Structural and functional analyses of the Porphyromonas gingivalis type IX secretion system PorN protein

Structural insights into the mechanism of protein transport by the Type 9 Secretion System 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 trap the T9SS of Flavobacterium johnsoniae in the process of substrate transport by disrupting the T9SS motor complex. Cryo-EM analysis of purified substrate-bound T9SS translocons reveals an extended translocon structure in which the previously described translocon core is augmented by a 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.

Structural Model of a Porphyromonas gingivalis type IX Secretion System Shuttle Complex

Porphyromonas gingivalis is a gram-negative oral anaerobic pathogen and is one of the key causative agents of periodontitis. P. gingivalis utilises a range of virulence factors, including the cysteine protease RgpB, to drive pathogenesis and these are exported and attached to the cell surface via the type IX secretion system (T9SS). All cargo proteins possess a conserved C-terminal signal domain (CTD) which is recognised by the T9SS, and the outer membrane β-barrel protein PorV (PG0027/LptO) can interact with cargo proteins as they are exported to the bacterial surface. Using a combination of solution nuclear magnetic resonance (NMR) spectroscopy, biochemical analyses, machine-learning-based modelling and molecular dynamics (MD) simulations, we present a structural model of a PorV:RgpB-CTD complex from P. gingivalis. This is the first structural insight into CTD recognition by the T9SS and shows how the conserved motifs in the CTD are the primary sites that mediate binding. In PorV, interactions with extracellular surface loops are important for binding the CTD, and together these appear to cradle and lock RgpB-CTD in place. This work provides insight into cargo recognition by PorV but may also have important implications for understanding other aspects of type-IX dependent secretion.

The type IX secretion system: Insights into its function and connection to glycosylation in Cytophaga hutchinsonii

The recently discovered type IX secretion system (T9SS) is limited to the Bacteroidetes phylum. Cytophaga hutchinsonii, a member of the Bacteroidetes phylum widely spread in soil, has complete orthologs of T9SS components and many T9SS substrates. C. hutchinsonii can efficiently degrade crystalline cellulose using a novel strategy, in which bacterial cells must be in direct contact with cellulose. It can rapidly glide over surfaces via unclear mechanisms. Studies have shown that T9SS plays an important role in cellulose degradation, gliding motility, and ion assimilation in C. hutchinsonii. As reported recently, T9SS substrates are N- or O-glycosylated at their C-terminal domains (CTDs), with N-glycosylation being related to the translocation and outer membrane anchoring of these proteins. These findings have deepened our understanding of T9SS in C. hutchinsonii. In this review, we focused on the research progress on diverse substrates and functions of T9SS in C. hutchinsonii and the glycosylation of its substrates. A model of T9SS functions and the glycosylation of its substrates was proposed.