Importance of the different beta subunits in the membrane expression of the alpha 1A and alpha 2 calcium channel subunits: Studies using a depolarization-sensitive alpha 1A antibody

The plasma membrane expression of the rat brain calcium channel subunits alpha 1A, alpha 2-delta and the beta subunits beta 1b, beta 2a, beta 3b and beta 4 was examined by transient expression in COS-7 cells. Neither alpha 1A nor alpha 2-delta localized to the plasma membrane, either alone or when coexpressed. However, coexpression of alpha 1A or alpha 2-delta/alpha 1A with any of the beta subunits caused alpha 1A and alpha 2 to be targetted to the plasma membrane. The alpha 1A antibody is directed against an exofacial epitope at the mouth of the pore, which is not exposed unless cells are depolarized, both for native alpha 1A channels in dorsal root ganglion neurons and for alpha 1A expressed with a beta subunit. This subsidiary result provides evidence that either channel opening or inactivation causes a conformational change at the mouth of the pore of alpha 1A. Immunostaining for alpha 1A was obtained in depolarized non-permeabilized cells, indicating correct orientation in the membrane only when it was coexpressed with a beta subunit. in contrast, beta 1b and beta 2a were associated with the plasma membrane when expressed alone. However, this is not a prerequisite to target alpha 1A to the membrane since beta 3 and beta 4 alone showed no differential localization, but did direct the translocation of alpha 1A to the plasma membrane, suggesting a chaperone role for the beta subunits.