Protein-induced inactivation and phosphorylation of rabbit muscle phosphofructokinase

Several previously untested proteins promote the reversible inactivation of rabbit skeletal muscle phosphofructokinase. Grouped in decreasing order of effectiveness, they include the following: skeletal muscle troponin C greater than troponin, the two smooth muscle myosin light chains, alpha-actinin, and S-100 much greater than parvalbumin and soybean trypsin inhibitor. The efficiency of troponin C in this process may even exceed that previously reported for calmodulin. Sequences near calcium binding site III are apparently involved in the troponin C-phosphofructokinase interaction. Troponin C and calmodulin exert calcium-dependent effects on the physical and chemical properties of muscle phosphofructokinase. When calcium is present, comigration with either protein allows the enzyme to enter the stacking gel during urea-polyacrylamide gel electrophoresis. Both enhance the phosphorylation of phosphofructokinase catalyzed by the cAMP-dependent protein kinase, with phosphate incorporations approaching 2 mol of P/mol of protomer. Reaction occurs at Ser774 and at Ser376--a novel site whose phosphorylation is highly sensitive to troponin C and less so to calmodulin. Maximum phosphorylation has slight effect on the catalytic activity of the enzyme under standard assay conditions. The troponin C induced or calmodulin-induced phosphorylation of phosphofructokinase requires calcium and is strongly inhibited by either fructose 2,6-bisphosphate or fructose 1,6-bisphosphate. Inactivation occurs in the presence or absence of calcium, with generally higher concentrations of effectors required for protection in the latter case. Liver and yeast phosphofructokinases shows little activity loss in the presence of either calmodulin or troponin C. We have developed and tested a general mathematical model for the protein-induced inactivation of phosphofructokinase which may find application to other systems.