Binding of phosphorylase a and b to skeletal muscle thin filament proteins

Phosphorylase plays an important role in energy generation during muscle contraction. We have demonstrated that purified rabbit skeletal muscle phosphorylase a and phosphorylase b bind to rabbit muscle F-actin, F-actin-tropomyosin, F-actin-tropomyosin-troponin, and myofibrils. Neither phosphorylase a nor phosphorylase b binds to myosin. Phosphorylase a and b bind to F-actin with S0.5 values of 1.5 X 10(-6) and 2.1 X 10(-6) M, respectively. At saturation, 0.035 mol of phosphorylase a and b is bound for every seven G-actin monomers in the F-actin polymer. Using the F-actin-tropomyosin-troponin complex as opposed to F-actin as a binding target, there are five- and threefold increases in the maximal binding capacity for phosphorylase a and phosphorylase b, respectively, without a significant change in the S0.5 value for either form of the enzyme. A similar stoichiometry and affinity of phosphorylase binding are observed when myofibrils are used as the binding target. Ca2+ ions and AMP increase the maximal binding capacity for phosphorylase a to myofibrils while ATP decreases the Bmax. Our study suggests that in skeletal muscle, phosphorylase a and phosphorylase b may interact with the thin filament, and that this binding to thin filament proteins may be controlled by changes in sarcoplasmic concentration of Ca2+ and ligands of phosphorylase during muscle contraction.