Ca2؉ has been proposed to regulate Na ؉ channels through the action of calmodulin (CaM) bound to an IQ motif or through direct binding to a paired EF hand motif in the Na v 1 C terminus. Mutations within these sites cause cardiac arrhythmias or autism, but details about how Ca 2؉ confers sensitivity are poorly understood. Studies on the homologous Ca v 1.2 channel revealed non-canonical CaM interactions, providing a framework for exploring Na ؉ channels. In contrast to previous reports, we found that Ca 2؉ does not bind directly to Na ؉ channel C termini. Rather, Ca 2؉ sensitivity appears to be mediated by CaM bound to the C termini in a manner that differs significantly from CaM regulation of Ca v 1.2. In Na v 1.2 or Na v 1.5, CaM bound to a localized region containing the IQ motif and did not support the large Ca -dependent conformational change in Na v 1.2 C terminus⅐CaM complex that was absent in the wild-type complex. In Na v 1.5, CaM modulates the Cterminal interaction with the III-IV linker, which has been suggested as necessary to stabilize the inactivation gate, to minimize sustained channel activity during depolarization, and to prevent cardiac arrhythmias that lead to sudden death. Together, these data offer new biochemical evidence for Ca 2؉

Perturbation of sodium channel inactivation, a finely tuned process that critically regulates the flow of sodium ions into excitable cells, is a common functional consequence of inherited mutations associated with epilepsy, skeletal muscle disease, autism, and cardiac arrhythmias. Understanding the structural basis of inactivation is key to understanding these disorders. Here we identify a novel role for a structural motif in the COOH terminus of the heart Na V 1.5 sodium channel in determining channel inactivation. Structural modeling predicts an interhelical hydrophobic interface between paired EF hands in the proximal region of the Na V 1.5 COOH terminus. The predicted interface is conserved among almost all EF hand-containing proteins and is the locus of a number of disease-associated mutations. Using the structural model as a guide, we provide biochemical and biophysical evidence that the structural integrity of this interface is necessary for proper Na ؉ channel inactivation gating. We thus demonstrate a novel role of the sodium channel COOH terminus structure in the control of channel inactivation and in pathologies caused by inherited mutations that disrupt it.Channelopathies, so named because they represent a set of diseases caused by mutations in genes coding for ion channels, are a new and growing class of human disorders that include but are not limited to diabetes, muscle disorders, neurological disease, and cardiac arrhythmias (1). A surprising number of channelopathies associated with a wide diversity of human disease are caused by similar mutation-induced changes in ion channel function. Inactivation of voltage-dependent Na ϩ channels is an example of a physiological process critically important in many tissues that, when altered by mutation, can result in muscle weakness, inherited epilepsies, autism, or cardiac arrhythmia (2-4). Clinical consequences of inherited mutations that disrupt Na ϩ channel inactivation provide the most direct link between ion channel biophysics and human physiology and pathophysiology. Conversely, investigation into the consequences of inherited mutations on ion channel function has, in many cases, provided insight into the physiological importance of novel regions and/or structures of ion channel proteins.In the heart, Na ϩ channels (Na V 1.5) 4 primarily underlie action potential initiation and propagation but more recently have been shown to be critical determinants of action potential duration, particularly in the setting of certain inherited channelopathies. Inherited mutations in SCN5A, the gene coding for Na V 1.5, are now known to underlie multiple inherited cardiac arrhythmias, including the congenital long QT syndrome variant 3, Brugada syndrome, and isolated conduction disease (5), and in most cases, these inherited mutations disrupt channel inactivation.Fast inactivation of Na ϩ channels is due to rapid block of the inner mouth of the channel pore by the cytoplasmic linker between domains III and IV that occurs within milliseconds of membrane depolarization...

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