Multiple Regions of RyR1 Mediate Functional and Structural Interactions with α1S-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: alpha(1S)DHPRs 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 alpha(1S)DHPRs. alpha(1S)DHPR and RyR1 are linked to each other, organizing alpha(1S)-DHPRs into groups of four, or tetrads. In cardiac muscle, however, alpha(1C)DHPR Ca2+ current is important for activation of RyR2 (the cardiac isoform of the ryanodine receptor) and alpha(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 much greater than R16 much greater than R9 much greater than RyR2. The rank-order for restoration of DHPR tetrads is RyR1 > R4 = R9 > R10 = R16 much greater than 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 alpha(1S)DHPRs and that the regions responsible for tetrad formation do not correspond exactly to the ones required for functional coupling.