A Unique Human Mycoplasma Protein that Generically Blocks Antigen-Antibody Union

We report the discovery and crystal structure of a human mycoplasma protein, Protein M, which binds with high affinity to antibodies, predominantly through attachment to the variable region of the κ and λ light chains. Protein M broadly blocks antibody-antigen union and its mechanism of inhibition is of considerable interest because, as a diversity system, the binding mode of each antibody is different. Protein M thus appears to function by a mechanism that is independent of the sequences of members of the extensive antibody repertoire. By anchoring to conserved regions of the antibody light chains, Protein M is in a position to extend its large C-terminal domain over the antibody combining site and block entrance to macromolecular antigens.