Protein Secretion Mechanisms in Bacteroidetes Pathogenesis

Bacteroidetes species employ specialised secretion systems to deliver a wide array of effector proteins that underpin colonisation, nutrient acquisition and host interaction. Central to pathogenic members such as Porphyromonas gingivalis is the Type 9 Secretion System (T9SS), a dedicated translocon spanning the outer membrane that recognises a conserved C-terminal domain (CTD) on cargo proteins. Following Sec-mediated translocation across the inner membrane and processing in the periplasm, T9SS exports virulence factors—most notably gingipain proteases—into the extracellular milieu or anchors them to the cell surface. Energy for secretion derives from an inner-membrane motor complex shared with the gliding motility apparatus of environmental Bacteroidetes. In parallel, periplasmic enzymes such as glutaminyl cyclase introduce post-translational modifications that stabilise secreted proteins. Together, these mechanisms enable Bacteroidetes to manipulate host tissues, evade immune responses and establish persistent infections, with broad implications for periodontal disease and other systemic conditions.

Research from Nature Portfolio

High-resolution cryogenic electron microscopy has captured intermediate states of the T9SS translocon from Flavobacterium johnsoniae, revealing an extended structure incorporating periplasmic chaperones and accessory proteins that guide substrate binding and release. Analyses indicate that release of the substrate–carrier complex from the pore is the rate-limiting, energy-dependent step in protein translocation. Complementary studies on gliding motility have uncovered a multi-rail filament network beneath the outer membrane, along which adhesin filaments are propelled by the same motor that energises T9SS. These findings illuminate the intimate structural and functional coupling between secretion and surface translocation, suggesting that assembly of the gliding track may coordinate with virulence factor export in pathogenic relatives.

Protein Secretion Mechanisms in Bacteroidetes Pathogenesis publication trend

The graph below shows the total number of articles in protein secretion mechanisms in bacteroidetes pathogenesis across all publications each year (not limited to Nature Index journals).

Technical terms

Type 9 Secretion System (T9SS): A specialised protein export apparatus in Bacteroidetes that transports substrates across the outer membrane after Sec-dependent inner-membrane translocation.

Sec system: The general secretory pathway that translocates unfolded proteins across the inner membrane into the periplasm.

C-terminal domain (CTD): A conserved signal sequence at the C-terminus of T9SS cargo proteins that directs them to the translocon.

Glutaminyl cyclase: A periplasmic enzyme that converts N-terminal glutamine residues into pyroglutamate, enhancing extracytoplasmic protein stability.

Response regulator PorX: A two-component signalling protein that controls T9SS activity via phosphorylation and zinc-mediated conformational change.

Periplasm: The compartment between inner and outer membranes in Gram-negative bacteria where proteins undergo folding and modification.

References

1. Structural insights into the mechanism of protein transport by the Type 9 Secretion System translocon. Nature Microbiology (2024).
2. Filamentous structures in the cell envelope are associated with bacteroidetes gliding machinery. Communications Biology (2023).
3. The Bacteroidetes Q-rule and glutaminyl cyclase activity increase the stability of extracytoplasmic proteins. mBio (2023).
4. Unveiling the molecular mechanisms of the type IX secretion system's response regulator: Structural and functional insights. PNAS Nexus (2024).