The binding interactions that maintain excitation-contraction coupling junctions in skeletal muscle

Calcium for contraction of skeletal muscles is released via tetrameric ryanodine receptor (RYR1) channels of the sarcoplasmic reticulum (SR), which are assembled in ordered arrays called couplons at junctions where the SR abuts T tubules or plasmalemma. Voltage-gated Ca²⁺ (Ca_V1.1) channels, found in tubules or plasmalemma, form symmetric complexes called Ca_V tetrads that associate with and activate underlying RYR tetramers during membrane depolarization by conveying a conformational change. Intriguingly, Ca_V tetrads regularly skip every other RYR tetramer within the array; therefore, the RYRs underlying tetrads (named V), but not the voltage sensor-lacking (C) RYRs, should be activated by depolarization. Here we hypothesize that the checkerboard association is maintained solely by reversible binary interactions between Ca_Vs and RYRs and test this hypothesis using a quantitative model of the energies that govern Ca_V1.1-RYR1 binding, which are assumed to depend on number and location of bound Ca_Vs. A Monte Carlo simulation generates large statistical samples and distributions of state variables that can be compared with quantitative features in freeze-fracture images of couplons from various sources. This analysis reveals two necessary model features: (1) the energy of a tetramer must have wells at low and high occupation by Ca_Vs, so that Ca_Vs positively cooperate in binding RYR (an allosteric effect), and (2) a large energy penalty results when two Ca_Vs bind simultaneously to adjacent RYR protomers in adjacent tetramers (a steric clash). Under the hypothesis, V and C channels will eventually reverse roles. Role reversal justifies the presence of sensor-lacking C channels, as a structural and functional reserve for control of muscle contraction.