The role of phosphorylation in the interaction of rabbit muscle phosphofructokinase with F-actin

The role of phosphorylation in the regulation of rabbit muscle phosphofructokinase was investigated by monitoring the effect of this covalent modification on the steady-state kinetics and complex formation between F-actin and the enzyme. Binding of phosphofructokinase to F-actin at pH 7.0 and 23° C was monitored by sedimentation. These experiments show that phosphorylated phosphofructokinase has a higher apparent affinity for F-actin than does the dephosphorylated form. Control experiments showed that the complex formation is specific. Steady-state kinetic measurements at pH 7.0, 23°C, showed that the presence of F-actin did not significantly affect the basic kinetic properties of the dephosphorylated form. Under identical conditions, F-actin acted as a positive effector of the phosphorylated form, and the effect of F-actin is specific. Results from these in vitro studies are consistent with in vivo observations which show that upon stimulation of muscle contraction, the enzyme is phosphorylated to a greater extent and the binding to the muscle matrix is increased. Hence, phosphorylation of phosphofructokinase does not only alter the kinetic behavior of the enzyme, but also serves as a means to regulate the compartmentalization of the enzyme in order to provide energy to the cellular component where it is needed.