Prokaryotic Cytoskeleton

Throughout all three domains of life, the cytoskeleton is a network of intracellular filaments that is critical for cell shape, division, and function, among other things. The simple cytoskeletons of prokaryotes are surprisingly adaptable in terms of composition, with none of the core filament-forming proteins being conserved across all evolutionary lines. In contrast, the eukaryotic cytoskeletal function has been greatly elaborated through the addition of accessory proteins as well as extensive gene duplication and specialization, resulting in a complex network of interconnected structures. A large portion of this complexity evolved before the last common ancestor of eukaryotes’ existence. The distribution of cytoskeletal filaments places restrictions on the prokaryotic lineage that is most likely to have made this transition from prokaryogenesis to eukaryogenesis.

FtsZ

Z-ring: The Z-ring is a filamentous ring structure formed by FtsZ, the first identified prokaryotic cytoskeletal element, that constricts during cell division, similar to the actin-myosin contractile ring in eukaryotes. FtsZ is the first identified prokaryotic cytoskeletal element.

 The Z-ring is a highly dynamic structure made up of numerous bundles of protofilaments that can expand and contract. However, the mechanism behind Z-ring contraction as well as the number of protofilaments involved is currently unknown.  FtsZ is a protein that functions as an organizer protein and is essential for cell division. In the process of cytokinesis, it is the first component of the septum to form, and it is responsible for recruiting all other known cell division proteins to the division site. 

Despite its structural and functional similarity to actin, FtsZ is a homolog of the eukaryotic tubulin protein. However, even though the primary structures of FtsZ and tubulin are very different, their three-dimensional structures are strikingly similar. Furthermore, monomeric FtsZ, like tubulin, is bound to GTP and polymerizes with other FtsZ monomers as a result of the hydrolysis of GTP, like tubulin dimerization, when GTP is hydrolyzed. 

MreB

MreB is a bacterial protein that is thought to be a homolog of the eukaryotic actin protein. In terms of primary structure, MreB and actin are dissimilar, but in terms of three-dimensional structure and filament polymerization, they are very similar to one another.

MreB is responsible for determining the shape of nearly all non-spherical bacteria. It takes about an hour or so for MreB to assemble into a network of filamentous structures just beneath the cytoplasmic membrane, which extends the entire length of the cell.  MreB regulates cell shape by mediating the position and activity of enzymes involved in the peptidoglycan synthesis process. In addition, MreB acts as a rigid filament under the cell membrane, exerting outward pressure to sculpt and bolster the cell. During the final stages of cell division in Caulobacter crescentus, MreB condenses from its normal helical network and forms a tight ring at the septum, a mechanism that is thought to aid in the localization of the organism’s off-centre septum. MreB is also important for determining polarity in polar bacteria, as it is responsible for the correct positioning of at least four different polar proteins in the bacterium C. crescentus. MreB is also important for determining polarity in polar bacteria. 

Crescentin

Crescentin (encoded by the creS gene) is a protein that functions as a homolog of eukaryotic intermediate filaments (IFs). The sequence of crescentin (creS) has a 25 percent identity match and 40 percent similarity to cytokeratin 19 and a 24 percent identity match and 40 percent similarity to nuclear lamin A, in contrast to the other homologous relationships discussed here. In addition to three-dimensional similarity, crescentin has significant primary homology with IF proteins. Furthermore, crescentin filaments have a diameter of approximately 10 nm, which places them within the diameter range of eukarya intermediate filaments (8-15 nm). Crescentin is a filamentous protein that forms a continuous filament from pole to pole along the inner, concave side of the crescent-shaped bacterium Caulobacter crescentus’s concave side. Several researchers believe that MreB and crescentin are required for C. crescentus to exist in its characteristic shape; it is thought that MreB shapes the cell into a rod shape and crescentin bends this shape into a crescent. 

ParM and SopA

ParM is a cytoskeletal element that is structurally homologous to actin, even though it functions more like tubulin in terms of function. Furthermore, it polymerizes bidirectionally and exhibits dynamic instability, both of which are characteristics of tubulin polymerization and should be taken into consideration.  Along with ParR and parC, it forms a system that is responsible for the separation of R1 plasmids. When ParM attaches to ParR, a DNA-binding protein that specifically binds to 10 direct repeats in the parC region of the R1 plasmid, the result is the formation of a fusion protein. This binding takes place on both ends of the ParM filament at the same time. This filament is then stretched out, separating the plasmids from one another.  Phylogenetic analysis shows that the system is analogous to that of eukaryotic chromosome segregation because ParM acts similarly to eukaryotic tubulin in the mitotic spindle, ParR acts similarly to the kinetochore complex, and parC acts similarly to chromosome centromere. When the F plasmid segregates, it does so in the same manner as the cytoskeletal filament, with SopA acting as the cytoskeletal filament and SopB binding to the sopC sequence in the F plasmid, acting as the centromere and kinetochore, respectively. 

MinCDE 

In Escherichia coli, the MinCDE system is a filament system that is responsible for correctly positioning the septum in the middle of the cell. According to Shih et al., MinC prevents the formation of the septum by interfering with the polymerization of the Z-ring during the formation process. It is composed of the minerals MinC, MinD, and MinE, which combine to form a helix structure that wraps around the cell and is attached to the membrane by MinD. The MinCDE helix occupies a pole and terminates in a filamentous structure known as the E-ring, which is composed of MinE, at the middle-most edge of the polar zone, which is referred to as the E-ring. The E-ring will contract and move toward that pole as a result of this configuration, disassembling the MinCDE helix as it does so. Additionally, the disassembled fragments will reassemble at the opposite polar end, reforming the MinCDE coil on the opposite pole at the same time as the current MinCDE helix is being deconstructed. Afterwards, the process is repeated, with the MinCDE helix oscillating from one pole to the opposite pole. During the cell cycle, this oscillation occurs repeatedly, resulting in MinC (and its septum inhibiting effect) remaining at a lower time-averaged concentration in the middle of the cell than at the ends of the cell for long periods. 

Conclusion

FtsZ, MreB, MinCDE etc are the structural components of the cytoskeleton of a cell. These structures give the cell its shape and assist in the organisation of the cell’s constituent parts. Aside from that, they serve as a foundation for cell movement and division. In contrast, the eukaryotic cytoskeletal function has been greatly elaborated through the addition of accessory proteins as well as extensive gene duplication and specialization, resulting in a complex network of interconnected structures. A large portion of this complexity evolved before the last common ancestor of eukaryotes’ existence. The distribution of cytoskeletal filaments places restrictions on the prokaryotic lineage that is most likely to have made this transition from prokaryogenesis to eukaryogenesis.