Evidence for plasticity and structural mimicry at the immunoglobulin light chain-protein L interface

The multidomain bacterial surface protein L (PpL) is a virulence factor expressed by only 10% of Peptostreptococcus magnus strains, and its expression is correlated with bacterial vaginosis. The molecular basis for its ability to recognize 60% of mammalian immunoglobulin light chain variable regions (V-L) has been described recently by x-ray crystallography, which suggested the presence of two V-L binding sites on each protein L domain (Graille, M., Stura, E. A., Housden, N.G., Beekingham, J.A., Bottomley, S. P., Beale, D., Taussig, M.J., Sutton, B.J., Gore, M. G., and Charbonnier, J. (2001) Structure, 679-687). Here, we report the crystal structure at 2.1 Angstrom resolution of a protein L mutant complexed to an Fab' fragment with only 50% of the VL residues interacting with PpL site 1 conserved. Comparison of the site 1 interface from both structures shows how protein L is able to accommodate these sequence differences and therefore bind to a large repertoire of Ig. The x-ray structure and NMR results confirm the existence of two V-L binding sites on a single protein L domain. These sites exhibit a remarkable structural mimicry of growth factors binding to their receptors. This could explain the protein L superantigenic activity on human B lymphocytes.