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FtsZ is a protein encoded by the ftsZ gene that assembles into a ring at the future site of the septum of bacterial cell division. FtsZ, named after filamenting temperature-sensitive mutant Z, is a prokaryotic homologue to the eukaryotic protein tubulin.

Discovery of the bacterial cytoskeleton is fairly recent. FtsZ was the first protein of the prokaryotic cytoskeleton to be identified. In 1991 it was discovered by Erfei Bi and Joseph Lutkenhaus that FtsZ assembled into the Z-ring. 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. FtsZ's role in cell division is analogous to that of actin in eukaryotic cell division, but unlike the actin-myosin ring in eukaryotes, FtsZ has no known motor protein and is not thought to provide the mechanical force that physically divides the cell. The force behind cytokinesis is thought to be provided by the synthesis of new cell wall that is localized to the Z-ring.

How the roles of tubulin-like proteins and actin-like proteins in cell division became reversed is an evolutionary mystery, but the use of the FtsZ ring in dividing chloroplasts and some mitochondria further establishes their prokaryotic ancestry.

Much is known about the dynamic polymerization activities of tubulin and microtubules, but little is known about these activities in FtsZ. While it is known that single-stranded tubulin protofilaments form into 13 stranded microtubules, the multistranded structure of the FtsZ containing Z-ring is not known. Furthermore, there is controversy over the apparent cooperativity of single-stranded FtsZ polymer assembly since all established theoretical models for cooperative assembly require multistranded polymers.

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