Insights into type IX secretion from PorKN cogwheel structure bound to PorG and attachment complexes

Area of Science:

• Microbiology
• Structural Biology
• Bacterial Physiology

Background:

• The Type IX Secretion System (T9SS) is crucial for protein export across the outer membrane in Bacteroidota.
• The precise molecular mechanisms governing T9SS function remain largely unelucidated.
• Key components include the Sov translocon, Attachment Complexes (PorQ, U, V, Z), PorLM motors, and PorKN rings.

Purpose of the Study:

• To elucidate the structural basis and mechanistic details of the Type IX Secretion System in Porphyromonas gingivalis.
• To determine the structure of the periplasmic PorKN rings and their interaction with other T9SS components.

Main Methods:

• Cryo-electron microscopy (cryo-EM) at ~3.5 Å resolution.
• Biochemical analysis to identify critical disulfide bonds.
• Localization studies of Attachment Complexes.

Main Results:

• A high-resolution cryo-EM structure of the PorKN periplasmic rings (32-33 subunits each) was determined.
• A critical disulfide bond between PorK and the outer membrane protein PorG was identified, essential for secretion.
• Attachment Complexes were shown to bind above the PorKN rings.

Conclusions:

• The PorKN rings function as a cogwheel, with PorN projections engaging the PorLM motor.
• This interaction drives the rotation of the PorKN-PorG complex, powering protein secretion.
• The system facilitates both protein export and coordinated cell surface attachment of secreted cargo.