Prokaryotic cytoskeleton

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 recent advances in visualization technology and structure determination have shown that filaments indeed exist in these cells. In fact, analogues for all major cytoskeletal proteins in eukaryotes have been found in prokaryotes. Cytoskeletal elements play essential roles in cell division, protection, shape determination, and polarity determination in various prokaryotes. [cite journal |author=Shih YL, Rothfield L |title=The bacterial cytoskeleton |journal=Microbiol. Mol. Biol. Rev. |volume=70 |issue=3 |pages=729–54 |year=2006 |pmid=16959967 |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=16959967 |doi=10.1128/MMBR.00017-06] [cite journal |author=Michie KA, Löwe J |title=Dynamic filaments of the bacterial cytoskeleton |journal=Annu. Rev. Biochem. |volume=75 |issue= |pages=467–92 |year=2006 |pmid=16756499 |url=http://www2.mrc-lmb.cam.ac.uk/SS/Lowe_J/group/PDF/annrev2006.pdf |doi=10.1146/annurev.biochem.75.103004.142452]

FtsZ

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.cite journal | author = Bi, E. | coauthors = Lutkenhaus, J. | year = 1991 | title = FtsZ ring structure associated with division in "Escherichia coli" | journal = Nature | volume = 354 | issue = 6349 | pages = 161–164 | doi = 10.1038/354161a0 ] 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.cite journal |author=Gitai Z |title=The new bacterial cell biology: moving parts and subcellular architecture |journal=Cell |volume=120 |issue=5 |pages=577–86 |year=2005 |pmid=15766522 |doi=10.1016/j.cell.2005.02.026] 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.cite journal | author = Graumann, P.L. | year = 2004 | title = Cytoskeletal elements in bacteria | journal = Current Opinion in Microbiology | volume = 7 | issue = 6 | pages = 565–571 | pmid = 17506674 | doi = 10.1016/j.mib.2004.10.010]

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.cite journal | author = Desai, A. | coauthors = Mitchison, T.J. | year = 1998 | title = Tubulin and FtsZ structures: functional and therapeutic implications | journal = Bioessays | volume = 20 | issue = 7 | pages = 523–527 | pmid = 9722999 | doi = 10.1002/(SICI)1521-1878(199807)20:7<523::AID-BIES1>3.0.CO;2-L | doilabel = 10.1002/(SICI)1521-1878(199807)20:7523::AID-BIES13.0.CO;2-L] Since FtsZ is essential for cell division in bacteria, this protein is a target for the design of new antibiotics. [cite journal |author=Haydon DJ, Stokes NR, Ure R, "et al" |title=An inhibitor of FtsZ with potent and selective anti-staphylococcal activity |journal=Science (journal) |volume=321 |issue=5896 |pages=1673–5 |year=2008 |month=September |pmid=18801997 |doi=10.1126/science.1159961]

MreB

MreB is a bacterial protein believed to be analogous to eukaryal actin. MreB and actin have a weak primary structure match, but are very similar in terms of 3-D structure and filament polymerization.

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.cite journal | author = Kurner, J. | coauthors = Medalia, O.; Linaroudis, A.A.; Baumeister, W. | year = 2004 | title = New insights into the structural organization of eukaryotic and prokaryotic cytoskeletons using cryo-electron tomography. | journal = Exp Cell Res | volume = 301 | issue = 1 | pages = 38–42 | url = http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=15501443&dopt=Citation | doi = 10.1016/j.yexcr.2004.08.005] 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.cite journal | author=Gitai, Z. | coauthors=Dye, N.; Shapiro, L. | title= An actin-like gene can determine cell polarity in bacteria |journal=Proc Natl Acad Sci USA | volume=101 | issue=23 | pages=8643–8648 | year=2004 | doi = 10.1073/pnas.0402638101] 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

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).cite journal | author = Ausmees, N. | coauthors = Kuhn, J.R.; Jacobs-Wagner, C. | year = 2003 | title = The Bacterial Cytoskeleton An Intermediate Filament-Like Function in Cell Shape | journal = Cell | volume = 115 | issue = 6 | pages = 705–713 | url = http://linkinghub.elsevier.com/retrieve/pii/S0092867403009358 | doi = 10.1016/S0092-8674(03)00935-8] 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.cite journal | author=Garner, E.C. | coauthors = Campbell, C.S.; Mullins, R. D. | title= Dynamic Instability in a DNA-Segregating Prokaryotic Actin Homolog | journal=Science | volume=306 | pages=1021–1025 | year=2004 | doi = 10.1126/science.1101313] 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.cite journal | author = Moller-Jensen, J. | coauthors = Jensen, R.B.; Löwe, J.; Gerdes, K. | year = 2002 | title = Prokaryotic DNA segregation by an actin-like filament | journal = The EMBO Journal | volume = 21 | pages = 3119–3127 | doi = 10.1093/emboj/cdf320 ] 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.cite journal | author = Gitai, Z. | year = 2006 | title = Plasmid Segregation: A New Class of Cytoskeletal Proteins Emerges | journal = Current Biology | volume = 16 | issue = 4 | pages = 133–136 | url = http://linkinghub.elsevier.com/retrieve/pii/S0960982206011171 | doi = 10.1016/j.cub.2006.02.007] 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.

MinCDE system

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.cite journal | author = Shih, Y.L. | coauthors = Le, T.; Rothfield, L. | year = 2003 | title = Division site selection in "Escherichia coli" involves dynamic redistribution of Min proteins within coiled structures that extend between the two cell poles | journal = Proceedings of the National Academy of Sciences | volume = 100 | issue = 13 | pages = 7865–7870 | doi = 10.1073/pnas.1232225100 ]

ee also

* Prokaryotes
* Cytoskeleton
* Protein filament
* Cell division
* Cytokinesis

References