Microfilament: Cytoplasmic filament, 5-7 nm thick, of F-actin (filamentous actin) that can be decorated with HMM (heavy meromyosin); may be laterally associated with other proteins (tropomyosin, α-actinin) in some cases, and may be anchored to the membrane. Microfilaments are conspicuous in adherens junctions.
The cytoskeleton was previously considered to be a feature only of eukaryotic cells, but recent research has revealed that homologues to all the major proteins of the eukaryotic cytoskeleton can also be found in prokaryotes. Although the evolutionary relationships are so distant that they are not obvious from protein sequence comparisons alone, the similarity of their three-dimensional structures provides strong evidence that the eukaryotic and prokaryotic cytoskeletons are truly homologous.
FtsZ, a relative of the eukaryotic tubulin, was the first protein of the prokaryotic cytoskeleton to be identified. Like tubulin, FtsZ forms filaments in the presence of GTP, but these filaments do not group into tubules. During cell division, FtsZ is the first protein to move to the division site, and is essential for recruiting other proteins that produce a new cell wall between the dividing cells.
Prokaryotic actin-like proteins, such as MreB, are involved in the maintenance of cell shape. All non-spherical bacteria have genes encoding actin-like proteins, and these proteins form a helical network beneath the cell membrane that guides the proteins involved in cell wall biosynthesis.
Some plasmids encode a partitioning system that involves an actin-like protein ParM. Filaments of ParM exhibit dynamic instability, and may partition plasmid DNA into the dividing daughter cells by a mechanism analogous to that used by microtubules during eukaryotic mitosis.
The bacterium Caulobacter crescentus contains a protein, crescentin, that is related to the intermediate filaments of eukaryotic cells. Crescentin is also involved in maintaining cell shape, but the mechanism by which it does this is currently unclear.