FtsZ

FtsZ is a protein encoded by the ftsZ gene that assembles into a ring at the future site of bacterial cell division. FtsZ is a prokaryotic homologue of the eukaryotic protein tubulin. The initials FtsZ mean 'Filamenting temperature-sensitive mutant Z'. The hypothesis was that cell division mutants of E. coli would grow as filaments due to the inability of the daughter cells to separate from one another. FtsZ is found in almost all bacteria, many archaea, all chloroplasts and some mitochondria, where it is essential for cell division. FtsZ assembles the cytoskeletal scaffold of the Z ring that, along with additional proteins, constricts to divide the cell in two. FtsZ is a protein encoded by the ftsZ gene that assembles into a ring at the future site of bacterial cell division. FtsZ is a prokaryotic homologue of the eukaryotic protein tubulin. The initials FtsZ mean 'Filamenting temperature-sensitive mutant Z'. The hypothesis was that cell division mutants of E. coli would grow as filaments due to the inability of the daughter cells to separate from one another. FtsZ is found in almost all bacteria, many archaea, all chloroplasts and some mitochondria, where it is essential for cell division. FtsZ assembles the cytoskeletal scaffold of the Z ring that, along with additional proteins, constricts to divide the cell in two. In the 1960s scientists screened for temperature sensitive mutations that blocked cell division at 42° C. The mutant cells divided normally at 30°, but failed to divide at 42°. Continued growth without division produced long filamentous cells (Filamenting temperature sensitive). Several such mutants were discovered and mapped to a locus originally named ftsA, which could be one or more genes. In 1980 Lutkenhaus and Donachie showed that several of these mutations mapped to one gene, ftsA, but one well-characterized mutant, PAT84, originally discovered by Hirota et al, was a separate, adjacent gene. They named this cell division gene ftsZ. In 1991 Bi and Lutkenhaus used immunogold electron microscopy to show that FtsZ localized to the invaginating septum at midcell. Subsequently immuno-fluorescence microscopy and GFP fusions showed that FtsZ assembled Z rings early in the cell cycle, well before the septum began to constrict. Other division proteins then assemble onto the Z ring and constriction occurs in the last part of the cell cycle. In 1992-3 three labs independently discovered that FtsZ was related to eukaryotic tubulin, which is the protein subunit that assembles into microtubules. This was the first discovery that bacteria have homologs of eukaryotic cytoskeletal proteins. Later work showed that FtsZ was present in, and essential for cell division, in almost all bacteria and in many but not all archaea. Mitochondria and chloroplasts are eukaryotic orgenelles that originated as bacterial endosymbionts, so there was much interest in whether they use FtsZ for division. Chloroplast FtsZ was first discovered by Osteryoung, and it is now known that all chloroplasts use FtsZ for division. Mitochondrial FtsZ was discovered by Beech in an alga; FtsZ is used for mitochondrial division in some eukaryotes, while others have replaced it with a dynamin-based machinery.   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 associated with it. The origin of the cytokinetic force, thus, remains unclear, but it is believed that the localized synthesis of new cell wall produces at least part of this force. In liposomes Osawa (2009) showed FtsZ is capable of exerting a contractile force with no other proteins present. Erickson (2009) proposed how the roles of tubulin-like proteins and actin-like proteins in cell division became reversed in an evolutionary mystery.The use of the FtsZ ring in dividing chloroplasts and some mitochondria further establishes their prokaryotic ancestry. L-form bacteria that lack a cell wall do not require FtsZ for division, which implies that bacteria may have retained components of an ancestral mode of cell division. 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. It is only speculated that the structure consists of overlapping protofilaments. Nevertheless, recent work with purified FtsZ on supported lipid bilayers as well as imaging FtsZ in living bacterial cells revealed that FtsZ protofilaments have polarity and move in one direction by treadmilling (see also below). Recently, proteins similar to tubulin and FtsZ have been discovered in large plasmids found in Bacillus species. They are believed to function as components of segrosomes, which are multiprotein complexes that partition chromosomes/plasmids in bacteria. The plasmid homologs of tubulin/FtsZ seem to have conserved the ability to polymerize into filaments. FtsZ has the ability to bind to GTP and also exhibits a GTPase domain that allows it to hydrolyze GTP to GDP and a phosphate group. In vivo, FtsZ forms filaments with a repeating arrangement of subunits, all arranged head-to-tail. These filaments form a ring around the longitudinal midpoint, or septum, of the cell. This ring is called the Z-ring.