Purification, crystallization and preliminary X-ray crystallographic analysis of human IL-18 and its extracellular complexes.

Interleukin-18 (IL-18), a pro-inflammatory cytokine belonging to the interleukin-1 (IL-1) family, is involved in the pathogenesis of autoimmune/autoinflammatory and allergic diseases such as juvenile idiopathic arthritis and bronchial asthma. IL-18 forms a signalling complex with the IL-18 receptor α (IL-18Rα) and β (IL-18Rβ) chains; however, the detailed activation mechanism remains unclear. Here, the IL-18-IL-18Rα binary and IL-18-IL-18Rα-IL-18Rβ ternary complexes were purified and crystallized as well as IL-18 alone. An X-ray diffraction data set for IL-18 was collected to 2.33 Å resolution from a crystal belonging to space group P21, with unit-cell parameters a = 68.15, b = 79.51, c = 73.46 Å, β = 100.97°. Crystals of both the IL-18 binary and ternary complexes belonging to the orthorhombic space groups P21212 and P212121, respectively, diffracted to 3.10 Å resolution. Unit-cell parameters were determined as a = 135.49, b = 174.81, c = 183.40 Å for the binary complex and a = 72.56, b = 111.56, c = 134.57 Å for the ternary complex.