Colicin-like bacteriocins are complex structures with an N-terminal beta-barrel translocation domain (Pfam:PF09000), a long double-alpha-helical receptor-binding domain (Pfam:PF11570) and this C-terminal RNAse/DNase domain with endonuclease activity. Their competitor bacteriocidal action is by a process that involves binding to a surface receptor, entering the cell, and, finally, killing it. The lethal action of colicin E3 is a specific cleavage in the ribosomal decoding A site. The crystal structure of colicin E3 reveals a Y-shaped molecule with the receptor binding domain forming a 100 Angstrom long stalk and the two globular heads of the translocation domain and this catalytic domain comprising the two arms [2].

1: The crystal structure of the DNase domain of colicin E7 in complex with its inhibitor Im7 protein. Ko TP, Liao CC, Ku WY, Chak KF, Yuan HS; Structure. 1999;7:91-102. PMID:10368275

2: Crystal structure of colicin E3: implications for cell entry and ribosome inactivation. Soelaiman S, Jakes K, Wu N, Li C, Shoham M; Mol Cell. 2001;8:1053-1062. PMID:11741540