Crystal structures of the Erp protein family members ErpP and ErpC from Borrelia burgdorferi reveal the reason for different affinities for complement regulator factor H

Borrelia burgdorferi is the causative agent of Lyme disease, which can be acquired after the bite of an infected Ixodes tick. As a strategy to resist the innate immunity and to successfully spread and proliferate, B. burgdorferi expresses a set of outermembrane proteins that are capable of binding complement regulator factorH(CFH), factor H-like protein 1 (CFHL-1) and factor H-related proteins (CFHR) to avoid complement-mediated killing. B. burgdorferi B31 contains three proteins that belong to the Erp (OspE/F-related) protein family and are capable of binding CFH and some CFHRs, namely ErpA, ErpC and ErpP. We have determined the crystal structure of ErpP at 2.53 Å resolution and the crystal structure of ErpC at 2.15 Å resolution. Recently, the crystal structure of the Erp familymember OspE fromB. burgdorferi N40was determined in complexwith CFH domains 19–20, revealing the residues involved in the complex formation. Despite the high sequence conservation between ErpA, ErpC, ErpP and the homologous protein OspE (78–80%), the affinity for CFH and CFHRs differsmarkedly among the Erp family members, suggesting that ErpC may bind only CFHRs but not CFH. A comparison of the binding site in OspE with those of ErpC and ErpP revealed that the extended loop region,which is only observed in the potential binding site of ErpC, plays an important role by preventing the binding of CFH. These results can explain the inability of ErpC to bind CFH, whereas ErpP and ErpA still possess the ability to bind CFH.Thisworkwas supported by the EC 7th Framework Programme project REGPOT-CT-2013-316149-InnovaBalt and ERDF grant 2DP/2.1.1.10/ 14/APIA/VIAA/013. We thank Dr. Gunter Stier and Dr. Huseyin Besir from the EMBL for providing the expression vector pETm-11. We would like to acknowledge the staff at the MAX-lab synchrotron for their professional support during the data collection