Gliding Bacteria: Myxococcus xanthus

Did you know that E. coli is not the only bacteria model organism? Myxococcus xanthus has been the subject of recent research. People are interested in their biofilm self-organization and their motility. An interesting trait is that M. xanthus do not have flagella. They move by twitching or gliding. The mechanisms involved in twitching motility have been characterized, but the mechanics behind gliding have been a mystery. One model suggests that gliding is conducted by substrate-bound motors that run along a track inside the cell. Recently, Sun and his colleagues discovered indirect evidence that can support this model.

AglZ is a regulatory factor involved in gliding mobility. Sun’s team observed AglZ fused with yellow florescence protein. AglZ localizes to the leading cell pole and focal adhesion complexes distributed along the cell body. In a moving Myxococcus xanthus cell, the focal adhesion complexes would remain fixed relative to the cell surface, even if the cell moved several microns. However, when observing cells immobilized to a surface, they found focal adhesion complexes moving from one pole to the other. They then observed small beads attached to the surface of immobilized cells and found they moved from the leading pole to the lagging cell pole. The beads colocalize with AglZ in the focal adhesion complexes.(1) This all proves that the traction force is generated at the focal adhesion complexes.

Sun et al. hypothesized that motors were moving on cytoskeletal filaments in the cytoplasm and transmitting force through the cell wall to dynamic adhesion complexes, causing the cell to move forward. After observing the involvement of the AglZ protein, they found that gliding mobility requires a protein gradient, suggesting that the bacterial gliding and swimming may be linked to a common form of molecular motor, a proton channel.(1)

Sun et al. searched the Myxococcus genome for a homolog of bacterial motors that could be involved in a proton channel and identified the aglRQS locus. aglRQS encodes for the AglQ/AglR/AglS protein complex. AglR is homologous to bacterial motors MotA/TolQ/ExbB; AglQ and AglS are homologous to MotB/TolR/ExbD.(2) After further analysis, they concluded that the aglRQS locus indeed encodes a proton-conducting channel essential for gliding motility.

What is the importance of gliding bacteria? By understanding gliding motility in M. xanthus, researchers can investigate similar gliding motor proteins in other bacteria and eukaryotes. Sun et al. discovered AglQ/AglR/AglS as the first bacterial motor able to move in a directed manner between subcellular regions. If aglQ/Aglr/AglS can move around a bacteria cell in a controlled manner, then it is possible that other proteins can too. Søgaard-Andersen suggests that it is possible that motor proteins similar to aglQ/Aglr/AglS may be involved in organizing bacteria cells by moving proteins, DNA, or mRNA to their subecellular addresses.(2) With the discovery of this prokaryotic gliding mechanism, Myxococcus xanthus has great potential in further our understanding of the mechanism and organization of bacterial cells.

Works cited:
1. Sun, Mingzhai and et al. “Motor-driven intracellular transport powers bacterial gliding motility.” Proceedings of the National Academy of Sciences of the United States of America. 108.18 (2011): 7559-7564. UC Davis University Library, Davis, CA. 25 May 2011 < http://www.pnas.org/content/108/18/7559.full>
2. Lotte Søgaard-Andersen “Directional intracellular trafficking in bacteria.” Proceedings of the National Academy of Sciences of the United States of America. 108.18 (3 May 2011): 7283-7284. UC Davis University Library, Davis, CA. 27 May 2011 < http://www.pnas.org/content/108/18/7283.long>.