Recombinant expression of TLR5 proteins by ligand supplementation and a leucine-rich repeat hybrid technique.

Vertebrate TLR5 directly binds bacterial flagellin proteins and activates innate immune responses against pathogenic flagellated bacteria. Structural and biochemical studies on the TLR5/flagellin interaction have been challenging due to the technical difficulty in obtaining active recombinant proteins of TLR5 ectodomain (TLR5-ECD). We recently succeeded in production of the N-terminal leucine rich repeats (LRRs) of Danio rerio (dr) TLR5-ECD in a hybrid with another LRR protein, hagfish variable lymphocyte receptor (VLR), and determined the crystal structure of its complex with flagellin D1-D2-D3 domains. Although the structure provides valuable information about the interaction, it remains to be revealed how the C-terminal region of TLR5-ECD contributes to the interaction. Here, we present two methods to obtain recombinant TLR5 proteins that contain the C-terminal region in a baculovirus expression system. First, production of biologically active full-length drTLR5-ECD was substantially enhanced by supplementation of expression culture with purified flagellin proteins. Second, we designed TLR5-VLR hybrids using an LRR hybrid technology by single and double LRR fusions and were able to express diverse regions of drTLR5-ECD, allowing us to detect a previously unidentified TLR5/flagellin interaction. The drTLR5-VLR hybrid technique was also successfully applied to human TLR5-ECD whose expression has been highly problematic. These alternative TLR5 expression strategies provide an opportunity to obtain a complete view of the TLR5/flagellin interaction and can be applied to other LRR proteins.