I Ks channels open in response to depolarization of the membrane voltage during the cardiac action potential, passing potassium ions outward to repolarize ventricular myocytes and end each beat. Here, we show that the voltage required to activate I Ks channels depends on their covalent modification by small ubiquitin-like modifier (SUMO) proteins. I Ks channels are comprised of four KCNQ1 poreforming subunits, two KCNE1 accessory subunits, and up to four SUMOs, one on Lys 424 of each KCNQ1 subunit. Each SUMO shifts the half-maximal activation voltage (V 1/2 ) of I Ks ∼ +8 mV, producing a maximal +34-mV shift in neonatal mouse cardiac myocytes or Chinese hamster ovary (CHO) cells expressing the mouse or human subunits. Unexpectedly, channels formed without KCNE1 carry at most two SUMOs despite having four available KCNQ1-Lys 424 sites. SUMOylation of KCNQ1 is KCNE1 dependent and determines the native attributes of cardiac I Ks in vivo.C ardiac I Ks channels pass the slow component of the delayed rectifier potassium current that is necessary to produce normal heart rate and rhythm. Thus, I Ks varies in magnitude and timing subject to neuronal and hormonal influences (1). Abnormal changes in I Ks function, due to inherited mutations in KCNQ1 or KCNE1, or suppression of the current by medications, can produce long QT syndrome (LQTS) and life-threatening cardiac arrhythmias (2). KCNQ1 (also called K V 7.1 and previously K V LQT) is a classical voltage-gated potassium α-subunit with six transmembrane segments and a single pore-forming P loop. Human KCNQ1 has variants up to 676 residues in length, whereas KCNE1 (minK) has just 129 residues and a single transmembrane span (Fig. 1A). Despite its small size, KCNE1 is essential to the operation of I Ks . Compared with channels formed by KCNQ1 subunits alone, KCNE1 produces a right shift in the half-maximal voltage required to activate the channel (V 1/2 ), slows the kinetics of activation and deactivation, increases the channel unitary conductance, alters the ion selectivity of the conduction pore and, modifies its pharmacology (3-8). KCNE1 also endows I Ks channel with a high affinity for PIP 2 (9) and allows responsiveness to PKA-mediated phosphorylation following β-adrenergic stimulation (10). Here, we demonstrate that the V 1/2 inherent to native cardiac I Ks channels results from, and is modified by, KCNE1-dependent SUMOylation of KCNQ1.SUMOylation is the enzyme-mediated linkage of one of three SUMO isoforms to the e-amino group of specific Lys residues on a target protein (11). Present in all eukaryotic cells, the pathway was recognized to regulate the activity of nuclear transcription factors when we discovered it resident at the plasma membrane (12, 13) and to regulate the excitability of cerebellar granule neurons via SUMOylation of K2P1 and Na V 1.2 (14, 15), and of hippocampal neurons via modification of K V 2.1 (16).This study was inspired by the work of Qi et al. (17), who describe partial deficiency of the deSUMOylating enzyme SENP2 in mice, showing it to pro...