Crystal Structures of Calcium-Loaded Calmodulin in Complex with C-Terminal Domains of Voltage-Gated Sodium Channels

Abstract

Calmodulin (CaM) is a ubiquitous calcium sensor that can influence the gating of several ion channels. In both voltage-gated sodium (NaV) and calcium channels (CaV), CaM can affect channel inactivation. The primary binding site is located in the C-terminal region (CT), which consists of an IQ domain downstream of an EF-hand domain. We solved a crystal structure of a fully Ca2+-occupied CaM, bound to the CT of NaV1.5. We find the C-terminal lobe to bind to a site ∼90° rotated relative to a previous site that has been reported for apoCaM in complex with the NaV1.5 CT and for ternary complexes containing fibroblast growth factor homologous factors (FHF). We show that the binding of FHFs changes the orientation of the EF-hand domain relative to the IQ domain, such that the EF-hand domain would clash with a Ca2+-occupied C-lobe of CaM. This highlights the role of the EF-hand domain in modulating the binding mode of CaM. The related NaV1.4 channel has been shown to undergo Ca2+-dependent inactivation (CDI) akin to CaVs. We were unable to crystallize the NaV1.4 CT with Ca2+/CaM, but instead solved a crystal structure of Ca2+/CaM bound to the NaV1.4 IQ domain. Superposing this with NaV CT structures shows that Ca2+/CaM would clash with the EF-hand domain, suggesting that binding of Ca2+/CaM to the NaV1.4 CT would cause a reorientation of the EF-hand domain. We postulate that this event is a possible conformational switch that underlies CDI.