Borrelia burgdorferi, a spirochete transmitted to human hosts during feeding of infected Ixodes ticks, is the causative agent of
Lyme disease. Serum-resistant B. burgdorferi strains cause a chronic, multisystemic form of the
disease and bind complement factor H (FH) and FH-like protein 1 (FHL-1) on the spirochete surface. Here we report the atomic structure for the key FHL-1- and FH-binding protein BbCRASP-1 and reveal a homodimer that presents a novel target for drug design.