Backbone resonance assignments of complexes of apo human calmodulin bound to IQ motif peptides of voltage-dependent sodium channels Na1.1, Na1.4 and Na1.7.

Human voltage-gated sodium (Na) channels are critical for initiating and propagating action potentials in excitable cells. Nine isoforms have different roles but similar topologies, with a pore-forming α-subunit and auxiliary transmembrane β-subunits. Na pathologies lead to debilitating conditions including epilepsy, chronic pain, cardiac arrhythmias, and skeletal muscle paralysis. The ubiquitous calcium sensor calmodulin (CaM) binds to an IQ motif in the C-terminal tail of the α-subunit of all Na isoforms, and contributes to calcium-dependent pore-gating in some channels. Previous structural studies of calcium-free (apo) CaM bound to the IQ motifs of Na1.2, Na1.5, and Na1.6 showed that CaM binding was mediated by the C-domain of CaM (CaM), while the N-domain (CaM) made no detectable contacts. To determine whether this domain-specific recognition mechanism is conserved in other Na isoforms, we used solution NMR spectroscopy to assign the backbone resonances of complexes of apo CaM bound to peptides of IQ motifs of Na1.1, Na1.4, and Na1.7. Analysis of chemical shift differences showed that peptide binding only perturbed resonances in CaM; resonances of CaM were identical to free CaM. Thus, CaM residues contribute to the interface with the IQ motif, while CaM is available to interact elsewhere on the channel.