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TitleStructure of the T9SS PorKN ring complex reveals conformational plasticity based on the repurposed FGE fold.
Journal, issue, pagesmBio, Vol. 16, Issue 9, Page e0179925, Year 2025
Publish dateSep 10, 2025
AuthorsXiangan Liu / John D Perpich / Liqiang Song / Christian Cambillau / Thierry Doan / Lei Zheng / Richard J Lamont / Eric Cascales / Bo Hu /
PubMed AbstractThe type IX secretion system (T9SS) is a protein secretion machinery unique to the Bacteroidetes-Chlorobi-Fibrobacteres superphylum, which plays crucial roles in bacterial pathogenesis and gliding ...The type IX secretion system (T9SS) is a protein secretion machinery unique to the Bacteroidetes-Chlorobi-Fibrobacteres superphylum, which plays crucial roles in bacterial pathogenesis and gliding motility. It is composed of >15 proteins, including the proton-motive force-dependent PorLM motor, the PorKN ring anchored to the outer membrane, and the Sov translocon. Here, we present the cryo-electron microscopy (EM) structure of the PorKN ring complex from at 3.2 Å resolution. Our structural analysis reveals that PorK contains a repurposed formylglycine-generating enzyme-like fold, which serves as a structural hub for complex assembly rather than enzymatic activity. The complex exhibits a 33-fold symmetry with PorK and PorN assembling two tightly packed and wedged subrings. The structure reveals previously uncharacterized N- and C-terminal helices in PorN that are crucial for PorK binding and complex stability. By combining our high-resolution structure with cryo-electron tomography data, we propose a mechanism whereby PorKN undergoes conformational changes during substrate transport, transitioning between 50° and 90° states relative to the membrane plane. Finally, structural predictions coupled to site-directed disulfide cross-linking identified contacts between PorM and the PorKN ring. Collectively, these findings provide crucial insights into the molecular architecture and dynamic behavior of the T9SS machinery, advancing our understanding of bacterial protein secretion mechanisms.IMPORTANCEThe bacterial type IX secretion system (T9SS) is essential for processes such as gliding motility and secretion of virulence factors. In , a major periodontal pathogen, the T9SS transports over 30 virulence-associated proteins, making it central to disease development. The T9SS core is composed of PorLM motors that are thought to energize the PorKN outer membrane-associated ring. However, the molecular architecture of the PorKN ring has remained unresolved. Here, we present its atomic-resolution cryo-EM structure, revealing a formylglycine-generating enzyme-like fold in PorK that mediates PorK-PorN interactions through specific insertion motifs. Our results show that the ring exhibits intrinsic structural plasticity, including dynamic flexibility and variable stoichiometry. AlphaFold models and disulfide cross-linking experiments further provide information on how PorLM motors are connected to the PorKN ring. These insights redefine our understanding of the T9SS mechanism of action and offer a structural framework for the development of targeted antimicrobial strategies.
External linksmBio / PubMed:40772760 / PubMed Central
MethodsEM (single particle)
Resolution3.2 - 8.6 Å
Structure data

EMDB-70849: C33 reconstruction for the PorKN (single) ring complex of Type IX Secretion System (T9SS)
Method: EM (single particle) / Resolution: 7.4 Å

EMDB-70850: C34 reconstruction for the PorKN (single) ring complex of Type IX Secretion System (T9SS)
Method: EM (single particle) / Resolution: 8.6 Å

EMDB-70851: C32 reconstruction for the PorKN (single) ring complex of Type IX Secretion System (T9SS)
Method: EM (single particle) / Resolution: 8.4 Å

EMDB-70853: C33 reconstruction for the PorKN (double) ring complex of Type IX Secretion System (T9SS)
Method: EM (single particle) / Resolution: 5.6 Å

EMDB-70854: C34 reconstruction for the PorKN (double) ring complex of Type IX Secretion System (T9SS)
Method: EM (single particle) / Resolution: 6.0 Å

EMDB-70856: C32 reconstruction for the PorKN (double) ring complex of Type IX Secretion System (T9SS)
Method: EM (single particle) / Resolution: 6.1 Å

70857

9ots

EMDB-70857, PDB-9ots:
Cryo-EM structure of the T9SS PORkN ring complex of P. Gingivalis
Method: EM (single particle) / Resolution: 3.2 Å

Source
  • porphyromonas gingivalis atcc 33277 (bacteria)
KeywordsMEMBRANE PROTEIN / T9SS / Type IX SECRETION SYSTEM / PORKN RING COMPLEX / PORPHYROMONAS GINGIVALIS / PROTEIN TRANSPORT

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