Timothy Syndrome-Associated Mutations Affect State-Dependent Contacts in L-Type Calcium Channel

Abstract

Timothy syndrome (TS) is a very rare disorder almost exclusively associated with mutations G402S and G406R in IS6 of Cav1.2. Recently, mutations R518C/H in IIS0 of the voltage sensing domain II (VSD-II) were described as a cause of cardiac-only Timothy syndrome. The three mutations decelerate the voltage-dependent inactivation (VDI). We describe a new case of cardiac-only Timothy syndrome caused by mutation R518C. We used Class Ia and Class II cryo-EM structures of presumably inactivated-state channel Cav1.1 to model Cav1.2 and explore possible impact of the three mutations on interdomain contacts. In both models, R518 and several other residues in VSD-II donated H-bonds to the IS6-linked α1-interaction domain (AID). We further employed steered Monte Carlo energy minimizations (MCM) to move helices S4-S5, S5 and S6 to positions seen in the X-ray structures of open and closed sodium channel NavAb. In the open-state models, positions of AID and VSD-II were similar to those in Cav1.1. In the closed-state models, AID moved along the β subunit (Cavβ) towards the pore axis and shifted AID-bound VSD-II. In all the models, R518 retained H-bonds with AID. Our models suggest that VSD-II rearrangement upon its activation would shift AID along Cavβ towards the pore axis. The AID-linked IS6 would bend at flexible G402 and G406, facilitating closure of the activation gate. Mutations R518C/H, which weakened the IIS0-AID contacts in our models, would retard the AID shift. Mutations G406R and G402S stabilized the open state and would resist the pore closure. Several Cav1.2 mutations associated with long QT syndromes are consistent with this scenario. Our results provide a mechanistic rationale for the VDI deceleration caused by TS-associated mutations. They also suggest targets for further studies of calcium channelopathies. Supported by grants to BSZ from RSF and NSERC.