MreB

MreB is a protein found in bacteria that has been identified as a homologue of actin, as indicated by similarities in tertiary structure and conservation of active site peptide sequence. The conservation of protein structure suggests the common ancestry of the cytoskeletal elements actin, found in eukaryotes, and MreB, found in prokaryotes. Indeed, recent studies have found that MreB proteins polymerize to form filaments that are similar to actin microfilaments.

Function
MreB controls the width of rod-shaped bacteria, such as Escherichia coli. A mutant E. coli that creates defective MreB proteins will be spherical instead of rod-like. Also, bacteria that are naturally spherical do not have the gene encoding MreB. Prokaryotes carrying the mreB gene can also be helical in shape. MreB filaments sit right beneath the bacterial cell membrane and give bacteria a strong, rigid structure, as if the bacterium were being stretched out from the inside.