Motion of the bacterial cytoskeleton is driven by cell wall synthesis

Motor-induced dynamics of the cytoskeleton is well-studied in eukaryotic cells but had not been observed in bacteria. Our experiments reveal that an important component of the bacterial cytoskeleton is dragged around the periphery of rod-shaped cells of Escherichia coli as new cell wall is synthesized. Computational modeling indicates that these dynamics are important for bacterial morphogenesis.

HFSP Cross-Disciplinary Fellow Sven vanTeeffelen and HFSP Grant holders Ned Wingreen, Joshua Shaevitz and Zemer Gitai and colleagues
authored on Thu, 03 November 2011

Bacteria were long regarded as unstructured bags of freely diffusing proteins and DNA.  Contrary to this view, bacterial cells are now known to display intricate sub-cellular localization and dynamics of their constituents.  As in eukaryotes, spatio-temporal organization is required for a variety of processes including cellular morphogenesis, proper DNA replication and segregation, efficient metabolic fluxes, etc.  Biophysically, one mechanism for achieving sub-cellular order relies on the bacterial cytoskeleton, which consists of polymeric proteins that are often distant homologs of the cytoskeletal proteins found in eukaryotic cells.  While many of these proteins have been identified and structurally characterized in recent years, their dynamics are poorly understood.  For example, no molecular motor that works similarly to myosin, dynein, or kinesin has been identified in bacteria.

 

Figure: Cartoon of the proposed model of cell-wall-synthesis-driven MreB rotation (front and side views):  A complex of cell-wall-synthesis-enzymes (yellow) polymerizes new glycan strands (green) and connects them with neighboring strands via peptide bonds (red) in the periplasm (between the cyan outer membrane and peach inner membrane) in order to expand the existing peptidoglycan cell wall.  Because of the proposed physical coupling MreB (magenta) rotates in the cytoplasm (indicated by the arrow).

In our work we studied the dynamics of the highly conserved cytoskeletal protein MreB, a distant homolog of eukaryotic actin, in the Gram-negative bacterium Escherichia coli. MreB forms polymers that are attached to the cytoplasmic side of the plasma membrane.  These polymers are required for intra-cellular protein localization and for bacterial morphogenesis.  By careful quantitative time-lapse microscopy of fluorescently labeled MreB, we found that this protein undergoes directed, persistent motion around the circumference of the cell.  This rotational motion is independent of MreB’s own polymerization and thus must be caused by other force-exerting enzymes.  While no single motor enzyme was found to be sufficient for moving MreB, we found that MreB motion requires the synthesis of the extra-cellular peptidoglycan cell wall.  The cell wall of Gram-negative bacteria consists of a thin meshwork of long glycan strands that run around the circumference of the cell in a hoop-like fashion and that are connected by short, flexible peptide bonds (see figure).  During longitudinal growth, new glycan strands are processively inserted in between existing glycan strands and connected to neighboring strands by a complex of synthesis enzymes; the expected trajectories of these synthesis complexes thus coincide with the observed circumferential trajectories of MreB polymers.  A model of cell-wall growth indicates that MreB and the synthesis complexes are physically linked and move as a unit around the circumference of the cell, as illustrated in the cartoon.  We hypothesize that one or multiple cell-wall synthesizing enzymes constitute the actual molecular motor that is responsible for physically dragging MreB around the cell. 

What is the function of the physical coupling of MreB to the cell-wall synthesis enzymes and of the observed cytoskeletal dynamics?  We have seen that MreB dynamics requires the enzymatic activity of the cell-wall synthesis machinery.  The localization of MreB is thus at least partially determined by cell-wall synthesis enzymes.  However, the localization of the cell-wall synthesis enzymes is thought at least in part to be determined by MreB, for example by MreB restricting the potential sites of synthesis initiation.  Using a computational model of MreB-guided cell-wall growth we showed that MreB-dependent patterning can stabilize a rod-like cell shape during longitudinal growth, which is essential for the viability and fitness of cells like E. coli

To conclude, our combined experiments and model suggest that active movement of  the cytoskeleton regulates bacterial morphogenesis.  Given the multitude of known interaction partners of MreB, the rotational dynamics we observed may have other functions, and it will be interesting to explore these in the future.  Moreover, MreB is only one of many cytoskeletal proteins in bacteria, even if a very important one.  We are confident that there will be more unconventional motor-driven dynamics to be discovered and that these discoveries will ultimately help understand the complex spatio-temporal programs executed by bacterial cells.

Reference

The bacterial actin MreB rotates, and rotation depends on cell-wall assembly. Sven van Teeffelen, Siyuan Wang, Leon Furchtgott, Kerwyn Casey Huang, Ned S. Wingreen, Joshua W. Shaevitz, and Zemer Gitai. Proceedings of the National Academy of Sciences of the USA (2011), 108, 15822-15827.

Pubmed link

PNAS link