Crystallization of the cytotoxic domain of a ribosome-inactivating colicinin complex with its immunity protein

The complex between the ribonuclease domain of the ribosome-inactivating co licin E3 and its protein inhibitor, the cognate immunity Im3, has been crys tallized and preliminary X-ray characterization has been performed. Single crystals of the 1:1 complex were grown from hanging-drop vapour-diffusion e xperiments using 2-propanol as a precipitant. The space group is P3(1)21 or P3(2)21, with unit-cell parameters a = b = 93.7, c = 76.2 Angstrom. When c ryocooled, these crystals diffract to a resolution of 2.4 Angstrom. A searc h for suitable conventional heavy-atom derivatives was unsuccessful and so Im3 mutants containing engineered cysteine or methionine residues have been produced for mercury soaks and selenomethionine-labelling experiments, res pectively.