Limited proteolysis at the carboxy end modifies interactions between the subunits of Escherichia coli...

The limited proteolysis of Escherichia coli phosphofructokinase by subtilisin involves the removal of a segment of 40-50 residues at the C-terminal end of each polypeptide chain [Le Bras, G., & Garel, J.R. (1985) J. Biol. Chem. 260, 13450-13453]. The time course of proteolysis has been followed by the appearance of shorter chains, the loss of allosteric inhibition by phosphoenolpyruvate, and the weakening of the tetrameric structure in the absence of fructose 6-phosphate. It is found that with only one shorter chain out of four the stability of the tetramer is altered so that it is no longer stable in the absence of fructose 6-phosphate. Also, the reduction in size of only two chains is sufficient to render the enzyme insensitive to allosteric effectors, albeit the protein still possesses the ability to bind such an effector (at least partially); the cleavage of all four chains is needed to lose all the effector binding ability. The C-terminal segment therefore plays an important role in subunit interactions as seen from the gradual changes in structural and functional properties which follow its removal from one, two, or four chains.