Third track model for coordination of septal peptidoglycan synthesis and degradation by FtsN in Escherichia coli.

In Escherichia coli, FtsN is thought to coordinate septal peptidoglycan (sPG) synthesis and degradation. Its E domain interacts with the sPG synthesis complex, FtsWIQLB, and its SPOR domain interacts with denuded glycan (dnG), intermediates of sPG degradation. Here we used single-molecule tracking of FtsN and FtsW to investigate how FtsN coordinates the two opposing processes. We found that the SPOR domain binds to dnG cooperatively. This binding sequesters FtsWIQLB on dnG, which we call the dnG-track, and prevents dnG degradation. SPOR domain's release from dnGs exposes dnGs to degradation, moves FtsN to the sPG synthesis track and activates FtsWIQLB. In addition, FtsN self-interacts through the SPOR domain, promoting the multimerization of FtsWIQLB on both tracks. This self-interaction may create a sensitive switch, regulating FtsN's partitioning between dnG- and sPG-tracks to coordinate sPG degradation and synthesis while also controlling the balance between sequestered and active populations of the sPG synthesis complex. Our data reveal a third track that plays an important role in sPG synthesis and degradation across space and time, complementing the previously discovered sPG-track and FtsZ-track in E. coli for robust septal cell wall constriction.