Delineation of Species-Specific Binding Properties of the CspZ Protein (BBH06) of Lyme Disease Spirochetes: Evidence for New Contributions to the Pathogenesis of Borrelia spp

Borrelia burgdorferi CspZ (TIGR open reading frame designation, BBH06) is part of a functionally related group of proteins that bind one or more members of the factor H (FH) protein family. In this report we assess the conservation, distribution, properties, and ligand binding abilities of CspZ from the three mainBorrelia species associated with Lyme disease infections in humans. CspZ (also referred to as BbCRASP-2 in the literature) was found to be highly conserved at the intraspecies level but divergent at the interspecies level. All CspZ orthologs that originated fromB. burgdorferi isolates bound FH from a diverse group of mammals. In contrast, CspZ derived fromB. garinii andB. afzelii did not. Regardless of theBorrelia species of origin, all CspZ proteins tested bound to unknown ∼60-kDa serum proteins produced by different mammals. To further define the molecular basis for the differential binding of CspZ orthologs to host proteins, DNA sequence, truncation, and site-directed mutagenesis analyses were performed. DNA sequence analyses revealed thatB. garinii andB. afzelii CspZ orthologs possess a 64-amino-acid N-terminal domain that is absent fromB. burgdorferi CspZ. However, binding analyses of recombinant proteins revealed that this domain does not in and of itself influence ligand binding properties. Truncation and mutagenesis analyses further revealed that the key determinants required for ligand binding are discontinuous and that the presentation of the ligand binding pocket is dependent on alpha helices with high coiled-coil formation probability. The data presented here provide insight into the molecular basis of CspZ-ligand interactions and suggest that CspZ orthologs from diverseBorrelia species can contribute to the host-pathogen interaction through their interaction with serum proteins.