Preliminary X-ray crystallographic analysis of the complex between the DNAase domain of colicin E9 and its cognate immunity protein

We have crystallized and performed preliminary X-ray characterization of the complex between the DNAase domain of the E9 colicin and its cognate immunity protein Im9. The dissociation constant for this complex, K_d = 1 × 10^-16 M, reveals it to be one of the highest affinity protein-protein interactions known. Single crystals of the 1:1 complex were grown from microseeding experiments using PEG 4K as precipitant. The space group is P2₁2₁2₁ with one molecule of complex in the asymmetric unit, and crystals contain approximately 43% solvent. These crystals are inherently non-isomorphous and so selenomethionine-derivatized protein has been prepared and crystals grown for MAD phasing experiments.