Dissection of functional domains of the voltage-dependent Ca²⁺ channel α₂δ subunit

Coexpression of the cloned voltage-dependent Ca²⁺ channel α₂δ subunit with the pore-forming α₁ subunit results in a significant increase in macroscopic current amplitude. To gain insight into the mechanism underlying this interaction, we have examined the regulatory effect of either the α₂δ complex or the δ subunit on the Ca²⁺ channel α₁ subunit. Transient transfection of tsA201 cells with the cardiac L-type α(1c) subunit alone resulted in the expression of inward voltage-activated currents as well as measurable [³H]-PN200-110 binding to membranes from transfected cells. Coexpression of the α₂δ subunit significantly increased the macroscopic current amplitude, altered the voltage dependence and the kinetics of the current, and enhanced [³H]-PN200-110 binding. Except for the increase in amplitude, coexpression of the δ subunit reproduced entirely the effects of the full-length α₂δ subunit on the biophysical properties of the α(1c) currents. However, no effect on specific [³H]-PN200-110 binding was observed on δ subunit coexpression. Likewise, profound effects on current kinetics of the neuronal α(1A) subunit were observed on coexpression of the α₂δ complex in Xenopus oocytes. Furthermore, by using a chimeric strategy, we localized the region involved in this regulation to the transmembrane domain of the δ subunit. These data strongly suggest that the molecular determinants involved in α₂δ regulation are conserved across L-type and non-L type Ca²⁺ channels. Taken together, our results indicate that the region of the α₂δ subunit involved in the modulation of the gating properties of the high voltage-activated calcium channels is localized in the δ domain of the protein. In contrast, the level of membrane expression of functional channels relies on the presence of the α₂ domain of the α₂δ complex.