E2 is a DNase which utilises the outer membrane receptor BtuB to bind to and enter the cell. This family of proteins is E2R135 (residues 321-443) which is the part of E2 which is responsible for binding to BtuB in a coiled coil formation [1].

1: Structure of the complex of the colicin E2 R-domain and its BtuB receptor. The outer membrane colicin translocon. Sharma O, Yamashita E, Zhalnina MV, Zakharov SD, Datsenko KA, Wanner BL, Cramer WA; J Biol Chem. 2007;282:23163-23170. PMID:17548346