This protein was likely replaced with dynamin in mitochondria review in Erickson However, FtsZ has been detected in the golden-brown alga Mallomonas splendens , localised in patches on the mitochondrial membrane Beech et al. Initial sequential analysis showed that FtsZ contains a short segment of amino acids that is virtually identical to the tubulin signature motif found in all alpha, beta and gamma tubulins de Boer et al.
Further analysis identified a dozen additional completely conserved residues. Structural analysis of FtsZ polymers shows that they are similar to tubulin polymers Erickson et al.
Prokaryotes - Missing a Nucleus
However, unlike microtubules, FstZ protofilaments do not assemble into higher-order tubules Fig. This combined analysis showed a double helical superstructure, which is unusual for tubulin Aylett et al. The protofilament would be attached to the membrane either via proteins or through direct association with lipids. Two new proteins designated FtsK and ZipA could participate in this process. ZipA is an integral membrane protein that is essential for E.
It contains an amino-terminal transmembrane anchor and a cytoplasmic domain consisting of three parts: a 60 amino acid charged domain, a amino acid domain rich in proline and asparagine and a amino acid globular carboxy-terminal domain. Some constriction may occur, most likely due to conformational changes taking place in the protofilament, which may exhibit a straight conformation once it is attached to the membrane and then adopt a curved conformation upon dissociation from the membrane Erickson In the presence of GTP, protofilaments are usually straightened, whereas in the presence of GDP, protofilaments favour a curved conformation.
It is important to note that FtsZ serves as a target for several auxiliary proteins. All of these proteins interact at the level of division site placement. In addition to FtsZ, other tubulin homologues have been found in some prokaryotes. The genes encoding these proteins were most likely transferred from a eukaryotic cell via horizontal gene transfer, diverging shortly after tubulin gene duplication Schlieper et al.
Because FstZ is involved in cell division, it has become a target for the development of new drugs able to inhibit prokaryotic proliferation. Special emphasis has been placed on the development of drugs that do not interfere with eukaryotic tubulin. Several other drugs, such as SRI, a taxane derivative, sanguinarine, a 3- 2-indolyl piperidine derivative, and a carboxybiphenyllindole derivative, have been shown to interact with FstZ review in Volmer Actin-like homologues - Since the late s, it has been known that the gene mreB from murein cluster e is involved in establishing and maintaining the shape of prokaryotic cells reviewed in Graumann A lack of mreB in E.
Other MreB-like proteins have been identified in B. It has been shown that MreB exists during most of the cell cycle and it splits into two equivalent structures Fig. Further studies have shown that these proteins are involved in the maintenance of cell shape and viability.
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Immunofluorescence microscopy of cells transfected with GFP-tagged MreB and Mb1 showed that they form filamentous structures in a helically arranged pattern localised below the bacterial membrane. Mb1 filaments extend from pole to pole, while MreB filaments were not detected at the poles Jones et al. These observations led to the suggestion that MreB is involved in the control of cell width, while Mb1 controls cell length.
A breakthrough in this area of inquiry came from work showing that MreB is a member of the actin family and that it forms actin-like double filaments that run in parallel and with the same linear orientation, in contrast to what is observed for actin, which adopts a helical organisation Van den Ent et al. MreE filaments are more rigid than actin filaments and they tend to assemble into bundles.
In contrast, actin filaments are assembled as single filaments Esue et al. The spacing between MreB proteins along protofilaments is 5. Other data also demonstrate the involvement of actin-like proteins in the segregation of plasmids and whole chromosomes Van den Ent et al. ParM belongs to another class of actin homologue that differs with respect to its sequence and polymerisation dynamics from the MreB class. While the MreB class is always encoded in the bacterial chromosome and regulates a wide array of cellular functions, the ParM class always interacts with extrachromosomal plasmids and appears to be dedicated to the segregation of plasmids, functioning as a motor.
As filaments grow, they push plasmids to the cell poles prior to cell division Moller-Jensen et al. Par A forms linear polymers in vitro. Par B is a centromere-binding protein that binds to and destabilises the Par A structure, causing centromere movement to opposite poles Ptacin et al. This observation suggests that prokaryotic cells use mechanisms resembling well-characterised mechanisms involved in the mitotic machinery operating in eukaryotic cells.
No other cytoskeletal motor proteins, such as dynein, kinesin or myosin, have been identified in prokaryotes thus far. Intermediate-like filament protein homologues - Studies using the bacterium C. The corresponding gene was identified and it was found that when it was deleted, cells still grew but lost their typical shape, resulting in straight cells Ausmees et al. Immunofluorescence microscopy showed that the protein localises in a filamentous form on the concave side of cells and along the inner cell curvature over the entire length of the helical cells.
The available data indicate that the CreS protein induces cell bending and creates asymmetry along the length of a cell. This protein has also been found in other species, including Helicobacter pylori.
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Further analysis of this protein showed that it exhibits a high content of coiled-coils and strutters, which are positions where the coiled-coils are disrupted. This organisation resembles that found in proteins such as keratin, which forms intermediate filaments. Indeed, isolated CreS protein assembles into 10 nm thick filaments in vitro without requiring cofactors or energy.
Prokaryote
A second type of intermediate-like filament has been characterised in several spirochete genera, including Treponema , Spirochaeta , Leptonema and Diplocalyx. It appears as a filamentous ribbon-like structure composed of two-six filaments, which are severed during cell division such that half of them remain in each daughter cell. The width of the filaments is approximately 6 nm and the space between them is approximately Based on the thickness of these filaments, they are very different from typical intermediate filaments of mammalian cells, which exhibit a thickness of approximately 10 nm.
There is no sequence similarity between CfpA and other known proteins and it does not contain any predicted extended coiled-coil domains Izard et al. Electron microscope tomography revealed that the filaments are part of a complex in which one set of components anchors the filaments to the membrane while another set establishes bridges between the filaments and cytoplasmic components.
In addition, the filaments do not contact each other Izard et al. Although this has not yet been studied in detail, other proteins have also been implicated in the assembly of intermediate filament-like structures presenting typical coiled-coil domains. These structures include the fibrillar structures found in Spiroplasma melliferum formed by a kDa protein.
The assembled filaments are located beneath the membrane and are arranged in parallel ribbons extending along the length of the spiral-shaped organism and their expression is dependent upon the cfpA gene Townsend et al. Another filamentous structure is related to the protein AgIZ. The C-terminal region of this protein includes heptad repeats, which are a characteristic of the rod region of coiled-coils of proteins, such as the myosin heavy chain observed in Myxococcus xanthus Yang et al.
Differences Between Prokaryotic and Eukaryotic Cells
Expression of this protein in E. Finally, another type of structure has been described based on cytoskeletal-like structures that have been observed in both intact and detergent-extracted Mycoplasma pneumoniae Regula et al. Protein glycosylation - For many years, protein glycosylation was considered to be a characteristic feature of eukaryotic cells, which possess an elaborate system of cisternae forming the endoplasmic reticulum and Golgi complex systems. Subsequently, other glycosylated proteins have been found in structures such as flagella and pili.
Glycoproteins appear to be modified primarily by short-chain carbohydrate moieties mono to trisaccharide substituents. There are several excellent reviews on this subject Schaffer et al. Protein complexes - At present there, is a great deal of evidence of the presence of protein complexes localised in the cytoplasm.
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These complexes usually consist of several proteins that associate with each other, forming a structure that can be recognised in thin sections using electron microscopy in many cases. If the complex is too small, it is not directly recognised, but can be isolated through differential centrifugation and then subjected to structural analysis. The biological way to form a protein complex is to combine proteins involved in a well-defined metabolic pathway. Therefore, we can consider protein complexes as defining a certain subcompartment of the cell. Because these complexes are not surrounded by a typical unit membrane, I prefer to refer to them as protein complexes rather than protein organelles, as described by others.
One good example of a protein complex in a eukaryotic cell is the proteasome, which is a structure involved in the degradation of proteins. Interest in this field increased when these bodies were identified and isolated from the chemoautotrophic bacterium Halobacterium neapolitanus and were shown to be composed of a surrounding shell and a matrix containing the enzyme ribulosebiphosphate carboxylase, also abbreviated as RuBisCO.
They were then designated as carboxysomes Shively et al.