Interaction of phosphofructokinase with tubulin and microtubules. Quantitative evaluation of the mutual effects.

The linked equilibria involved in the binding of phosphofructokinase (EC 2.7.1.11, ATP:D-fructose-6-phosphate 1-phosphotransferase) to tubulin and microtubules were studied at high ionic strength in vitro. The concentration-dependent dissociation of phosphofructokinase was analyzed in the absence and presence of tubulin or microtubules, and the binding of kinase to the tubulin dimer and microtubules was compared. Enzyme activity of phosphofructokinase was inhibited by both tubulin and microtubules: the relative inhibition increased with decreasing enzyme concentration. The complex formation between phosphofructokinase and tubulin was demonstrated by means of fluorescent anisotropy. Concentration-dependent copelleting of the kinase with taxol-stabilized microtubules revealed binding of the enzyme to microtubules as well as phosphofructokinase-enhanced pelleting of microtubules. The binding data agree with the enzyme kinetic findings that the inactive dissociated forms of phosphofructokinase (monomer-dimer) are involved in the heterologous complex formation. Microtubule reorganization (bundle formation) by phosphofructokinase was established by turbidity measurements and sedimentation experiments. The binding data are consistent with a simple molecular model for the interactions in phosphofructokinase-tubulin/microtubules systems.