Expression, purification and crystallization of the Cmi immunity protein from Escherichia coli

Many bacteria kill related bacteria by secretion of bacteriocins. In Escherichia coli, the colicin M protein kills E. coli after uptake into the periplasm. Self-protection from destruction is provided by the co-expressed immunity protein. The colicin M immunity protein (Cmi) was cloned, overexpressed and purified to homogeneity. The correct fold of purified Cmi was analyzed by activity tests and circular-dichroism spectroscopy. Crystallization trials yielded crystals, one of which diffracted to a resolution of 1.9 A in the orthorhombic space group C222(1). The crystal packing, with unit-cell parameters a = 66.02, b = 83.47, c = 38.30 A, indicated the presence of one monomer in the asymmetric unit with a solvent content of 53%.