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 Na(v)1 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 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 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 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 Ca2+-dependent conformational change seen in the Ca(v)1.2 . CaM complex. Furthermore, CaM binding to Na(v)1 C termini lowered Ca2+ binding affinity and cooperativity among the CaM-binding sites compared with CaM alone. Nonetheless, we found suggestive evidence for Ca2+/CaM-dependent effects upon Na(v)1 channels. The R1902C autism mutation conferred a Ca2+-dependent conformational change in Na(v)1.2 C terminus . CaM complex that was absent in the wild-type complex. In Na(v)1.5, CaM modulates the C-terminal 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.