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

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, β_1a 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.