Initial crystallographic analysis of a recombinant human interleukin-1 receptor antagonist protein

We report the crystallization of samples of a recombinant preparation of human interleukin-1 receptor antagonist protein (IRAP) and solution of the crystal structure by isomorphous replacement methods. Crystals were obtained by the hanging-drop vapor-diffusion method at 277 K from solutions of PEG 4000 containing sodium chloride, dithiothreitol and PIPES [sodium piperazione-N,N'-bis(2-ethanesulfonate)] buffer at pH 7.0. Crystals appear within about a week and grow as truncated tetragonal bipyramids to 0.3-0.6 mm on an edge. X-ray diffraction data from these crystals specify space group P4₃2₁2 and unit-cell dimensions of a = b = 72.35(26), c = 114.7(8) Å and Z = 16 (two molecules per asymmetric unit). Fresh crystals diffract to about 2.3 Å resolution. The search for heavy-atom derivatives has produced two, potassium gold cyanide and trimethyl lead chloride, as same-site, single-site derivatives. Inspection of an electron-density map at 4 Å resolution calculated with these derivatives confirms that the IRAP molecule is a member of the interleukin-1 structural family.