Structural Differences in Calmodulin Bound to Voltagate-Gated Sodium Channel IQ Motifs

an N-terminal extracellular domain that adopts an immunoglobulin fold, a single transmembrane helix, and a short cytoplasmic C-terminal tail. We previously solved the crystal structure of the Navb4 extracellular domain, and the structure of the corresponding Navb3 domain has also been reported. We now report the crystal structure of the Navb2 extracellular domain at high resolution (1.4A). A direct comparison of all three available Navb structures shows that each displays unique loop conformations, different lengths of b strands, and a different distribution of disulfide bonds. Although all three contain a central buried disulfide bond, both Navb2 and Navb3 contain an additional bridge, albeit at different locations. Both Navb2 and Navb4 contain an exposed cysteine at identical locations that appears crucial for interaction with the a subunit. However, the surface features around the crucial cysteine differ appreciably among both isoforms. These differences affect the surface properties, and likely underlie the differential effects of Navb subunits on the gating and pharmacological properties of the a subunits. We also solved the structure of a glycosylated form of Navb4, which shows that there is a minimal effect of glycosylation on the final structure. We provide a detailed structural comparison of different Navb subunits in wild type and mutant forms.