γ1 subunit interactions within the skeletal muscle L-type voltage-gated calcium channels

Abstract

Voltage-gated calcium channels mediate excitation-contraction coupling in the skeletal muscle. Their molecular composition, similar to neuronal channels, includes the pore-forming α1 and auxiliary α2δ, β, and γ subunits. The γ subunits are the least characterized, and their subunit interactions are unclear. The physiological importance of the neuronal γ is emphasized by epileptic stargazer mice that lack γ2. In this study, we examined the molecular basis of interaction between skeletal γ1 and the calcium channel. Our data show that the α1,1.1, β1a and α2δ subunits are still associated in γ1 null mice. Reexpression of γ1 and γ2 showed that γ1, but not γ2, incorporates into γ1 null channels. By using chimeric constructs, we demonstrate that the first half of the γ1 subunit, including the first two transmembrane domains, is important for subunit interaction. Interestingly, this chimera also restores calcium conductance in γ1 null myotubes, indicating that the domain mediates both subunit interaction and current modulation. To determine the subunit of the channel that interacts with γ1 we examined the channel in muscular dysgenesis mice. Cosedimentation experiments showed that γ1 and α2δ are not associated. Moreover, α11.1 and γ1 subunits form a complex in transiently transfected cells, indicating direct interaction between the γ1 and α11.1 subunits. Our data demonstrate that the first half of γ1 subunit is required for association with the channel through α11.1. Because subunit interactions are conserved, these studies have broad implications for γ heterogeneity, function and subunit association with voltage-gated calcium channels.