1.C.1.4.1
Colicin E2 or E9 (Mosbahi et al., 2002). Colicin E2 is still in contact with its receptor and import machinery when its nuclease domain enters the cytoplasm (Duche, 2007). Colicin E3 is almost identical to Colicin E3 (RNAase).  The crystal structure  of Colicin E3 with bound BtuB and with the N-terminal translocation (T) domain of E3 and E9 (DNAase) inserted into the OM OmpF porin has been solved (Cramer et al. 2018) revealing: (I) Details of the initial interaction of the colicin central receptor (R)- and N-terminal T-domain with OM receptors/translocators. (II) Features of the translocon include: (a) high-affinity (K _d ≈ 10^-9 M) binding of the E3 receptor-binding R-domain E3 to BtuB; (b) insertion of disordered colicin N-terminal domain into the OmpF trimer; (c) binding of the N-terminus, documented for colicin E9, to the TolB protein on the periplasmic side of OmpF. Reinsertion of the colicin N-terminus into the second of the three pores in OmpF implies a colicin anchor site on the periplasmic side of OmpF. (III) Studies on the insertion of nuclease colicins into the cytoplasmic compartment imply that translocation proceeds via the C-terminal catalytic domain, proposed here to insert through the unoccupied third pore of the OmpF trimer, consistent with in vitro occlusion of OmpF channels by the isolated E3 C-terminal domain (Cramer et al. 2018).