The gene for the major outer surface protein A (OspA) from several clinically obtained strains of Borrelia burgdorferi, the cause of
Lyme disease, has been cloned, sequenced, and expressed in Escherichia coli by using a T7-based expression system (J. J. Dunn, B. N. Lade, and A. G. Barbour, Protein Expr. Purif. 1:159-168, 1990). All of the OspAs have a single conserved tryptophan at residue 216 or, in some cases, 217; however, the region of the protein flanking the tryptophan is hypervariable, as determined by a moving-window population analysis of ospA from 15 European and North American isolates of B. burgdorferi. Epitope-mapping studies using chemically cleaved OspA and a TrpE-OspA fusion have indicated that this hypervariable region is important for immune recognition. Biophysical analysis, including fluorescence and circular dichroism spectroscopy, have indicated that the conserved tryptophan is buried in a hydrophobic environment. Polar amino acid side chains flanking the tryptophan are likely to be exposed to the hydrophilic solvent. The hypervariability of these solvent-exposed amino acid residues may contribute to the antigenic variation in OspA. To test this, we have performed site-directed mutagenesis to replace some of the potentially exposed amino acid side chains in the B31 protein with the analogous residues of a Borrelia garinii strain, K48. The altered proteins were then analyzed by Western blot (immunoblot) with monoclonal antibodies which bind OspA on the surface of the intact B31 spirochete. Our results indicate that specific amino acid changes near the tryptophan can abolish the reactivity of OspA to these monoclonal antibodies, which is an important consideration in the design of vaccines based on recombinant OspA.