Structural Insights Into Recognition Of IL-33by Its Receptors

Interleukin-1(IL-1) is the one of earliest discoverd cytokines with a history ofnearly40years. So far, IL-1family has got11members. They are all playingimportant roles in human innate immune.Interleukin33（IL-33） is the latest member of the IL-1family that haspleiotropic activities in innate and adaptive immune responses in immunity and disease.The signaling of IL-33depends on its binding to the primary receptor ST2andsubsequent recruitment of accessory receptor IL-1RAcP, both of which are members ofthe IL-1receptor family. The ligand-binding induced receptor hetero-dimerizationresults in the juxtaposition of the intracellular TIR domains of both receptors, which isnecessary and sufficient to activate NF-κB and MAPK pathways in the target cells.Studies showed that IL-33could not only fuction as a cytokine but also as atranscription factor involved in the immune response. The maturation mechanism ofIL-33is qutie different from the other members, making IL-33a very special one inIL-1family. Some progress has already be made in both functional and structuralstudies of IL-33,while a lot questions are still need to answer.Structural study of IL-33and its receptor could provide both great insight into mechanism of IL-1familycytokines recognition by their receptors and a lot of help to treatment withIL-33-related diseases.Here we present the crystal structure of IL-33in complex with the ectodomain ofST2at a resolution of3.27. With mutagenesis and SPR binding assay, it defines themolecular mechanism by which ST2specifically recognizes IL-33. Small-angle X-rayscattering （SAXS） analysis showed that the architecture of IL-33/ST2recruitingIL-1RAcP to form the ternary complex using a “left” model similar with previouslydetermined IL-1β-IL-1RII-IL-1RAcP and IL-1β-IL-1RI-IL-1RAcP crystal structures.SAXS data also revealed that ST2is much more flexible than IL-1RAcP between theD3domain and “D1D2” module in unbound state in solution. And binary and ternarycomplex in solution both maintain in a single stable comformer, which provides newinsights into domain-level conformational changes of IL-1primary receptors uponligand binding and the inability of IL-1RAcP to bind ligands directly. Our work also showed that why the ligand binding is nessasery for activing signaling through tworeceptors.