2.A.130. The Type 9 Secretory System (T9SS) Family
The type 9 secretion system (T9SS) is a protein export pathway in bacteria of the Gram-negative Fibrobacteres-Chlorobi-Bacteroidetes superphylum and is an essential determinant of pathogenicity in severe periodontal disease. The central element of the T9SS is a protein-conducting translocon located in the bacterial outer membrane. Using cryo-electron microscopy, Lauber et al. 2018 provided structural evidence that the translocon is the T9SS protein SprA. SprA forms a large (36-strand) single polypeptide transmembrane β-barrel. The barrel pore is capped on the extracellular end, but has a lateral opening to the external membrane surface. Structures of SprA bound to different components of the T9SS showed that partner proteins control access to the lateral opening and to the periplasmic end of the pore. The SprA porin has a distinctive architecture that uses an alternating access mechanism in which the two ends of the protein-conducting channel are open at different times in response to the proton motive force (pmf).
T9SS substrates are large (100–650 kDa) multi-domain proteins that fold in the periplasm before being exported through the outer membrane translocon. Substrates are targeted to the translocon by means of a ~100 aa C-terminal folded signal domain (CTD). Distinct type A and type B CTDs differ in the T9SS components required for their export (Lauber et al. 2018).
The Bacteroidete T9SS contains at least 15 proteins. GldK, GldL, GldM and GldN form a trans-periplasmic motor complex that is thought to use the inner membrane pmf to drive secretion at the outer membrane. PorQ, PorU, PorV and PorZ form a cell surface-exposed attachment complex that proteolytically removes the substrate CTD following transport and covalently links some substrate proteins to a lipopolysaccharide anchor. A pool of PorV proteins in the outer membrane may shuttle substrate proteins from the translocon to the attachment complex (Lauber et al. 2018). The phylum Bacteroidetes is large and diverse, with rapid gliding motility and the ability to digest macromolecules associated with many genera and species. A novel protein secretion system, the Por secretion system (PorSS; Type IX secretions system (T9SS)), has been identified in two members of the phylum, the gliding bacterium Flavobacterium johnsoniae and the nonmotile oral pathogen Porphyromonas gingivalis (McBride and Zhu 2013).
The components of the T9SS complex are not similar in sequence to those of other well-studied bacterial secretion systems. The F. johnsoniae T9SS genes are a subset of the gliding motility genes, suggesting a role for the secretory system in motility. F. johnsoniae T9SS is needed for assembly of the gliding motility apparatus and for secretion of a chitinase, and the P. gingivalis T9SS is involved in secretion of gingipain protease virulence factors. Comparative analysis of 37 genomes of members of the phylum Bacteroidetes revealed the widespread occurrence of gliding motility genes and T9SS genes. Genes associated with other bacterial protein secretion systems were less common. It has been suggested that gliding motility is common among bacteria. Microscopic observations confirmed that organisms previously described as nonmotile, including Croceibacter atlanticus, 'Gramella forsetii,' Paludibacter propionicigenes, Riemerella anatipestifer, and Robiginitalea biformata, exhibit gliding motility. Three genes (gldA, gldF and gldG) that encode an apparent ATP-binding cassette transporter required for F. johnsoniae gliding were absent from two related gliding bacteria, suggesting that this transporter is not central to gliding motility (McBride and Zhu 2013).
The transport reaction believed to be catalyzed by the T9SS, energized by the pmf, is:
Folded protein (periplasm) → Folded protein (extrenal medium).