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 Å in the orthorhombic space group C2221. The crystal packing, with unit‐cell parameters a = 66.02, b = 83.47, c = 38.30 Å, indicated the presence of one monomer in the asymmetric unit with a solvent content of 53%.