Crystal structure of colicin E3: Implications for cell entry and ribosome inactivation

Colicins kill E. coil by a process that involves binding to a surface recep tor, entering the cell, and, finally, intoxicating it. The lethal action of colicin E3 is a specific cleavage in the ribosomal decoding A site. The cr ystal structure of colicin E3, reported here in a binary complex with its i mmunity protein (IP), 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 (T) and the catalytic domain (C) comprising the two a rms. Active site residues are D510, H513, E517, and R545. IP is buried betw een T and C. Rather than blocking the active site, IP prevents access of th e active site to the ribosome.