FtsN activates septal cell wall synthesis by forming a processive complex with the septum-specific peptidoglycan synthase in E. coli
The FtsN protein of Escherichia coli and other proteobacteria is an essential and highly conserved bitopic membrane protein that triggers the inward synthesis of septal peptidoglycan (sPG) during cell division. Previous work has shown that the activation of sPG synthesis by FtsN involves a series of interactions of FtsN with other divisome proteins and the cell wall. Precisely how FtsN achieves this role is unclear, but a recent study has shown that FtsN promotes the relocation of the essential sPG synthase FtsWI from an FtsZ-associated track (where FtsWI is inactive) to an sPG-track (where FtsWI engages in sPG synthesis). Whether FtsN works by displacing FtsWI from the Z-track or capturing/retaining FtsWI on the sPG-track is not known. Here we use single-molecule imaging and genetic manipulation to investigate the organization and dynamics of FtsN at the septum and how they are coupled to sPG synthesis activity. We found that FtsN exhibits a spatial organization and dynamics distinct from those of the FtsZ-ring. Single FtsN molecules move processively as a single population with a speed of [~] 9 nm s-1, similar to the speed of active FtsWI molecules on the sPG-track, but significantly different from the [~] 30 nm s-1 speed of inactive FtsWI molecules on the FtsZ-track. Furthermore, the processive movement of FtsN is independent of FtsZs treadmilling dynamics but driven exclusively by active sPG synthesis. Importantly, only the essential domain of FtsN, a three-helix bundle in the periplasm, is required to maintain the processive complex containing both FtsWI and FtsN on the sPG-track. We conclude that FtsN activates sPG synthesis by forming a processive synthesis complex with FtsWI exclusively on the sPG-track. These findings favor a model in which FtsN captures or retains FtsWI on the sPG-track rather than one in which FtsN actively displaces FtsWI from the Z-track.