Abstract
The type IX secretion system (T9SS) is specific to the Bacteroidetes phylum. Porphyromonas gingivalis, a keystone pathogen for periodontitis, utilises the T9SS to transport many proteins—including its gingipain virulence factors—across the outer membrane and attach them to the cell surface. Additionally, the T9SS is also required for gliding motility in motile organisms, such as Flavobacterium johnsoniae. At least nineteen proteins have been identified as components of the T9SS, including the three transcription regulators, PorX, PorY and SigP. Although the components are known, the overall organisation and the molecular mechanism of how the T9SS operates is largely unknown. This review focusses on the recent advances made in the structure, function, and organisation of the T9SS machinery to provide further insight into this highly novel secretion system.
Highlights
The Bacteroidetes are an important group of bacteria found in the digestive tract of animals and humans and are widespread in the environment
PorL and PorM are embedded in the inner membrane [7]; PorF, PorG, PorP, Sov, PorV, PorQ and PorT form outer membrane beta-barrel structures [20,23,26,33]; PorK, PorE and PorW are lipoproteins associated with the outer membrane and localised in the periplasm [7,22,23]; PorU and PorZ are cell surface proteins anchored by PorV and PorQ [31], respectively; PorN is a periplasmic protein anchored to the outer membrane via PorK [23]
Since the in-situ structure of the PorK/N ring was found to be tightly bound to the outer membrane, we have proposed that a function of the PorK/N rings is to form a barrier to localise all the outer membrane T9SS components in an outer membrane island to harmonise secretion and cell surface attachment of the cargo proteins [34]
Summary
The Bacteroidetes are an important group of bacteria found in the digestive tract of animals and humans and are widespread in the environment. The T9SS is important in the periodontal pathogen, Porphyromonas gingivalis, for the secretion of at least 30 proteins, including its major virulence factors called gingipains (Kgp, RgpA, RgpB) [7,8,9]. PorL and PorM are embedded in the inner membrane [7]; PorF, PorG, PorP, Sov, PorV, PorQ and PorT form outer membrane beta-barrel structures [20,23,26,33]; PorK, PorE and PorW are lipoproteins associated with the outer membrane and localised in the periplasm [7,22,23]; PorU and PorZ are cell surface proteins anchored by PorV and PorQ [31], respectively; PorN is a periplasmic protein anchored to the outer membrane via PorK [23]. Motor stator Rotor and shaft/pinion Forms ring structure with PorK Binds to PorE and PG1035
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