Calmodulin Mediates Ca2+ Sensitivity of Sodium Channels.

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 Nav1 C terminus. Mutations within these sites cause cardiac arrhythmias or autism, but details about how Ca2+ confers sensitivity are poorly understood. Studies on the homologous Cav1.2 channel revealed non-canonical CaM interactions, providing a framework for exploring Na+ channels. In contrast to previous reports, we found that Ca2+ does not bind directly to Na+ channel C termini. Rather, Ca2+ sensitivity appears to be mediated by CaM bound to the C termini in a manner that differs significantly from CAM regulation of Cav1.2. In Nav1.2 or Nav1.5, CaM bound to a localized region containing the IQ motif and did not support the large Ca2+-dependent conformational change seen in the Cav1.2·CaM complex. Furthermore, CaM binding to Nail C termini lowered Ca2+ binding affinity and cooperativity among the CaM-bindlng sites compared with CaM alone. Nonetheless, we found suggestive evidence for Ca2+/CaM-dependent effects upon Nail channels. The R1902C autism mutation conferred a Ca2+-dependent conformational change in Nav1.2 C terminus·CaM complex that was absent in the wild-type complex. In Nav1.5, 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 Ca2+/CaM modulation of Na+ channel function. [ABSTRACT FROM AUTHOR]