Low-resolution structure of immunoglobulins IgG1, IgM and rheumatoid factor IgM-RF from solution X-ray scattering data

Low-resolution structures of immunoglobulins IgG, IgM and rheumatoid factor IgM-RF in solution were analyzed using synchrotron radiation small-angle X-ray scattering and the macromolecular shapes were restored ab initio from the scattering data. The shape of IgG agrees well with the distorted Y-type crystallographic model but has a swollen appearance reflecting flexibility of the molecule in solution. The structures of pentameric IgM and IgM-RF were reconstructed assuming a five-fold symmetry. The IgM displays a flat star-like shape with observable F(ab)₂ regions. The overall shape of the IgM-RF is similar to that of the IgM but with distinctly asymmetric F(ab)₂ regions. This result agrees with the earlier observed functional dissimilarity of the Fab fragments in the rheumatoid factor and points to their structural dissimilarity.