This domain is found twice in Colicin S4, a toxic protein produced by some strains of Escherichia coli that belongs to the group of pore- forming colicins. This protein interacts specifically with OmpW, OmpF, and the Tol system of E. coli strains that lack the corresponding immunity protein. Colicin S4 is organised into four independent compact domains: a N-terminal translocation domain, a C-terminal activity domain and two nearly identical receptor-binding domains. The central domains, which show the highest variability in sequence and structure, are responsible for binding an outer membrane receptor of the target cell. These domains arose from a recent domain duplication event and form the arms of the Y-shaped structure of the protein. Both consist of a three-stranded antiparallel beta sheet followed by three helices [1, 2].

1: Structure and function of colicin S4, a colicin with a duplicated receptor-binding domain. Arnold T, Zeth K, Linke D; J Biol Chem. 2009;284:6403-6413. PMID:19056731

2: Characterization of colicin S4 and its receptor, OmpW, a minor protein of the Escherichia coli outer membrane. Pilsl H, Smajs D, Braun V; J Bacteriol 1999;181:3578-3581. PMID:10348872