Investigation of human IL18 structures towards understanding the dynamics and self-assembly of IL18-IL18BP heterodimers
Interleukin-18 (IL18) is an inflammatory cytokine involved in the regulation of both innate and adaptive immunity. IL18 activity is tightly regulated by its constitutively secreted inhibitor, IL18 binding protein (IL18BP). All known IL18BPs from human to poxviruses share an identical protein fold. Investigation of known crystal structures of human IL18 illustrated that the region between the 68th and 81st amino acids is either missing or disordered in all IL18-IL18BP complexes while adopting a 310 helical structure in the free or signaling complexes of IL18. The C74 residue in this region was reported to form a novel intermolecular disulfide bond in the human IL18-IL18BP tetrameric assembly. Yet, the impact of this small surface epitope on the stability and self-assembly of IL18 is unclear. First, we investigated the dynamics of this small surface region of IL18-IL18BP heterodimers by molecular dynamics simulations. In simulations, we used all known crystal structures of IL18-IL18BP and two additional computer models generated by AlphaFold2 and homology modeling to observe the differential folding of small surface regions. Next, we generated the proposed human and Yaba-like disease virus tetramers and analyzed them with or without intermolecular disulfide bonds. We found that the helical structure of small surfaces stabilizes the backbone of the complex through reducing its flexibility. While our computational model folded into a more stable and less flexible short IL18 epitope, similar to free IL18 or bound IL18 on the signal complex, IL18 from the complexes with human or virus IL18BPs displayed flexible regions with a less stable backbone. The core of the human crystal loosened, becoming more exposed to solvent in the human crystal structure. Particularly, a salt bridge stabilized the human IL18 structure at the helix configuration, linking the helix to the core region. We observed that the bivalent binding mode for the human tetramer, which is disturbing one side of the tetramer by breaking the disulfide bonds, did not affect the stability of the complex, and thereby the tetramer remained intact. Hence, the tetramer formation of IL18-IL18BP can be beneficial to the host as it provides an additional stability advantage. Overall, our results show that the short IL18 epitope between the amino acids 68 and 81 mediates stability and self-assembly of IL18-IL18BP heterodimers.