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 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.
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.
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