Isolation and characterization of a novel IgD-binding protein from Moraxella catarrhalis

A novel surface protein of the bacterial species Moraxella catarrhalis that displays a high affinity for IgD (MID) was solubilized in Empigen and isol ated by ion exchange chromatography and gel filtration. The apparent molecu lar mass of monomeric MID was estimated to similar to 200 kDa by SDS-PAGE. The mid gene was cloned and expressed in Escherichia coli. The complete mid nucleotide gene sequence was determined, and the deduced amino acid sequen ce consists of 2123 residues. The sequence of MID has no similarity to othe r Ig-binding proteins and differs from all previously described outer membr ane proteins of M. catarrhalis. MID was found to exhibit unique Ig-binding properties. Thus, in ELISA, dot blots, and Western blots, MID bound two pur ified IgD myeloma proteins, four IgD myeloma sera, and finally one IgD stan dard serum. No binding of MID was detected to IgG, IgM, IgA, or IgE myeloma proteins. MID also bound to the surface-expressed B cell receptor IgD, but not to other membrane molecules on human PBLs. This novel Ig-binding reage nt promises to be of theoretical and practical interest in immunological re search.