Conformational change in the N-terminal domain of the Escherichia coli tryptophan synthase beta 2 subunit induced by its interactions with monoclonal antibodies.

A fluorescence energy transfer signal was used to follow conformational changes occurring in two types of protein-protein complexes. The first complex studied was the native-like beta 2 subunit of Escherichia coli tryptophan synthase reconstituted by reassembly of the N- and C-terminal proteolytic domains of the beta chain. The other complexes were formed by the association of the N-terminal fragment (F1) with a monoclonal antibody that recognizes the native dimeric protein; four such complexes, obtained with different antibodies that recognize four distinct antigenic sites on native beta 2, were investigated. It was shown that a structural readjustment, which the isolated F1 domain was unable to undergo alone, was imposed upon F1 by interdomain interactions. Furthermore, with three of the four antibodies studied, the same conformational change in F1 also occurred after formation of the F1-antibody complex. These results demonstrate that, through an "induced fit mechanism", antigen-antibody stereospecific assembly can force the polypeptide chain to adopt a structure more closely resembling the conformation it has in the native protein.