly binding to the C terminus of KCNQ1 to allow the pore to open. The channel is most sensitive to ATP near its physiological concentration, and lowering ATP concentration in cardiac myocytes results in I_Ks reduction and action potential prolongation. Multiple mutations that suppress I_Ks by decreasing the ATP sensitivity of the channel are associated with the long QT (interval between the Q and T waves in electrocardiogram) syndrome that predisposes afflicted individuals to cardiac arrhythmia and sudden death. A cluster of basic and aromatic residues that may form a unique ATP binding site are identified; ATP activation of the wild-type channel and the effects of the mutations on ATP sensitivity are consistent with an allosteric mechanism. These results demonstrate the activation of an ion channel by intracellular ATP binding, and ATP-dependent gating allows I_Ks to couple myocyte energy state to its electrophysiology in physiologic and pathologic conditions.Significant energy is required to sustain both the electrical and contractile events that accompany each heart beat, suggesting that the level of ATP is one key to normal cardiac functions. Not surprisingly, a reduction in ATP concentration ([ATP]) plays a key role in the pathogenesis and progression of ischemic heart diseases, including heart failure. Intracellular ATP is important not only in providing energy (1) and as a substrate for protein kinases (2), but also as signaling molecules to bind and modulate proteins. Only a handful of results have shown that intracellular ATP serves as a signal for membrane channels and transporters (3). The best-studied example is the K_ATP channel (4, 5). This channel is inhibited in physiologic conditions by [ATP]s of 5–10 mM (6), but when the ATP levels drop to submillimolar concentrations, as in cardiac ischemia, the K_ATP channels open, shortening the action potential duration and providing metabolic protection against the insult of ischemia (7). However, at normal physiologic conditions, whether and how ATP serves as a signal connecting the energetic state of the cell to membrane excitability is still unknown.The slowly activating K⁺ channel I_Ks plays an important role in controlling the action potential duration (APD) in cardiac myocytes; it opens in response to depolarization to conduct potassium ions out of the cell, which contributes to repolarization of the membrane, terminating the cardiac action potential and thereby the myocyte contraction. The I_Ks channel consists of pore-forming KCNQ1 subunits and the single-transmembrane auxiliary subunits KCNE1 (8, 9). Loss-of-function mutations in either KCNQ1 or KCNE1 lead to prolongation of ventricular action potentials and long QT syndrome (LQTS) that manifests as QT (interval between the Q and T waves in electrocardiogram) prolongation in the electrocardiogram. LQTS predisposes patients to cardiac arrhythmias that lead to syncope and sudden death (10). Previous studies show that ATP is required for activation of I_Ks channels (11), but the molecular mechanism and the physiologic function of this ATP modulation is not known. It was the purpose of this study to investigate the mechanism by which ATP regulated I_Ks. Our results show that ATP directly binds to the KCNQ1 protein to regulate channel function at concentrations ([ATP]) close to the physiologic intracellular [ATP] in cardiac myocytes. Our studies also reveal a unique ATP binding site and mechanism for modulation of ion channel function. Further, we found that several LQT-associated mutations alter I_Ks function by reducing ATP sensitivity. These results demonstrate that ATP regulation is vital for I_Ks channel function; a disruption of these regulations predisposes to life-threatening cardiac arrhythmias.