Bacterial cells were initially considered simple, randomly organized structures lacking a cytoskeleton. However, the discovery of cytoskeleton homologs in bacteria led to the change of this opinion. Bacterial cytoskeletal filaments regulate the cell shape, cell polarity, cell division, and partitioning of plasmids during cell division. It was later discovered that bacterial cytoskeletal proteins, mainly actin and tubulin homologs, are diverse compared to their eukaryotic counterparts. On the other hand, intermediate filaments are extremely rare, with only a single homolog Crescentin, being reported.
Bacterial Actin
Bacterial actins, as per their gene sequences, are known to be highly diverse. They are classified into distinct families based on phylogenetic analysis, structural features, and functional similarities. Bioinformatics analysis has led to the classification of up to 35 different families of bacterial actin. Structural variations in bacterial actin filaments include—a dramatic change in the twist, where the intersubunit angles vary by around +/- 10o in a filament; changes in strand number; and the possibility of antiparallel strands. These differences in filament structure ultimately lead to differences in filament dynamics and, thereby, their function. Despite their diversity, the important point is that the individual subunits have a similar tertiary structure and evolved from the same ancestor as eukaryotic actins.
Bacterial Tubulin
Microtubules formed from bacterial tubulin-like proteins are structurally different from the 13 protofilament structure common amongst most eukaryotes. Bacterial tubulins like FtsZ are known to form single-stranded filaments or twisted filament pairs instead of hollow tubes. Other homologs like BtubA/BtubB, mainly found in Prosthecobacter species, include a 5-protofilament structure. Although discovered later, it is hypothesized that bacterial microtubules came before the 13-protofilament eukaryotic structure, and longitudinal bonds in single-stranded filaments evolved before the lateral interaction required for the protofilament structure. And with the 5-protofilament structure found in Prosthecobacter species, the only reported bacterial protofilament is hypothesized to be an outcome of horizontal gene transfer.
Bacterial cells have diverse cytoskeletal proteins including eukaryotic homologs of actin, tubulin, and intermediate filaments, along with a unique fourth group, the MinD-ParA proteins.
Actin homologs, MreB and Mbl are found abundantly in rod and spiral-shaped bacteria. These proteins form spiral scaffolds that guide the formation of the peptidoglycan cell wall.
Another actin homolog, ParM, is encoded by non-genomic antibiotic-resistant plasmids in some bacteria and spontaneously assembles into filaments.
During cell division, the growing filament polymers push the plasmid copies apart to the opposite ends of the cell.
Tubulin homologs, FtsZ and BtubA/B, polymerize into filaments or rings.
FtsZ filaments assemble into the Z-ring at the middle of the cell to initiate cell division.
Crescentin subunits are the only intermediate filament-like proteins identified in bacteria. They create a crescent shape in some bacteria like Caulobacter crescentus. In the absence of crescentin subunits, these bacteria turn rod-shaped.
MinD and ParA are ATPases unique to bacterial cells and are required for cell division.
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