Dynamic movement of actin‐like proteins within bacterial cells

Abstract

Actin proteins are present in pro-and eukaryotes, and have been shown to perform motor-like functions in eukaryotic cells in a variety of processes. Bacterial actin homologues are essential for cell viability and have been implicated in the formation of rod cell shape, as well as in segregation of plasmids and whole chromosomes. We have generated functional green fluorescent protein fusions of all three Bacillus subtilis actin-like proteins (MreB, Mbl and MreBH), and show that all three proteins form helical filaments underneath the cell membrane, the pattern of which is distinct for each protein. Time-lapse microscopy showed that the filaments are highly dynamic structures. A number of separate filaments of MreB and Mbl continuously move through the cell along helical tracks underneath the cell membrane. The speed of extension of the growing end of filaments is within the range of known actin polymerization (0.1 lm/s), generating a potential poleward or centreward pushing velocity at 0.24 lm/min for MreB or Mbl, respectively. During nutritional downshift and a block in topoisomerase IV activity, the filaments rapidly disintegrated, showing that movement occurs only in growing cells. Contrary to Mbl and MreBH filaments, MreB filaments were generally absent in cells lacking DNA, providing a further distinction between the three orthologues.

An intracellular actin motor in bacteria?

“…Indeed, our recent results show that GFPMreB and GFP-Mbl filaments rapidly move along helical tracks around the whole cell diameter within about a minute, corresponding to a speed of 0.1 mm/s. (24) Given such dynamics, it is clear that bacterial actins could do more than just providing a helical cytoskeletal element: MreB or Mbl could in fact constitute an intracellular motor that might push molecules through the cell. Movement of Mbl could direct translocation of cell wall precursors and/or membrane proteins.…”

“…Mbl filaments extend through the whole cell from pole to pole and have a longer lead (axial advance during one complete revolution of the helix) than MreB helices, while MreB filaments are usually absent from the cell poles. (16,24) Thus, the longer Mbl cables could dictate the cell's axis, while the more-central MreB filaments might control the width of the cell. How this control of rod cell shape works on molecular terms is still unknown.…”

“…Like the other two actin orthologues, MreBH forms filamentous helical structures assembling close to the cell surface. (24) On the one hand, therefore, bacterial actin appears to form an intracellular cytoskeleton in many bacteria, which dictates the shape of the cell. On the other hand, rather than through an actin-based contractile ring, cell division is mastered by the tubulin orthologue FtsZ (Table 1), which forms a ring structure at the cell middle, to which all other known cell division protein are recruited.…”

Determinants of Bacterial Morphology: From Fundamentals to Possibilities for Antimicrobial Targeting

“…Originally, this movement was proposed to be caused by treadmilling of the MreB filaments (Soufo and Graumann, 2004; Kim et al, 2006). A revised hypothesis posited that the movement of the elongasome depends on PG synthesis, either by insertion of new glycan strands of PG by a bifunctional PBP in a pushing movement or by pulling of hydrolases degrading old PG strands (Errington, 2015).…”

Prokaryotic cells: structural organisation of the cytoskeleton and organelles

“…These proteins change their pattern of localisation according to the cell cycle stage (Carballido-Lopez & Errington 2003, Figge et al 2004). Bundles of filaments move continuously and in parallel along helical tracks (Defeu-Soufo & Graumann 2004). It has also been suggested that these proteins are involved in the organisation of the cell wall, leading to the cells acquiring their typical rod-like shape (Daniel & Errington 2003).…”