Ca-dependent regulation of sodium channels Na1.4 and Na1.5 is controlled by the post-IQ motif.

Skeletal muscle voltage-gated Na channel (Na1.4) activity is subject to calmodulin (CaM) mediated Ca-dependent inactivation; no such inactivation is observed in the cardiac Na channel (Na1.5). Taken together, the crystal structures of the Na1.4 C-terminal domain relevant complexes and thermodynamic binding data presented here provide a rationale for this isoform difference. A Ca-dependent CaM N-lobe binding site previously identified in Na1.5 is not present in Na1.4 allowing the N-lobe to signal other regions of the Na1.4 channel. Consistent with this mechanism, removing this binding site in Na1.5 unveils robust Ca-dependent inactivation in the previously insensitive isoform. These findings suggest that Ca-dependent inactivation is effected by CaM's N-lobe binding outside the Na C-terminal while CaM's C-lobe remains bound to the Na C-terminal. As the N-lobe binding motif of Na1.5 is a mutational hotspot for inherited arrhythmias, the contributions of mutation-induced changes in CDI to arrhythmia generation is an intriguing possibility.