Fig. 3.

Type IV substrate translocation signals and accessory factors. Most substrates are translocated through the T4CP-dependent pathway. DNA substrates are processed by Dtr processing factors through assembly of the relaxosome at the origin of transfer sequence (oriT). Lower left: Relaxases responsible for nicking at oriT can carry positively-charged C-terminal or internal translocation signals (TS’s) for docking with the T4CP receptor. A conserved sequence found in the TraI_R1 TSA motif and present in TSA and TSB motifs of MobA_R1162, TraI_R1, and TrwC_R388 is shown. Upper left: A model of the TSA motif from TraI_R1 generated from an X-ray structure [64]. The motif bears structural homologies to SF1B helicase domains. Mutations of residues shown in stick representation and color-coded disrupted TSA translocation function: red, significant impairment; pink, moderate impairment; yellow, no effect; blue, stimulation of TSA translocation function. Molecular graphics and analyses were performed with the UCSF Chimera package [158] using the PDB ID (4L0J) of the TraI_R1 TSA domain structure [64]. Lower right: DNA substrates and effector proteins can be recruited directly to the T4CP (Chaperone-independent) or via association with chaperones or spatial adaptors. Protein substrates can carry one or more of a combination of C-terminal, N-terminal or internal translocation signals; chaperones and spatial adaptors are in parantheses and the relevant species or plasmid is in parantheses (A.t., A. tumefaciens VirB/VirD4; L.p., L. pneumophila Dot/Icm; B.h., Bartonella henselae VirB/VirD4; H.p., H. pylori Cag). Upper right: In the T4CP-independent pathway, B. pertussis S1–S5 subunits are secreted across the inner membrane by the Sec system and assemble in the periplasm as the pertussis toxin (PT), which then docks by an unknown mechanism with the Ptl T4SS for extrusion across the outer membrane. Most T4SS substrates dock with the T4CP and are delivered in one step through the IMC/OMC channel.