Dual function of the voltage-dependent Ca²⁺ channel α₂δ subunit in current stimulation and subunit interaction

Voltage-dependent Ca²⁺ channels are modulated by complex interactions with the α₂δ subunit. In vitro translation was used to demonstrate a single transmembrane topology of the α₂δ subunit in which all but the transmembrane sequence and 5 carboxy-terminal amino acids are extracellular. The glycosylated extracellular domain is required for current stimulation, as shown by coexpression of truncated α₂δ subunits with α_1A and β₄ subunits in Xenopus oocytes and deglycosylation with peptide-N-glycosidase F. However, coexpression of the transmembrane domain-containing δ subunit reduced the stimulatory effects of full-length α₂δ subunits and substitution of a different transmembrane domain resulted in a loss of current stimulation. These results support a model whereby the α₂δ transmembrane domain mediates subunit interactions and the glycosylated extracellular domain enhances current amplitude.