E1163

CRYSTAL STRUCTURE OF OUTER SURFACE PROTEIN A (OspA) OF BORRELIA BURGDORFERI COMPLEXED WITH A MURINE MONOCLONAL Fab. Hong Li¹, John Dunn¹, Benjamin J.Luft², & Catherine L. Lawson¹, ¹Biology Department, Brookhaven National Laboratory, Upton, N.Y. 11973, ²Division of Infectious Diseases, School of Medicine, State University of Stony Brook, Stony Brook, N.Y. 11794-8153

OspA is an abundant outer surface protein of Borrelia burgdorferi that is currently being tested in several forms as a prophylactic vaccine against Lyme disease. The crystal structure of recombinant OspA (with amino-terminal lipidated cysteine replaced by non-lipidated alanine) was solved in a complex with the Fab fragment of an agglutinating monoclonal antibody. The structure of OspA will be useful for mapping sequence variability and for defining antigenic epitopes to aid in vaccine design.

OspA consists almost entirely of antiparallel ß-strands with short turns or small loops. Nine strands (5-13) form a central ß-sheet with right-handed twist. Strands 1 to 4 pack against the central sheet in an orthogonal sandwich motif. Strands 14 to 17 and 18 to 21 form two sheets that pack against the sole a-helix terminating at lysine 273, forming a ß/ß/a barrel. A homology search of the protein structure data base (DALI) suggests that OspA does not share structure homology with any known proteins. The Fab184.1 binds to loops in the region of the N-terminal sandwich and may stabilize an otherwise flexible N-terminal region. Sequence variability maps to two C-terminal loops on one edge of the central sheet, plus one loop of the C-terminal barrel.

The structure was solved by a combination of multiple isomorphous replacement and molecular replacement methods. The refined model consists of residues 22 to 273 of OspA (28 kD), all residues of Fab184.1 (50 kD), and 324 waters (current R-factor is 22.9 % and R free is 29.4 at 1.95 Å).