Evidence for an α-helical epitope on outer surface protein A from the Lyme disease spirochete, Borrelia burgdorferi: An application of steady-state and time-resolved fluorescence quenching techniques

Outer surface protein A (OspA) is a major antigen of Borrelia burgdorferi, the etiological agent of Lyme disease. A recombinant form of OspA (OspA-257) from B. burgdorferi, strain B31, contains 257 amino acids and a single tryptophan residue at position 216 (Trp-216). Mapping studies indicate that Trp-216 is involved in the epitope for the agglutinating monoclonal antibody 105.5 However, the fluorescence emission maximum of the native protein is 330 nm, indicating that Trp-216 is not solvent-exposed. Primary structure analysis suggests and α-helical conformation for residues approx. 204-217, which, if located on the protein surface, would allow Trp-216 to be buried, while leaving hydrophilic residues on the opposite side of the helix exposed. This helix would plate Lys-212 within approx. 6 Å of Trp-216; the presence of such a positively-charged residue can, in principle, be ascertained from fluorescence quenching studies. Stern-Volmer plots confirm that Trp-216 is indeed buried in the native protein, but is readily accessible to the small polar quencher, Cs⁺, while the minor component undergoes static quenching by I^-, indicating the proximity of a positively-charged residue. These data are consistent with the existence of an α-helix from residues 204-217 in the predicted orientation at the protein surface, hence indicating the structure of the antigentic determinant.