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, Kd = 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.