The Type IX Secretion System (T9SS): Highlights and Recent Insights into Its Structure and Function.

Area of Science:

• Microbiology
• Bacterial Physiology
• Molecular Biology

Background:

• Protein secretion systems are essential for bacterial functions, including nutrient acquisition, communication, defense, and pathogenesis.
• The type IX secretion system (T9SS) is a specialized translocon unique to the Bacteroidetes phylum.
• T9SS has dual roles: mediating gliding motility in environmental species and translocating virulence factors in pathogens.

Purpose of the Study:

• To provide a comprehensive review of the recently discovered type IX secretion system (T9SS).
• To summarize current knowledge on T9SS components, cargo recognition, and functional mechanisms.
• To highlight T9SS's significance in diverse bacterial lifestyles and its potential as a therapeutic target.

Main Methods:

• Literature review of studies on T9SS in representative species like Flavobacterium johnsoniae and Porphyromonas gingivalis.
• Analysis of conserved protein domains, particularly the C-terminal domain (CTD) directing cargo to the T9SS.
• Examination of the post-translocation modifications, including CTD cleavage and LPS attachment.

Main Results:

• T9SS involves at least 18 proteins, including those for cargo modification and translocation across the outer membrane.
• Cargo proteins are recognized via a conserved CTD, which is processed post-translocation.
• T9SS mediates extracellular secretion or surface attachment of cargo proteins, regulated by a two-component system.

Conclusions:

• T9SS is a crucial system for both environmental adaptation and pathogenicity in Bacteroidetes.
• Understanding T9SS structure and regulation is key to developing novel therapeutic strategies against bacterial pathogens.
• Ongoing research is rapidly expanding our knowledge of T9SS, with the translocon structure remaining a key area for future exploration.