The fellowship of the Z ring

TerBush and Osteryoung explore how two cytoskeletal proteins combine to promote chloroplast division.

FtsZ proteins are a family of tubulin-like GTPases that polymerize into “Z rings” at the equator of bacteria and chloroplasts, facilitating cell or organelle division. Bacteria have a single FtsZ gene, whereas plants express two, FtsZ1 and FtsZ2, which have unique functions in chloroplast fission despite coassembling into the organelle’s Z ring. The nature of these functions is unknown, however, so TerBush and Osteryoung investigated the properties of the plant FtsZ proteins by expressing them in fission yeast.

FtsZ1 and FtsZ2 self-assembled into a variety of different structures—including rings—inside yeast cells. FtsZ1 typically assembled into long cables, whereas FtsZ2 formed more elaborate networks. When the two proteins were coexpressed, they coassembled into structures resembling the FtsZ2 meshworks, suggesting that FtsZ2 is the main determinant of polymer morphology. FtsZ1, on the other hand, appears to control polymer dynamics. Photobleaching experiments revealed that subunit turnover was faster in FtsZ1 filaments than in FtsZ2 structures, but FtsZ2 became more dynamic when copolymerized with FtsZ1. FtsZ1 may therefore help remodel the Z ring as it constricts during chloroplast division.

Turnover is thought to depend on the GTPase activity of FtsZ proteins, and filaments formed by a GTPase-deficient version of FtsZ2 were completely immobile. A GTPase-dead FtsZ1 mutant still formed dynamic filaments, however, suggesting that FtsZ1-containing structures may be less stable because FtsZ1 subunits form weaker interactions with their neighbors. The researchers now want to test this hypothesis by investigating the structure of the interface between FtsZ proteins.