Properties of Calmodulin Binding to Na V 1.2 IQ Motif and Its Autism-Associated Mutation R1902C

Voltage-gated sodium channels (VGSCs) are fundamental to the initiation and propagation of action potentials in excitable cells. Ca 2+ /calmodulin (CaM) binds to VGSC type II (Na V 1.2) isoleucine and glutamine (IQ) motif. An autism-associated mutation in Na V 1.2 IQ motif, Arg1902Cys (R1902C), has been reported to affect the combination between CaM and the IQ motif compared to that of the wild type IQ motif. However, the detailed properties for the Ca 2+ -regulated binding of CaM to Na V 1.2 IQ (1901Lys-1927Lys, IQ wt ) and mutant IQ motif (IQ R1902C ) remains unclear. Here, the binding ability of CaM and CaM's constituent proteins including N- and C lobe to the IQ motif of Na V 1.2 and its mutant was investigated by protein pull-down experiments. We discovered that the combination between CaM and the IQ motif was U-shaped with the highest at [Ca 2+ ] ≈ free and the lowest at 100 nM [Ca 2+ ]. In the IQ R1902C mutant, Ca 2+ -dependence of CaM binding was nearly lost. Consequently, the binding of CaM to IQ R1902C at 100 and 500 nM [Ca 2+ ] was increased compared to that of IQ wt . Both N- and C lobe of CaM could bind with Na V 1.2 IQ motif and IQ R1902C mutant, with the major effect of C lobe. Furthermore, CaMKII had no impact on the binding between CaM and Na V 1.2 IQ motif. This research offers novel insight to the regulation of Na V 1.2 IQ wt and IQ R1902C motif, an autism-associated mutation, by CaM.