Modulation of the smooth-muscle L-type Ca2+ channel α1 subunit (α1C-b) by the β2a subunit:: a peptide which inhibits binding of β to the I-II linker of α1 induces functional uncoupling

Modulation of the smooth-muscle Ca2+ channel alpha 1C-b subunit by the auxiliary beta 2a subunit was studied in the HEK 293 (cell line from human embryonic kidney cells) expression system. In addition, we tested whether the alpha 1-beta interaction in functional channels is sensitive to an 18-amino-acid synthetic peptide that corresponds to the sequence of the defined major interaction domain in the cytoplasmic I-II linker of alpha 1C (AID-peptide). Ca2+ channels derived by co-expression of alpha 1C-b and beta 2a subunits exhibited an about 3-fold higher open probability (P-0) than alpha 1C-b channels. High-P-0 gating of alpha 1C-b.beta 2a channels was associated with the occurrence of long-lasting channel openings [mean open time (tau) > 10 ms] which were rarely observed in alpha 1C-b channels. Modulation of fast gating by the beta 2a subunit persisted in the cell-free, inside-out recording configuration. Biochemical experiments showed that the AID-peptide binds with appreciable affinity to beta 2 subunits of native Ca2+ channels. Binding of the beta 2 protein to immobilized AID-peptide was specifically inhibited (K-i of 100 nM) by preincubation with free (uncoupled) AID-peptide, but not by a corresponding scrambled peptide. Administration of the AID-peptide (10 mu M) to the cytoplasmic side of inside-out patches induced a substantial reduction of P-0 of alpha 1C-b.beta 2a channels. The scrambled control peptide failed to affect alpha 1C-b.beta 2a channels, and the AID-peptide (10 mu M) did not modify alpha 1C-b channel function in the absence of expressed beta 2a subunit. Our results demonstrate that the beta 2a subunit controls fast gating of alpha 1C-b channels, and suggest the alpha 1-beta interaction domain in the cytoplasmic I-II linker of alpha 1C (AID) as a possible target of modulation of the channel. Moreover, our data are consistent with a model of alpha 1-beta interaction that is based on multiple interaction sites, including AID as a determinant of the affinity of the alpha 1-beta interaction.