Fig. 1.

Calcium channel α₁, β, and α₂δ subunits, and their topology. (A) Nomenclature of calcium channel subunits, including gene name, initial names of cloned α1 subunits, rationalised protein names (Ca_V nomenclature), and names used in physiological discovery of the channels. HVA, LVA = classical definition of channels as high- or low-voltage-activated. (B) Calcium channel α₁, β, and α₂δ subunit topology. The α₁ subunit has 24 transmembrane segments, comprising four homologous domains, labelled I–IV. Each domain has six transmembrane segments (S1–S6), including the S4 voltage sensor (yellow), and the S5–S6 pore-forming segments (blue).