Binding of MgATP to the allosteric site of phosphofructokinase-2 promotes a dimer to tetramer conversion. In the presence of Fru-6-P the enzyme remains as a dimer. Limited proteolysis in the presence of MgATP completely protects the enzyme against inactivation and cleavage, while Fru-6-P provides a partial protection. A 28-kDa proteolytic fragment containing the N-terminus of the protein is inactive, but retains the ability to bind Fru-6-P and the allosteric effector MgATP. The fragment remains as a dimer but does not form a tetramer in the presence of MgATP. The results suggest major conformational changes of the enzyme upon ligand binding that confer a higher degree of compactness to the monomers in the dimer and in the tetramer, demonstrate the presence of the active and allosteric sites in this N-terminus fragment, and stress the importance of the C-terminus region of the protein for catalytic activity and ligand-induced oligomerization.