The discovery of intracellular transport motors in bacteria

Molecular-motor driven intracellular transport has not previously been documented in bacteria. Here, studying a motility mechanism in Myxococcus xanthus, we identify a membrane-localized PMF-driven molecular motor that transports the motility complexes directionally down the cell axis, creating traction forces at the cell surface and thus locomotion.

HFSP Young Investigator Grant holders Tâm Mignot and Joshua Shaevitz and colleagues
authored on Mon, 16 May 2011

In recent years, it has become clear that the bacterial cell is organized by a complex eukaryotic-like cytoskeleton involving actin-, tubulin- and intermediate filament-like polymers involved in critical cellular processes such as cell wall synthesis, cell division, chromosome segregation, cell polarity control and motility. However, despite extensive searches, cytoskeletal motors akin to myosins or kinesins have not been identified in bacteria.

Some bacteria move across solid surfaces in the absence of gross morphological changes. In Myxococcus xanthus, “gliding” motility was proposed to rely on eukaryotic-like focal adhesion complexes (FACs) coupled to an actin-like cytoskeleton via an as-yet undiscovered molecular motor.  Through a combination of biological and biophysical approaches, we found that these motors consist of proton-channels homologous to the MotAB channel from the E. coli flagellar motor.

 

Figure– Left: Bead transport at the surface of immobilized cells. The fluorescent signal corresponds to AglZ-YFP, a cytosolic FAC-associated protein that co-tracks with the beads. Right: cartoon of the motility machinery as presently characterized. The actin-like cytoskeleton (blue) cooperates with the AglRQS motor (yellow) through AglZ (green) and an as yet uncharacterized envelope-spanning complex (white).

We first tested whether traction forces are indeed exerted at the location of FACs. In cells immobilized on an artificial substrate, FACs trafficked down the cell axis at velocities matching those of gliding cells. Polystyrene beads on the cell exterior near FACs where similarly transported along the cell surface, showing unambiguously that traction forces are exerted at FACs. To search for the FAC-energizing molecular motor, we used microfluidics and metabolic inhibitors to show that the proton-motive force drives motility. A search of the Myxococcus genome for predicted motor channels successfully identified a proton channel system (AglRQS) that is essential for gliding motility. As expected, motor mutants were unable to exert traction forces at the cell surface and transport FAC-associated proteins. Cytological studies using mCherry and GFP fluorescent fusions showed that the motor complex localizes within FACs, providing direct evidence that AglRQS is the motility motor.

This work identifies the first bacterial transport motor and proves that gliding motility is powered by distributed machineries that harvest the PMF to produce traction forces at the cell surface. Many questions remain unresolved: how is the motor linked to the cytoskeleton, how is directionality achieved and what protein complex transduces its activity to the cell surface? From a general perspective, this discovery highlights the existence of a class of processive bacterial motors that can transport intracellular proteins. In the future, it will be interesting to determine if AglRQS-like motor complexes have been adapted to other cell biology purposes, for example protein targeting to given subcellular sites.

  

Reference:

Motor-driven intracellular transport powers bacterial gliding motility. Sun M., Wartel M., Cascales E., Shaevitz J.W. & Mignot T.Proc. Natl. Acad. Sci. U.S.A (2011) 108: 7559-7564.

 

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