Conformational change in the N-terminal domain of the Escherichia coli tryptophan synthase β2 subunit induced by its interactions with monoclonal antibodies

Abstract

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 β 2 subunit of Escherichia coli tryptophan synthase reconstituted by reassembly of the N- and C-terminal proteolytic domains of the β chain. The other complexes were formed by the association of the N-terminal fragment (F 1) 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 β 2, were investigated. It was shown that a structural readjustment, which the isolated F 1 domain was unable to undergo alone, was imposed upon F 1 by interdomain interactions. Furthermore, with three of the four antibodies studied, the same conformational change in F 1 also occurred after formation of the F 1-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.