Segmental flexibility in an antibody molecule

Abstract

The rotational motions of an antibody molecule were investigated by nanosecond fluorescence polarization techniques. The fluorescent chromophore, ϵ-dansyl- l-lysine, was specifically bound to the combining sites of antidansyl antibody. The emission anisotropy of the bound dansyl group was measured as an explicit function of time following excitation by a nanosecond light pulse. The F( (ab′)2) and F ab fragments of the antibody were studied in the same way. The striking finding is that the antibody molecule displays a discrete mode of flexibility in the nanosecond time range. The F ab portions of the intact antibody molecule are free to rotate over an angular range of the order of 33 degrees in times of nanoseconds. The rotational correlation time for this restricted segmental motion is 33 nsec. The over-all rotational motion of the antibody molecule has a correlation time of 168 nsec, indicating that the antibody has a markedly non-spherical shape. Our findings are consistent with an important feature of the model proposed by Valentine & Green (1967), namely that there is a flexible joint at the junction of the F ab segments. This joint may be biologically significant in facilitating the formation of antibody-antigen complexes. The facile segmental flexibility exhibited by the antibody exemplifies a structural design which is probably used in other large proteins and in organized macromolecular assemblies.