The prokaryotic cytoskeleton is the collective name for all structural filaments in prokaryotes. It was once thought that prokaryotic cells did not possess cytoskeletons, but advances in visualization technology and structure determination led to the discovery of filaments in these cells in the early 1990s. Not only have analogues for all major cytoskeletal proteins in eukaryotes been found in prokaryotes, cytoskeletal proteins with no known eukaryotic homologues have also been discovered. Cytoskeletal elements play essential roles in cell division, protection, shape determination, and polarity determination in various prokaryotes.
FtsZ, the first identified prokaryotic cytoskeletal element, forms a filamentous ring structure located in the middle of the cell called the Z-ring that constricts during cell division, similar to the actin-myosin contractile ring in eukaryotes. The Z-ring is a highly dynamic structure that consists of numerous bundles of protofilaments that extend and shrink, although the mechanism behind Z-ring contraction and the number of protofilaments involved are unclear. FtsZ acts as an organizer protein and is required for cell division. It is the first component of the septum during cytokinesis, and it recruits all other known cell division proteins to the division site.
Despite this functional similarity to actin, FtsZ is homologous to eukaryal tubulin. Although comparison of the primary structures of FtsZ and tubulin reveal a weak relationship, their 3-dimensional structures are remarkably similar. Furthermore, like tubulin, monomeric FtsZ is bound to GTP and polymerizes with other FtsZ monomers with the hydrolysis of GTP in a mechanism similar to tubulin dimerization. Since FtsZ is essential for cell division in bacteria, this protein is a target for the design of new antibiotics. There currently exist several models and mechanisms that regulate Z-ring formation.One of the model of Z-ring formation was proposed by Rashid, Aijaz and Sing, Perminder. Critical Concentration Gradient Model of Bacterial Cell Division. Available from Nature Precedings <http://hdl.handle.net/10101/npre.2010.4659.1> (2010).According to this model positive and negative regulators of FtsZ assembly form a gradient inside the bacterial cell. The gradient is such that critical concentration of FtsZ required for FtsZ assembly is minimum at medial position of cell,hence Z-ring and ultimately divisome complex is formed at middle of cell.
Almost all non-spherical bacteria rely on MreB to determine their shape. MreB assembles into a helical network of filamentous structures just under the cytoplasmic membrane, covering the whole length of the cell. MreB determines cell shape by mediating the position and activity of enzymes that synthesize peptidoglycan and by acting as a rigid filament under the cell membrane that exerts outward pressure to sculpt and bolster the cell. MreB condenses from its normal helical network and forms a tight ring at the septum in Caulobacter crescentus right before cell division, a mechanism that is believed to help locate its off-center septum. MreB is also important for polarity determination in polar bacteria, as it is responsible for the correct positioning of at least four different polar proteins in C. crescentus.
Crescentin (encoded by creS gene) is an analogue of eukaryotic intermediate filaments (IFs). Unlike the other analogous relationships discussed here, crescentin has a rather large primary homology with IF proteins in addition to three-dimensional similarity - the sequence of creS has a 25% identity match and 40% similarity to cytokeratin 19 and a 24% identity match and 40% similarity to nuclear lamin A. Furthermore, crescentin filaments are roughly 10 nm in diameter and thus fall within diameter range for eukaryal IFs (8-15 nm). Crescentin forms a continuous filament from pole to pole alongside the inner, concave side of the crescent-shaped bacterium Caulobacter crescentus. Both MreB and crescentin are necessary for C. crescentus to exist in its characteristic shape; it is believed that MreB molds the cell into a rod shape and crescentin bends this shape into a crescent.
ParM and SopA
ParM is a cytoskeletal element that possesses a similar structure to actin, although it behaves functionally like tubulin. Further, it polymerizes bidirectionally and it exhibits dynamic instability, which are both behaviors characteristic of tubulin polymerization. It forms a system with ParR and parC that is responsible for R1 plasmid separation. ParM affixes to ParR, a DNA-binding protein that specifically binds to 10 direct repeats in the parC region on the R1 plasmid. This binding occurs on both ends of the ParM filament. This filament is then extended, separating the plasmids. The system is analogous to eukaryotic chromosome segregation as ParM acts like eukaryotic tubulin in the mitotic spindle, ParR acts like the kinetochore complex, and parC acts like the centromere of the chromosome. F plasmid segregation occurs in a similar system where SopA acts as the cytoskeletal filament and SopB binds to the sopC sequence in the F plasmid, like the kinetochore and centromere respectively.
The MinCDE system is a filament system that properly positions the septum in the middle of the cell in Escherichia coli. According to Shih et al., MinC inhibits the formation of the septum by prohibiting the polymerization of the Z-ring. MinC, MinD, and MinE form a helix structure that winds around the cell and is bound to the membrane by MinD. The MinCDE helix occupies a pole and terminates in a filamentous structure called the E-ring made of MinE at the middle-most edge of the polar zone. From this configuration, the E-ring will contract and move toward that pole, disassembling the MinCDE helix as it moves along. Concomitantly, the disassembled fragments will reassemble at the opposite polar end, reforming the MinCDE coil on the opposite pole while the current MinCDE helix is broken down. This process then repeats, with the MinCDE helix oscillating from pole to pole. This oscillation occurs repeatedly during the cell cycle, thereby keeping MinC (and its septum inhibiting effect) at a lower time-averaged concentration at the middle of the cell than at the ends of the cell.
The dynamic behavior of the Min proteins has been reconstituted in vitro using an artificial lipid bilayer as mimic for the cell membrane. MinE and MinD self-organized into parallel and spiral protein waves by a reaction-diffusion like mechanism.
Bactofilin is a cytoskeletal element that forms filaments throughout the cells of the rod-shaped proteobacterium Myxococcus xanthus. The bactofilin protein, BacM, is required for proper cell shape maintenance and cell wall integrity. M. xanthus cells lacking BacM have a deformed morphology characterized by a bent cell body, and bacM mutants have decreased resistance to antibiotics targeting the bacterial cell wall. M. xanthus BacM protein is cleaved from its full-length form to allow polymerization. Bactofilins have been implicated in cell shape regulation in other bacteria, including curvature of Proteus mirabilis cells, stalk formation by Caulobacter crescentus, and helical shape of Helicobacter pylori.
Crenactin is an actin homologue unique to the archaeal kingdom Crenarchaeota that has been found in the orders Thermoproteales and Candidatus Korarchaeum. It has the highest sequence similarity to eukaryotic actins of any known actin homologue. Crenactin has been well characterized in Pyryobaculum calidifontis and shown to have high specificity for ATP and GTP. Species containing crenactin are all rod or needle shaped and in P. calidifontis Crenactin has been shown to form helical structures that span the length of the cell, suggesting a role for crenactin in shape determination similar to that of MreB in other prokaryotes.
- Gitai, Z. (2005). "The New Bacterial Cell Biology: Moving Parts and Subcellular Architecture". Cell 120 (5): 577–586. doi:10.1016/j.cell.2005.02.026. PMID 15766522.
- Bi, E.; Lutkenhaus, J. (1991). "FtsZ ring structure associated with division in Escherichia coli". Nature 354 (6349): 161–164. doi:10.1038/354161a0. PMID 1944597.
- Wickstead B, Gull K (2011). "The evolution of the cytoskeleton". The Journal of Cell Biology 194 (4): 513–25. doi:10.1083/jcb.201102065. PMC 3160578. PMID 21859859.
- Shih YL, Rothfield L (2006). "The bacterial cytoskeleton". Microbiol. Mol. Biol. Rev. 70 (3): 729–54. doi:10.1128/MMBR.00017-06. PMC 1594594. PMID 16959967.
- Michie KA, Löwe J (2006). "Dynamic filaments of the bacterial cytoskeleton" (PDF). Annu. Rev. Biochem. 75: 467–92. doi:10.1146/annurev.biochem.75.103004.142452. PMID 16756499.[dead link]
- Graumann, P.L. (2004). "Cytoskeletal elements in bacteria". Current Opinion in Microbiology 7 (6): 565–571. doi:10.1016/j.mib.2004.10.010. PMID 17506674.
- Desai, A.; Mitchison, T.J. (1998). "Tubulin and FtsZ structures: functional and therapeutic implications". BioEssays 20 (7): 523–527. doi:10.1002/(SICI)1521-1878(199807)20:7<523::AID-BIES1>3.0.CO;2-L. PMID 9722999.
- Haydon DJ, Stokes NR, Ure R et al. (September 2008). "An inhibitor of FtsZ with potent and selective anti-staphylococcal activity". Science 321 (5896): 1673–5. doi:10.1126/science.1159961. PMID 18801997.
- Kurner, J.; Medalia, O.; Linaroudis, A.A.; Baumeister, W. (2004). "New insights into the structural organization of eukaryotic and prokaryotic cytoskeletons using cryo-electron tomography.". Exp Cell Res 301 (1): 38–42. doi:10.1016/j.yexcr.2004.08.005. PMID 15501443.
- Gitai, Z.; Dye, N.; Shapiro, L. (2004). "An actin-like gene can determine cell polarity in bacteria". Proc Natl Acad Sci USA 101 (23): 8643–8648. doi:10.1073/pnas.0402638101. PMC 423248. PMID 15159537.
- Ausmees, N.; Kuhn, J.R.; Jacobs-Wagner, C. (2003). "The Bacterial Cytoskeleton An Intermediate Filament-Like Function in Cell Shape". Cell 115 (6): 705–713. doi:10.1016/S0092-8674(03)00935-8. PMID 14675535.
- Garner, E.C.; Campbell, C.S.; Mullins, R. D. (2004). "Dynamic Instability in a DNA-Segregating Prokaryotic Actin Homolog". Science 306 (5698): 1021–1025. doi:10.1126/science.1101313. PMID 15528442.
- Moller-Jensen, J.; Jensen, R.B.; Löwe, J.; Gerdes, K. (2002). "Prokaryotic DNA segregation by an actin-like filament". The EMBO Journal 21 (12): 3119–3127. doi:10.1093/emboj/cdf320. PMC 126073. PMID 12065424.
- Gitai, Z. (2006). "Plasmid Segregation: A New Class of Cytoskeletal Proteins Emerges". Current Biology 16 (4): 133–136. doi:10.1016/j.cub.2006.02.007. PMID 16488865.
- Shih, Y.L.; Le, T.; Rothfield, L. (2003). "Division site selection in Escherichia coli involves dynamic redistribution of Min proteins within coiled structures that extend between the two cell poles". Proceedings of the National Academy of Sciences 100 (13): 7865–7870. doi:10.1073/pnas.1232225100. PMC 164679. PMID 12766229.
- Loose M, Fischer-Friedrich E, Ries J, Kruse K, Schwille P (2008). "Spatial Regulators for Bacterial Cell Division Self-Organize into Surface Waves in Vitro.". Science 320 (5877): 789–792. doi:10.1126/science.1154413. PMID 18467587.
- Koch, M.K.; McHugh, C.A.; Hoiczyk, E. (2010). "BacM, an N-terminally processed bactofilin of Myxococcus xanthus, is crucial for proper cell shape". Molecular Microbiology 80 (4): 1031–1051. doi:10.1111/j.1365-2958.2011.07629.x. PMC 3091990. PMID 21414039.
- Hay, N.A.; Tipper, D.J.; Gygi, D.; Hughes, C. (1999). "A novel membrane protein influencing cell shape and multicellular swarming of Proteus mirabilis". Journal of Bacteriology 181 (7): 2008–2016. PMC 93611. PMID 10094676.
- Kühn, J.; Briegel, A.; Mörschel, E.; Kahnt, J.; Leser, K.; Wick, S.; Jensen, G.J.; Thanbichler, M. (2010). "Bactofilins, a ubiquitous class of cytoskeletal proteins mediating polar localization of a cell wall synthase in Caulobacter crescentus". The EMBO Journal 29 (2): 327–339. doi:10.1038/emboj.2009.358. PMC 2824468. PMID 19959992.
- Sycuro, L.K.; Pincus, Z.; Gutierrez, K.D.; Biboy, J.; Stern, C.A.; Vollmer, W.; Salama, N.R. (2010). "Peptidoglycan crosslinking relaxation promotes Helicobacter pylori 's helical shape and stomach colonization". Cell 141 (5): 822–833. doi:10.1016/j.cell.2010.03.046. PMC 2920535. PMID 20510929.
- Ettema, T.J.; Lindas, A.; Bernander, R. (2011). "An actin-based cytoskeleton in archaea". Molecular Microbiology 80 (4): 1052–61. doi:10.1111/j.1365-2958.2011.07635.x. PMID 21414041.
- Yutin, N.; Wolf,M.Y.; Wolf, Y.I., Koonin, E.V. (2009). "The origins of phagocytosis and eukaryogenesis". Biology Direct 4: 9. doi:10.1186/1745-6150-4-9. PMC 2651865. PMID 19245710.