Interaction between glycogen phosphorylase and sarcoplasmic reticulum membranes and its functional implications

Abstract

Skeletal muscle glycogen phosphorylase b binds to sarcoplasmic reticulum (SR) membranes with a dissociation constant of 1.7 ± 0.6 mg of phosphorylase/ml at 25 °C at physiological pH and ionic strength. Raising the temperature to 37 °C produced a 2-3-fold decrease in the dissociation constant. The SR membranes could bind up to 1.1 ± 0.1 mg of glycogen phosphorylase b/mg of SR protein, whereas liposomes prepared with endogenous SR lipids and reconstituted Ca2+-ATPase were unable to bind glycogen phosphorylase. Binding of glycogen phosphorylase b to SR membranes is accompanied by inhibition of its activity in the presence of AMP. The V(max) for glycogen phosphorylase b associated with SR membranes is 40 ± 5% of that for purified glycogen phosphorylase and shows a decreased affinity for its allosteric activators, AMP and IMP. These kinetic effects are also observed with purified glycogen phosphorylase b when starch or α-amylose is used as substrate instead of glycogen. Treatment of SR membranes with α-amylase produced dissociation of glycogen phosphorylase b from the SR membranes. Thus, linear polysaccharide fragments of glycogen bound to the SR membranes are likely mediating the binding of glycogen phosphorylase b to these membranes.