While eukaryotic cells are typically large and morphologically complex, bacterial cells are usually only a few micrometers long and assume simple shapes such as spheres or rods. Bacteria also lack elaborate networks of intracellular membrane-enclosed organelles. Historically, biologists assumed that a cytoskeleton was not necessary in such simple cells. We now know, however, that bacteria contain homologs of all three of the eukaryotic cytoskeletal filaments. Furthermore, bacterial actins and tubulins are more diverse than their eukaryotic versions, both in the types of assemblies they form and in the functions they carry out.
Nearly all bacteria and many archaea contain a homolog of tubulin called FtsZ, which can polymerize into filaments and assemble into a ring (called the Z-ring) at the site where the septum forms during cell division (Figure 16–7). Although the Z-ring persists for many minutes, the individual filaments within it are highly dynamic, with an average filament half-life of about thirty seconds. As the bacte rium divides, the Z-ring becomes smaller until it has completely disassembled. FtsZ filaments in the Z-ring are thought to generate a bending force that drives the membrane invagination necessary to complete cell division. The Z-ring may also serve as a site for localization of enzymes required for building the septum between the two daughter cells.
Many bacteria also contain homologs of actin. Two of these, MreB and Mbl, are found primarily in rod-shaped or spiral-shaped cells where they assemble to form dynamic patches that move circumferentially along the length of the cell (Figure 16–8A). These proteins contribute to cell shape by serving as a scaffold to direct the synthesis of the peptidoglycan cell wall, in much the same way that microtubules help organize the synthesis of the cellulose cell wall in higher
Figure 16–7 The bacterial FtsZ protein, a tubulin homolog in prokaryotes. (A) A band of FtsZ protein forms a ring in a dividing bacterial cell. This ring has been labeled by fusing the FtsZ protein to green fluorescent protein (GFP), which allows it to be observed in living E. coli cells with a fluorescence microscope. (B) FtsZ filaments and circles, formed in vitro, as visualized using electron microscopy. (C) Dividing chloroplasts (red) from a red alga also cleave using a protein ring made from FtsZ (yellow). (A, from X. Ma,
D.W.
Ehrhardt and W. Margolin, Proc. Natl Acad. Sci. USA 93:12998–13003, 1996; B, from H.P. Erickson et al., Proc. Natl Acad. Sci. USA 93:519–523, 1996. Both with permission from National Academy of Sciences; C, from
S.
Miyagishima et al., Plant Cell 13:2257–2268, 2001, with permission from (C)American Society of Plant Biologists.) 1 µm
Figure 16–8 Actin homologs in bacteria determine cell shape. (A) The MreB protein forms abundant patches made up of many short, interwoven linear or helical filaments that are seen to move circumferentially along the length of the bacterium and are associated with sites of cell wall synthesis. (B) The common soil bacterium Bacillus subtilis normally forms cells with a regular rodlike shape when viewed by scanning electron microscopy (left). In contrast, B. subtilis cells lacking the actin homolog MreB or Mbl grow in distorted or twisted shapes and eventually die (center and right). (A, from P. Vats and L. Rothfield, Proc. Natl Acad. Sci. USA 104:17795–17800, 2007. With permission from National Academy of Sciences; B, from A. Chastanet and