Rory Hennell James explores the motor behind bacterial gliding motility

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Many bacteria of the Bacteroidetes phylum use a protein machine called the  Type IX Secretion System (T9SS) to allow them to secrete proteins and to move. Overlapping sets of proteins are used to secrete proteins across the outer membrane and then drive them along the cell surface. The energy for these processes comes from the proton-motive force across the inner membrane and is transduced to the outer membrane by the GldLM motor complex.

To understand how this complex functions we solved the structure of a truncated GldLM complex from Flavobacterium johnsoniae. Five copies of GldL surround two copies of GldM in the detergent micelle. No channel for proton flow was visible. However, we did find several conserved protonatable residues in the interface between the GldL and GldM transmembrane helices, some of which were essential for function in vivo. Interactions between these residues is charge-dependent, so proton flow could make and break these interactions to drive conformational changes. The periplasmic domain of GldM could then transmit these movements to the outer membrane to power secretion and motility.

Better understanding of T9SS may allow us to target it for therapies in T9SS+ pathogens and to better exploit it in biotechnological applications.

Hennell James, R., Deme, J., Kjær, A., Alcock, F., Silale, A., Lauber, F., Johnson, S., Berks, B. and Lea, S. (2021) Structure and mechanism of the proton-driven motor that powers Type 9 secretion and gliding motility Nature Microbiology in press