Flavobacterium T9SS Extended Translocon

Secretion Systems are protein export machines that enable bacteria to exploit their environment through the release of protein effectors. The Type 9 Secretion System (T9SS) is responsible for protein export across the outer membrane (OM) of bacteria of the phylum Bacteroidota. Here we use deletion of the T9SS motor protein to trap the T9SS Flavobacterium johnsoniae in the process of substrate transport. CryoEM analysis of purified substrate-bound T9SS translocons revealed an extended translocon compared to previous analyses. The translocon core is augmented by a five-subunit periplasmic structure incorporating the proteins SprE, PorD, and a homologue of the canonical periplasmic chaperone Skp. Substrate proteins bind to the extracellular loops of a carrier protein within the translocon pore. As transport intermediates accumulate on the translocon when energetic input is removed, we deduce that release of the substrate-carrier protein complex from the translocon is the energy-requiring step in T9SS transport.