Bacteriocins are proteins secreted by many bacterial cells to kill related bacteria of the same niche. To avoid their own suicide through reuptake of secreted bacteriocins, these bacteria protect themselves by co-expression of immunity proteins in the compartment of colicin destination. In Escherichia coil the colicin M (Cma) is inactivated by the interaction with the Cma immunity protein (Cmi). We have crystallized and solved the structure of Cmi at a resolution of 1.95 angstrom by the recently developed ab initio phasing program ARCIMBOLDO. The monomeric structure of the mature 10 kDa protein comprises a long N-terminal alpha-helix and a four-stranded C-terminal beta-sheet. Dimerization of this fold is mediated by an extended interface of hydrogen bond interactions between the alpha-helix and the four-stranded beta-sheet of the symmetry related molecule. Two intermolecular disulfide bridges covalently connect this dimer to further lock this complex. The Cmi protein resembles an example of a 3D domain swapping being stalled through physical linkage. The dimer is a highly charged complex with a significant surplus of negative charges presumably responsible for interactions with Cma. Dimerization of Cmi was also demonstrated to occur in vivo. Although the Cmi-Cma complex is unique among bacteria, the general fold of Cmi is representative for a class of YebF-like proteins which are known to be secreted into the external medium by some Gram-negative bacteria. (C) 2012 Elsevier Inc. All rights reserved.