Molecular endpoints of Ca²⁺/calmodulin- and voltage-dependent inactivation of Ca_v1.3 channels

Ca²⁺/calmodulin- and voltage-dependent inactivation (GDI and VDI) comprise vital prototypes of Ca²⁺ channel modulation, rich with biological consequences. Although the events initiating GDI and VDI are known, their downstream mechanisms have eluded consensus. Competing proposals include hinged-lid occlusion of channels, selectivity filter collapse, and allosteric inhibition of the activation gate. Here, novel theory predicts that perturbations of channel activation should alter inactivation in distinctive ways, depending on which hypothesis holds true. Thus, we systematically mutate the activation gate, formed by all S6 segments within Ca_v1.3. These channels feature robust baseline CDI, and the resulting mutant library exhibits significant diversity of activation, CDI, and VDI. For CDI, a clear and previously unreported pattern emerges: activation-enhancing mutations proportionately weaken inactivation. This outcome substantiates an allosteric CDI mechanism. For VDI, the data implicate a "hinged lidshield" mechanism, similar to a hinged-lid process, with a previously unrecognized feature. Namely, we detect a "shield" in Ca_v1.3 channels that is specialized to repel lid closure. These findings reveal long-sought downstream mechanisms of inactivation and may furnish a framework for the understanding of Ca²⁺ channelopathies involving S6 mutations.