Structures of FtsZ from a cell-wall less bacterium Spiroplasma provide a mechanism for kinetic polarity

FtsZ, the tubulin homolog essential for bacterial cell division, assembles as Z-ring at the division site, and directs peptidoglycan synthesis by treadmilling. To obtain insights into fundamental features of FtsZ assembly dynamics independent of peptidoglycan synthesis, we characterized the FtsZ from the cell wall-less bacteria, Spiroplasma melliferum (SmFtsZ). SmFtsZ was found to be a slower GTPase and has higher critical concentration (CC) for polymerization compared to Escherichia coli FtsZ (EcFtsZ). In FtsZs, a conformational switch from R (close)- to T (open)- state favors polymerization. In FtsZs, a conformational switch from R (close)- to T (open)- state favors polymerization. We identified a residue, Phe224, located at the cleft between N-terminal domain (NTD) and C-terminal domain (CTD) of SmFtsZ, which is crucial for R- to T-state transition. The mutation F224M in SmFtsZ cleft resulted in higher GTPase activity and lower CC, whereas the corresponding M225F in EcFtsZ resulted in cell division defects in E. coli. Our results demonstrate that relative rotation of the domains is a rate-limiting step of polymerization. Our structural analysis of interdomain interactions suggests that R- to T-state transition likely follows addition of a GTP-bound monomer to the filament through interaction of the preformed NTD. Hence, the addition of monomers to the NTD-exposed end of filament is slower in comparison to the C-terminal domain end, thus supporting the phenomenon of kinetic polarity in a single protofilament assembly.