Crosslinking by ZapD drives the assembly of short FtsZ filaments into toroidal structures in solution

In most bacteria, division depends on a cytoskeletal structure, the Z ring, which serves as a scaffold for recruiting additional proteins, with which it forms the machinery responsible for division, the divisome. The detailed architecture of the ring, in particular the mechanisms of assembly, stabilization, and disassembly, are still largely unknown. Here, we highlight the role of FtsZ-associated proteins (Zaps) in stabilizing the Z ring by crosslinking the filaments. Among Zap proteins, ZapD binds the C-terminal domain of FtsZ, which serves as a hub for its regulation. We demonstrate that ZapD crosslinks FtsZ filaments in solution into toroidal structures formed by an arrangement of short, curved filaments. Using cryo-electron tomography combined with biochemical analysis, we reveal the three-dimensional organization of FtsZ within the toroids, shedding light on the crosslinking mechanism by ZapD. In spite of the compositional simplicity of our reconstituted system, the structural organization of the FtsZ polymers by ZapD appears to be compatible with the current model of the Z ring in the bacterial cell.