Importance of the Different |β Subunits in the Membrane Expression of the α1A and α2 Calcium Channel Subunits: Studies Using a Depolarization-sensitive α1A Antibody

The plasma membrane expression of the rat brain calcium channel subunits α1A, α2-Δ and the β subunits β1b, β2a, β3b and β4 was examined by transient expression in COS-7 cells. Neither α1A nor α2-Δ localized to the plasma membrane, either alone or when coexpressed. However, coexpression of α1A or α2-Δ/α1A with any of the p subunits caused α1A and α2 to be targetted to the plasma membrane. The α1A antibody is directed against an exofacial epitope at the mouth of the pore, which is not exposed unless cells are depolarized, both for native α1A channels in dorsal root ganglion neurons and for α1A expressed with a β subunit. This subsidiary result provides evidence that either channel opening or inactivation causes a conformational change at the mouth of the pore of α1A. Immunostaining for α1A was obtained in depolarized non-permeabilized cells, indicating correct orientation in the membrane only when it was coexpressed with a subunit. In contrast, β1b and β2a were associated with the plasma membrane when expressed alone. However, this is not a prerequisite to target α1A to the membrane since β3 and β4 alone showed no differential localization, but did direct the translocation of α1A to the plasma membrane, suggesting a chaperone role for the β subunits