Review
The push and pull of the bacterial cytoskeleton

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A crucial function for eukaryotic cytoskeletal filaments is to organize the intracellular space: facilitate communication across the cell and enable the active transport of cellular components. It was assumed for many years that the small size of the bacterial cell eliminates the need for a cytoskeleton, because simple diffusion of proteins is rapid over micron-scale distances. However, in the last decade, cytoskeletal proteins have indeed been found to exist in bacteria where they have an important role in organizing the bacterial cell. Here, we review the progress that has been made towards understanding the mechanisms by which bacterial cytoskeletal proteins influence cellular organization. These discoveries have advanced our understanding of bacterial physiology and provided insight into the evolution of the eukaryotic cytoskeleton.

Section snippets

Bacterial cytoskeletal proteins

Cytoskeletal elements have been identified in prokaryotes based on structural and functional homology to eukaryotic proteins (Table 1 and Ref. [1]). Although there is virtually no primary sequence similarity between prokaryotic and eukaryotic cytoskeletal elements, prokaryotic proteins with actin- and tubulin-like folds have been found. Many of these proteins have been shown to polymerize in vitro in a nucleotide-dependent manner. Furthermore, genetic and cell biological data suggest that, in

A push in the right direction

The most basic way to influence the localization of cellular components is to physically move them through the cytoplasm to a specific location. In eukaryotic cells, there are motor proteins that directly transport vesicles, mRNA and proteins along tracks of actin or tubulin. However, actin and tubulin themselves can also function as motors to propel objects through the cytoplasm (Box 2). In bacteria, the plasmid-encoded actin homolog, ParM, is thought to force actively two clusters of plasmid

A mitotic-like pull

Just as polymerization can generate a pushing force, depolymerization can generate a pulling force. In eukaryotic cells, for example, the energy released upon depolymerization of microtubules can be harnessed by a complex of proteins attached to chromosomes, driving their segregation [14]. Recent evidence from the bacterium Vibrio cholerae suggests an analogous pulling mechanism might exist in prokaryotes as well [15]. V. cholerae has two chromosomes, each encoding its own partitioning genes

Dynamic scaffolding

Cytoskeletal proteins do not have to physically move proteins or DNA through the cytoplasm to influence cellular organization. A higher order cellular structure formed by cytoskeletal proteins can act as a scaffold to direct the localization of other cellular components. Just one of many possible examples from eukaryotic cells is the microtubule network in plants, which forms a scaffold for cell wall synthesis, directing the placement of the synthetic enzymes and the orientation of the

Concluding remarks

Both prokaryotes and eukaryotes seem capable of using cytoskeletal polymers to actively and dynamically organize their intracellular space. Moreover, archeal homologs of FtsZ and MreB have been identified, although little is known about their function in vivo. Nonetheless, the presence of this activity in eukaryotic and prokaryotic organisms argues that there is a fundamental requirement for dynamic polymers in cellular life and ancient mechanisms of polymer motors and scaffolds.

Dynamic

Acknowledgements

We thank Erin Barnhart, Matt Footer, Ethan Garner, Erin Goley, Antonio Iniesta, Kinneret Keren, Julie Theriot and Esteban Toro for critical reading of the manuscript and inspiring discussions.

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