Structure and immunomodulatory property relationship in NapA of Borrelia burgdorferi

NapA from Borrelia burgdorferi is a member of the Dps-like protein family with specific immunomodulatory properties; in particular, NapA is able to induce the expression of IL-23 in neutrophils and monocytes, as well as the expression of IL-6, IL-1 beta, and transforming growth factor beta (TGF-beta) in monocytes, via Toll-like receptor (TLR) 2. Such an activity on innate immune cells triggers a synovial fluid Th17 response. Here we report the crystal structure of NapA, determined at 2.6 A resolution, which shows that the quaternary structure of the protein is that of a dodecamer with 23 symmetry, typical of the proteins of the family. We also demonstrate that the N- and C-terminal tails, which are flexible and not visible in the crystal, are not relevant for its pro-Th17 activity. Based on the crystal structure and on the comparison with the structure of the orthologous protein from Helicobacter pylori, HP-NAP, we hypothesize that the charge distributions on the two proteins' surfaces are responsible for the interaction with TLR2 and for the different behaviors in modulating the immune response. (C) 2010 Elsevier B.V. All rights reserved.