Eukaryotes possess three major cytoskeletal elements: actin filaments, microtubules, and intermediate filaments. Actin filaments are made from actin, and microtubules are made from tubulin. Intermediate filaments are composed of a mixture of one or more members of different classes of proteins. Homologues of all three types of eukaryotic proteins have been identified in bacteria.
The bacterial cytoskeletal proteins are structurally similar to their eukaryotic counterparts and carry out similar functions: they participate in cell division, localize proteins to certain sites in the cell, and determine cell shape. In addition, some bacterial cytoskeletal proteins appear to be unique. Thus, it is likely that the evolution of the cytoskeleton was an early event in the history of life on Earth.
The cytoskeletons of Escherichia coli, Bacillus subtilis, and Caulobacter crescentus are the best studied and are the focus of our discussion. These three organisms are important bacterial model systems for several reasons.
E. coli is a Gram-negative rod that has been extensively studied and can be easily manipulated. B. subtilis is a Gram-positive rod found in soil. It is an endospore-forming bacterium, making it a good model for cellular differentiation. Both were models for cell wall structure C. crescentus is a curved rod found in aquatic habitats. It is interesting in part because it exhibits a complex life cycle that includes two different stages: a motile swarmer cell and a sessile, stalked cell that attaches to surfaces by a holdfast.
The best studied bacterial cytoskeletal proteins are FtsZ, MreB, and CreS (also known as crescentin). FtsZ was one of the first bacterial cytoskeletal proteins identified and has since been found in most bacteria. FtsZ is a homologue of the eukaryotic protein tubulin.
It forms a ring at the center of a dividing cell and is required for the formation of the septum that will separate the daughter cells. MreB and its relative Mbl are actin homologues. Their major function is to determine cell shape in rod-shaped cells; MreB is not found in cocci.
MreB and Mbl determine cell shape by properly positioning the machinery needed for peptidoglycan synthesis. CreS (crescentin) was discovered in C. crescentus and is responsible for its curved shape. CreS is a homologue of lamin and keratin, two intermediate filament proteins.