Epitope localization in antigen-monoclonal-antibody complexes by small-angle X-ray scattering. An approach to domain organization in the beta 2 subunit of Escherichia coli tryptophan synthase.

Each polypeptide chain of the beta 2 subunit of Escherichia coli tryptophan synthase (EC 4.2.1.20) is made of two domains, F1 (N-terminal) and F2 (C-terminal). To determine the relative position of these domains in the native protein, complexes between beta 2 and Fab fragments from two monoclonal antibodies, one specific for F1 (68-1) and the other for F2 (93-6), have been prepared and purified. Small-angle X-ray scattering measurements have been made on solutions of each complex. From the experimental scattering curves obtained, computer modeling leads to structural models of the two beta 2-Fab complexes. Though relatively low, the resolution of these models allows the localization on beta 2 of the antigenic sites recognized by the two antibodies, to show that the C-terminal F2 domains lie at the distal ends of the elongated beta 2 protein, and to show how steric hindrance prevents beta 2, though structurally and functionally dimeric, from binding more than one Fab 93-6 fragment per dimer.