Project Summary/Abstract Cardiac arrhythmias are the leading cause of morbidity in adults and can result in sudden cardiac death, a leading cause of mortality. The heart is an electromechanical pump which depends on voltage-gated potassium (K+) channels (VGKCs) and K+ regulatory proteins to maintain normal heart rhythm and contractility. Human mutations in voltage-gated K+ channels and K+ regulatory proteins are associated with arrhythmias, syncope, and sudden death. The identification of disease-associated genes and variants represents a significant challenge towards the early diagnosis and treatment of arrhythmogenic cardiovascular diseases. Recently, we discovered a small cardiac-enriched transmembrane micropeptide encoded by a small open reading frame which we named KCNEmini, due to its sequence and structural homology with members of the KCNE family of VGKC regulators. Preliminary studies have shown that KCNEmini co-localizes with VGKCS and functions as a novel regulator of the K+ channel hERG in cell-based assays. Interestingly, disruption of KCNEmini in mice results in QT interval prolongation and cardiac hypertrophy. In this application we will determine the function of KCNEmini: as a direct regulator of cardiac voltage-gated K+ channels using electrophysiology and cell-based experiments (Aim 1), its role in controlling cardiac rhythm using novel KCNEmini knockout mice in vivo (Aim 2), and its potential to mitigate the pathological electrical and cellular remodeling that occurs in response to cardiac hypertrophy (Aim 3). These studies will shed light on the role of a previously unrecognized regulator of K+ handling in the heart, which may be an important future therapeutic target for the diagnosis and treatment of lethal cardiac rhythm disorders. !