MD-2 and Der p 2 - a tale of two cousins or distant relatives?

MD-2, an LPS-binding protein is essential for the recognition of LPS by TLR4. MD-2 belongs to the ML superfamily of lipid-binding proteins. The tertiary structure of mite allergen protein Der p 2 was identified as having the protein fold most compatible with the sequence of MD-2. Comparison of MD-2 and Der p 2 reveals that they have many common biochemical characteristics: they are both rich in beta-structure and they are both very stable proteins as they both unfold only above 90 degrees C. In Der p 2, six cysteine residues form three disulfide bridges. We determined one free cysteine residue per recombinant biologically active MD-2 molecule, supporting similar disulfide topology with three disulfides bridges as in Der p 2. MD-2 binds LPS with high affinity; however, only weak binding of LPS was detected with Der p 2. Comparison of electrostatic potentials of the structural model of MD-2 and Der p 2 indicates a region of high positive potential on MD-2 and its absence in Der p 2, which may be the reason for its weak binding of LPS. We suggest that Der p 2 and its homologues probably do not have a role in response to Gram-negative bacteria in insects and that MD-2 family members with their specific role in innate immunity probably evolved from an ML ancestor only in higher vertebrates.