A journey with type IX secretion system effectors: selection, transport, processing and activities

Abstract

The type IX secretion system (T9SS) is a multiprotein machine distributed in Bacteroidota and responsible for the secretion of various proteins across the outer membrane. Secreted effectors can be either delivered into the medium or anchored to the cell surface. The T9SS is composed of a transenvelope complex consisting of proton-motive force-dependent motors connected to a membrane-associated ring and outer membrane translocons, and a cell-surface anchoring complex that processes effectors once translocated. The T9SS is involved in pathogenesis, metal acquisition, carbohydrate degradation, S-layer biogenesis and gliding motility. The broad spectrum of functions is linked to a highly versatile repertoire of effectors including metallophores, enzymes, toxins and adhesins, that all possess specific signatures to be recruited and transported by the apparatus. This review summarizes the current knowledge on T9SS substrate secretion signals, transport, processing and activities.

Keywords: effectors; gliding motility; protein transport; secretion signal; toxins; type IX secretion.

Fig. 1.

Schematic representation of the type IX secretion system (T9SS) and of associated proteins and modules. The major subunits of the T9SS are schematized (shade of blue), highlighting the LM rotary motor, KN ring, the SprE/PorW lipoprotein and the Sov/SprA translocon associated with Plug (grey). For the rotary motor, the glutamate residue that is thought to harvest the proton (H⁺) gradient during motor function is shown in red. The PorV β-barrel and the PorQUZ attachment complex are shown in purple. Additional modules associated with type IX secretion or gliding motility are depicted in green. IM, inner membrane; PG, peptidoglycan; OM, outer membrane.

Fig. 2.

Schematic representation of the T9SS substrate transport pathway. The substrate, which possesses a signal peptide (SP), is exported to the periplasm via the Sec machinery (a). Once in the periplasm, the substrate folds, revealing a C-terminal domain (CTD, pink) that is recognized by the T9SS (b). The substrate is conveyed to the Sov/SprA translocon in the open conformation (c) where it is transferred to the PorV shuttle (d) and transported to the processing/attachment PorQUZ complex. The CTD is then cleaved by the PorU sortase and the protein is either released in the medium or anchored to the cell surface (e).

Fig. 3.

T9SS secretion signals. Sequence alignments of randomly selected type A (a), type B (c) and type C (d) CTDs from various Bacteroidotoa species using muscle [126] and edited using Jalview [127]. Residues with similar functional side-chains are shown in colour (blue, hydrophobic residue; orange, glycine; purple, negatively charged residue; red, positively charged residue; green, serine and threonine; yellow, proline). The different conserved motifs are framed. For type A CTDs, the residues upstream and downstream from the cleavage site, when experimentally validated, are framed in purple. (b) Surface representation (left) and ribbon representation (right) of the structure of the type A CTD from PorZ (PDB: 5M11 [50]). The A, B, D and E motifs are shown in colour [red, blue, green and orange, respectively; same colour as the frames in (a)] and with sticked side-chains in the ribbon representation.

Fig. 4.

T9SS effector processing by the PorQUZ attachment complex. The T9SS effector is conveyed by the PorV shuttle to the PorQUZ complex. The PorU proteolytic domain cleaves the CTD, forming a thioester-bound complex between PorU and the processed substrate. The CTD is then either released in the medium or attached to the A-LPS after the nucleophilic attack of the free amino groups of primary amines, amino acids, short peptides or A-LPS. The A-LPS is synthesized by the Wba/Vim pathway and transported to PorU by the LPS transport machinery (pink) and PorZ.