Interaction between the dihydropyridine receptor Ca2+ channel β-subunit and ryanodine receptor type 1 strengthens excitation-contraction coupling

Previous studies have shown that the skeletal dihydropyridine receptor (DHPR) pore subunit Cav1.1 (alpha 1S) physically interacts with ryanodine receptor type 1 (RyR1), and a molecular signal is transmitted from alpha 15 to RyR1 to trigger excitation-contraction (EC) coupling. We show that the beta-subunit of the skeletal DHPR also binds RyR1 and participates in this signaling process. A novel binding site for the DHPR beta 1a-subunit was mapped to the M-3201 to W-3661 region of RyR1. In vitro binding experiments showed that the strength of the interaction is controlled by (KKKRR)-K-3495 _ _ R-3502, a cluster of positively charged residues. Phenotypic expression of skeletal-type EC coupling by RyR1 with mutations in the (KKKRR)-K-3495 _ _ R-3502 cluster was evaluated in dyspedic myotubes. The results indicated that charge neutralization or deletion severely depressed the magnitude of RyR1-mediated Ca2+ transients coupled to voltage-dependent activation of the DHPR. Meantime the Ca2+ content of the sarcoplasmic reticulum was not affected, and the amplitude and activation kinetics of the DHPR Ca2+ currents were slightly affected. The data show that the DHPR beta-subunit, like alpha 1S, interacts directly with RyR1 and is critical for the generation of high-speed Ca2+ signals coupled to membrane depolarization. These findings indicate that EC coupling in skeletal muscle involves the interplay of at least two subunits of the DHPR, namely alpha 1S and beta 1a, interacting with possibly different domains of RyR1.