Structural Differences In Calmodulin Bound To Voltagate-Gated Sodium Channel Iq Motifs

Voltage-gated sodium channels (NaV) control the influx of Na+ and the rising phase of action potentials in excitable cells. They bind calmodulin (CaM) at an IQ motif in their C-terminal domain (CTD), but calcium-mediated regulation is poorly understood. Five high-resolution structures of apo (calcium-depleted) CaM bound to the IQ motif of NaV1.2, NaV1.5 and NaV1.6 show the NaV primarily contacts the C-domain of CaM (CaMC), which is in a “semi-open” tertiary conformation. In these structures, the IQ motif adopts the same orientation relative to apo CaMC. In two structures (4JPZ/Xray and 2M5E/NMR) of Ca2+-CaM bound to IQ-containing fragments of NaV1.2, Ca2+-CaM primarily makes contacts with residues 1905-1921. However, the orientation of the IQ motif relative to CaMC in 2M5E is opposite to that of 4JPZ. In 4JPZ (X-ray), the NaV1.2 CTD (residues 1777-1937, including the EF-like domain and IQ region), binds “semi-open” CaMC, whereas in 2M5E (NMR), CaMC is “open” when bound to NaV1.2 residues 1901-1927 (I1912 and Q1913 are the IQ residues). These extremely different CaM-IQ complexes may both be populated in the cell. To explore how the NaV1.2 sequence outside the IQ motif may determine these differences, we used solution (NMR) studies to compare full-length human Ca2+-CaM bound to two NaV1.2 fragments - residues 1777-1937 and 1901-1927. In both complexes, Ca2+-CaMC adopted the “open” conformation (observed in 2M5E and other Ca2+-CaMstructures), suggesting that differences in the sequences flanking the NaV1.2 IQ motif are not sufficient to explain the differences between 4JPZ and 2M5E. Future structural studies will explore how different solution conditions used in the 4JPZ (X-ray) and 2M5E (NMR) structures may select for different conformations of CaM bound to IQ motifs. Supported by NIH R01 GM57001.