Investigation of gliding motility in Bacteroidetes

Disease-causing bacteria invade and colonise the host organism using protein molecules on their cell surface or secreted into their environment. Specific transport mechanisms are needed to transport these protein molecules from their site of synthesis inside the cell to the cell exterior. These protein transporters are, thus, important pathogenicity factors in bacterial diseases.
The Type 9 Secretion System (T9SS) is a recently-discovered bacterial protein export system that is essential for the pathogenesis of Porphyromonas gingivalis and related dental pathogens that cause severe periodontal disease. These bacteria have also been implicated in the etiology of rheumatoid arthritis and alzheimers disease.
The evolutionary origin of the T9SS lies in the machinery used by gliding bacteria to move rapidly over solid surfaces. In these bacteria the proto-T9SS exports proteins that bind to the solid surface and then moves these `adhesins' along the cell body on helical tracks located in the outer membrane.
It has recently been shown that the T9SS requires an energetic input to extract substrate proteins from the outer membrane transporter. Similarity between T9SS and gliding motility components suggests that this is accomplished using a cut-down version of the gliding track. The mechanism of this crucial step in T9SS transport will be elucidated using the movement of adhesins on tracks in gliding bacteria as our experimental model.
Our rationale for using the gliding motility model organism, Flavobacterium johnsoniae, is that the mechanical process of movement on tracks needs to be studied in intact cells. It has previously been shown that the movements of fluorescently-labelled adhesins along the cell body on gliding tracks can be followed by single molecule imaging methods in actively gliding cells. By contrast, the cut-down gliding tracks of the P. gingivalis T9SS are too small for movement on the tracks to be optically resolved for study.
The organisation and mechanism of gliding tracks will be probed using in-cell imaging complemented by biochemical, structural, and computational characterisation of track components. The insights obtained from this work will then be used to guide experiments to directly test T9SS function. These studies will substantially increase our understanding of the mechanism of the T9SS.