Multiple regions of RyR1 mediate functional and structural interactions with α_1S-dihydropyridine receptors in skeletal muscle

Excitation-contraction (e-c) coupling in muscle relies on the interaction between dihydropyridine receptors (DHPRs) and RyRs within Ca²⁺ release units (CRUs). In skeletal muscle this interaction is bidirectional: α_1SDHPRs trigger RyR1 (the skeletal form of the ryanodine receptor) to release Ca²⁺ in the absence of Ca²⁺ permeation through the DHPR, and RyR1s, in turn, affect the open probability of α_1SDHPRs. α_1SDHPR and RyR1 are linked to each other, organizing α_1S-DHPRs into groups of four, or tetrads. In cardiac muscle, however, α_1CDHPR Ca²⁺ current is important for activation of RyR2 (the cardiac isoform of the ryanodine receptor) and α_1C-DHPRs are not organized into tetrads. We expressed RyR1, RyR2, and four different RyR1/RyR2 chimeras (R4: Sk1635-3720, R9: Sk2659-3720, R10: Sk1635-2559, R16: Sk1837-2154) in 1B5 dyspedic myotubes to test their ability to restore skeletal-type e-c coupling and DHPR tetrads. The rank-order for restoring skeletal e-c coupling, indicated by Ca²⁺ transients in the absence of extracellular Ca²⁺, is RyR1 > R4 > R10 ≫ R16 ≫ R9 ≫ RyR2. The rank-order for restoration of DHPR tetrads is RyR1 > R4 = R9 > R10 = R16 ≫ RyR2. Because the skeletal segment in R9 does not overlap with that in either R10 or R16, our results indicate that multiple regions of RyR1 may interact with α_1SDHPRs and that the regions responsible for tetrad formation do not correspond exactly to the ones required for functional coupling.