During its evolution, the gram-negative bacteria Myxococcus xanthus has developed an original property : the ability to move on surfaces in the absence of extracellular appendages such as flagella or pili. This process, called gliding, is mediated by bacterial Focal adhesion sides (FAs). As in eukaryotic cells, FAs sites temporary create a connection between the surface and the bacterial membrane in order to induce the cell movement. The FA sites contain a molecular machinery containing more than fourteen different proteins, distributed between the various cell compartments : external membrane, periplasm, internal membrane and cytosol.

This work was conducted by a tight collaboration between the group of Marcelo Nöllmann at the CBS and the team of Tâm Mignot at the LCB (CNRS, Marseille). JB Fiche, Laura Faure and León Espinosa are the lead authors.

For this study, several microscopy techniques (2-color TIRF, 3D imaging based on astigmatism, RICM, ...) were used to characterize the architecture and formation of FA sites during cell gliding. We show that the motility complex is divided into three distinct functional groups:

*a molecular motor producing proton motive force for the movement
*an inner membrane/cytosolic group acting as a platform for FAs assembly
*a periplasm/outer membrane group allowing the connection with the surface and the force transmission

We discovered that the motor complex moves intracellularly along a right-handed helical path and, when it becomes stationary at FA sites, it powers a left-handed rotation of the cell around its long axis.

The reason underlying this helical movement of the cell are not fully understood yet. However, we hypothesize that the peptidoglycan (PG) is most probably the best candidate because the glycan strands have a global right-handed helical ordering that could serve as a track to guide the motility complex. Also, the rigidity of the PG layer could act both as a transient anchor for the motor complex and opposing contractions to propel the cell.

Reference:

The mechanism of force transmission at bacterial focal adhesion complexes
Laura M. Faure*, Jean-Bernard Fiche*, Leon Espinosa*, Adrien Ducret, Vivek Anantharaman, Jennifer Luciano, Sébastien Lhospice, Salim T. Islam, Julie Tréguier, Mélanie Sotes, Erkin Kuru, Michael S. Van Nieuwenhze, Yves V. Brun, Olivier Théodoly, Aravind L, Marcelo Nöllmann# & Tâm Mignot#.
[* Co-authors]
[# Co-corresponding authors]
Nature. 2016 Oct 5. doi: 10.1038/nature20121.