The α1-β-Subunit Interaction That Modulates Calcium Channel Activity Is Reversible and Requires a Competent α-Interaction Domain

High voltage-gated calcium channels consist of a pore-forming subunit (α1) and three nonhomologous subunits (α2/δ, β, and γ). Although it is well established that the β-subunit promotes traffic of channels to the plasma membrane and modifies their activity, the reversible nature of the interaction with the α1-subunit remains controversial. Here, we address this issue by examining the effect of purified β2a protein on CaV1.2 and CaV2.3 channels expressed in Xenopus oocytes. The β2a-subunit binds to the α1-interaction domain (AID) in vitro, and when injected into oocytes, it shifts the voltage dependence of activation and increases charge movement to ionic current coupling of CaV1.2 channels. This increase depended on the integrity of AID but was not abolished by bafilomycin, demonstrating that the α1-β interaction through the AID site can take place at the plasma membrane. Furthermore, injection of β2a protein inhibited inactivation of CaV2.3 channels and converted fast inactivating CaV2.3/β1b channels to slow inactivating channels. Inhibition of inactivation required larger concentration of β2a in oocytes expressing CaV2.3/β1b channels than expressing CaV2.3 alone but reached the same maximal level as expected for a competitive interaction through a single binding site. Together, our data show that the α1-β interaction is reversible in intact cells and defines calcium channels β-subunits as regulatory proteins rather than stoichiometric subunits.