SetB: an integral membrane protein that affects chromosome segregation in Escherichia coli

Bacterial Mitotic Machineries

Here, we review recent progress that yields fundamental new insight into the molecular mechanisms behind plasmid and chromosome segregation in prokaryotic cells. In particular, we describe how prokaryotic actin homologs form mitotic machineries that segregate DNA before cell division. Thus, the ParM protein of plasmid R1 forms F actin-like filaments that separate and move plasmid DNA from mid-cell to the cell poles. Evidence from three different laboratories indicate that the morphogenetic MreB protein may be involved in segregation of the bacterial chromosome.

An intracellular actin motor in bacteria?

Actin performs structural as well as motor-like functions in eukaryotic cells. Orthologues of actin have also been identified in bacteria, where they perform an essential function during cell growth. Bacterial actins are implicated in the maintenance of rod-shaped cell morphology, and appear to form a cytoskeletal structure, localising as helical filaments underneath the cell membrane. Recently, a plasmid-borne actin orthologue has been shown to perform a mitotic-like function during segregation of a plasmid, and chromosomally encoded actin proteins were found to play an important role in chromosome segregation. Based on the findings that actin filaments are dynamic structures in two bacterial species, we propose that actins perform motor functions rather than a purely structural role in bacteria. We suggest that an intracellular motor exists in bacteria that could be derived from an ancestral actin motor that was present in cells early in evolution.