Direct demonstration of multiple VH allotopes on rabbit Ig molecules: allotope characteristics and Fab arm rotational flexibility revealed by immunoelectron microscopy.

Immune complexes composed of rabbit IgG and Fab fragments of antibody specific for the VH framework allotypes were analyzed by molecular immunoelectron microscopy. In this manner, the number of allotypic epitopes ( allotopes ) and their approximate topological location could be determined. A monoclonal anti-allotype Fab was used to show that relatively fine details of allotope location and orientation are demonstrable with this technique. Each of the three VH allotypes (a1, a2 and a3) was found to express at least two spacially separated allotopes . The locations of the allotopes varied for each of the three allotypes. Some allotopes were observed near the VH-CH1 switch region while others were in close proximity to the complementarity-determining region or were at intermediate positions. In addition to the information concerning allotope topology, the variety of configurations resulting from the interaction of monoclonal antibody Fab with IgG suggest considerable rotational flexibility (twisting) of the Fab arm of IgG about its long axis.