Colicins are plasmid-encoded protein antibiotics, or bacteriocins, produced by strains of Escherichia coli that kill closely related bacteria. Colicins are classified according to the cell-surface receptor they bind to, colicin E3 binding to the BtuB receptor involved in vitamin B12 uptake. The lethal action of colicin E3 arises from its ability to inactivate the ribosome by site-specific RNase cleavage of the 16S ribosomal RNA, which is carried out by the catalytic, or ribonuclease domain. Colicin E3 is comprised of three domains, each domain being involved in a different stage of infection: receptor binding, translocation and cytotoxicity. Colicin E3 is a Y-shaped molecule with the receptor-binding middle domain forming the stalk, the N-terminal translocation domain forming the two globular heads , and the C-terminal catalytic domain forming the two globular arms. To neutralise the toxic effects of colicin E3, the host cell produces an immunity protein, which binds to the C-terminal end of the ribonuclease domain and effectively suppresses its activity.
This entry represents the ribonuclease domain (also called catalytic or cytotoxic domain) found in various colicins. This domain confers cytotoxic activity to proteins, enabling the formation of nucleolytic breaks in 16S ribosomal RNA. The structure of the domain reveals a highly twisted central beta-sheet elaborated with a short N-terminal alpha-helix [10986462, 11741540].