FtsZ

FtsZ assembles the cytoskeletal scaffold of the Z ring that, along with additional proteins, constricts to divide the cell in two.

Several such mutants were discovered and mapped to a locus originally named ftsA, which could be one or more genes.

In 1991 Bi and Lutkenhaus used immunogold electron microscopy to show that FtsZ localized to the invaginating septum at midcell.

[3] Subsequently, the Losick and Margolin groups used immuno-fluorescence microscopy[4] and GFP fusions[5] to show that FtsZ assembled Z rings early in the cell cycle, well before the septum began to constrict.

In 1992-3 three labs independently discovered that FtsZ was related to eukaryotic tubulin, which is the protein subunit that assembles into microtubules.

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 organelles that originated as bacterial endosymbionts, so there was much interest in whether they use FtsZ for division.

Supporting this, in E. coli the rate of division is affected by mutations in cell wall synthesis.

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.

Recently, proteins similar to tubulin and FtsZ have been discovered in large plasmids found in Bacillus species.

Computer models and in vivo measurements suggest that single FtsZ filaments cannot sustain a length more than 30 subunits long.

This is akin to the creating of a temporary structure by construction workers to access hard-to-reach places of a building.

The scaffold theory is supported by information that shows that the formation of the ring and localization to the membrane requires the concerted action of a number of accessory proteins.

Recent super-resolution imaging in several species supports a dynamic scaffold model, in which small clusters of FtsZ protofilaments or protofilament bundles move unidirectionally around the ring's circumference by treadmilling, anchored to the membrane by FtsA and other FtsZ-specific membrane tethers.

However, other species including Streptococcus pneumoniae and Myxococcus xanthus seem to use positive regulators that stimulate FtsZ assembly at mid-cell.

So far, two proteins have been identified in E. coli and B. subtilis that prevent division over the nucleoid region: Noc and SlmA.

The mechanism is not well understood, but thought to involve sequestration of FtsZ, preventing polymerization over the nucleoid region.