Roles of L-type Ca^(2+) Channel Subunits of Skeletal Muscle in Dihydropyridine Binding : Evidence for Functional Interaction among α₁, β, γ and α₂δ Subunits

다국어 초록 (Multilingual Abstract)

Purified skeletal muscle 1,4-dihydropyridine (DHP) receptois are composed of five polypeptides termed α₁, β, γ, α₂, and 5. Among these, the α₁ subunit is known to be sufficient to function as a voltage-dependent Ca^(2+) channel and a DHP receptor. The α₁ alone exhibits very similar allosteric regulation of DHP binding to that found in skeletal muscle T-tubules. However, we previously showed that in the absence of Ca^(2+), DHP binding to α₁ alone at subsaturating concentrations was reduced, which could be restored by the (-) stereoisomer of a phenylalkylamine, D600. We hypothesized that this difference is due to lack of other regulatory subunits, specifically the β, γ and α₂δ components that copurify with α₁.
In order to test our hypothesis, we coexpressed the α₁ subunit with non-α₁ components in mouse fiboblasts, L cells, and monkey kidney cells, COS.M6 in various combinations-and compared their DHP binding activity for the effect of (-)D600 in the absence of Ca^(2+). Coexpression of 13 with α₁ did not normalize the abnormal enhancing effect of (-)D600 on DHP binding. Coexpression of either γ or α₂δ, partially reduced the enhancing effect of (-)D600. Importantly, coexpression of all the components, that is, when the receptor was composed of α₁β γα₂δ, completely abrogated the abnormal effect of (-)D600. Thus, our data clearly demonstrate that all the component copurifying with α₁ are essential to constitute the functional DHP receptor.