8.B.24 The Colicin Immunity Protein (ColIP) Functional Family
Proteins conferring immunity against pore-forming colicins are localized in the E. coli inner membrane. Their protective effects are mediated by direct interaction with the C-terminal domain of their cognate colicins. Cai, the immunity protein protecting E. coli against colicin A, contains four cysteine residues. Cai forms homodimers with four TMSs per subunit, and dimerization occurs via the third TMS. Co-expression of Cai with its target, the colicin A pore-forming domain (pfColA), in the inner membrane prevents the formation of intermolecular and intramolecular disulfide bonds, indicating that pfColA interacts with the dimer of Cai and modifies its conformation. When Cai is locked by disulfide bonds, it is no longer able to protect cells against colicin A (Zhang et al. 2010).
This family includes several subfamilies with very different protein sizes, topologies (from 0 to 4 TMSs) and sequences, and their target proteins inhibit colicins that act in very different ways. Homology between members of these different subfamilies has not been established, so the ColIP family should be consider to be a functional superfamily.