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 th e complex between the DNAase domain of the E9 colicin and its cognate immun ity protein Im9. The dissociation constant for this complex, K-d = 1 x 10(- 16) M, reveals it to be one of the highest affinity protein-protein interac tions known, Single crystals of the 1:1 complex were grown from microseedin g experiments using PEG 4K as precipitant. The space group is P2(1)2(1)2(1) with one molecule of complex in the asymmetric unit, and crystals contain approximately 43% solvent. These crystals are inherently non-isomorphous an d so selenomethionine-derivatized protein has been prepared and crystals gr own for MAD phasing experiments.