A key step in bacterial cell division is the formation of the Z-ring composed of polymers of the tubulin-like protein FtsZ. The Z-ring constricts and through interaction with other components engages in remodelling the cell wall and membranes in order to yield two daughter cells. During this process, FtsZ is known to interact with FtsA, which is an early component of the Z-ring, and then recruits other components of the divisome, the cell division apparatus. Analysis of FtsA sequences revealed a conserved C-terminal motif, which is predicted to form an amphipathic helix and has been shown to localise FtsA (and with it FtsZ) to the membrane. FtsA belongs to the actin/HSP70 family of proteins, although its fold was shown to deviate by a subdomain deletion and addition. Many members of this family are able to form filaments, and it is a long-standing question whether FtsA is able to polymerise.

In this study, I reconstituted the tethering of FtsZ to the membrane via FtsA’s C-terminal amphipathic helix in vitro using T. maritima proteins. A crystal structure of the FtsA:FtsZ interaction revealed 16 amino acids of the FtsZ tail bound to subdomain 2B of FtsA. The same structure and a second crystal form of FtsA revealed that FtsA forms actin-like protofilaments with a repeat of 48 Å. An identical repeat length was observed when FtsA was polymerised using a lipid monolayer surface and FtsAs from three organisms formed polymers in cells when overexpressed, as observed by electron cryo-tomography. Mutants that disrupt polymerisation also showed a significant cell division phenotype in a temperature sensitive FtsA background, demonstrating the importance of filament formation for FtsA’s function in the Z-ring.