Fig. 8: A working model for the self-enhancing effect of the E. coli divisome.

In newborn cells, FtsZ filaments anchored to the membrane by FtsA (and ZipA, not shown) accumulate at midcell as a loosely organized structure (nascent Z ring). Zap proteins promote Z ring condensation by crosslinking FtsZ filaments. Once a mature Z ring is formed, the sPG cleavage complex FtsEX-EnvC, the sPG synthase complex FtsQLBWI, and FtsN are recruited sequentially to form the complete divisome. FtsN activates sPG synthesis by switching FtsA and the FtsQLBWI complex to the active state. Newly synthesized sPG is processed by the FtsEX-EnvC-amidase system to denuded glycan strands, which recruit additional FtsN molecules via the SPOR domain to activate more FtsQLBWI complexes, forming a positive feedback loop of sPG synthesis (sPG loop). Meanwhile, sPG serves as a beacon for FtsZ filaments via the interaction between the FtsQLBWIN complex on the sPG track and FtsA, which anchors FtsZ filaments to the membrane, thus promoting Z ring condensation and divisome stability. In the absence of Zap proteins, FtsZ filaments cannot condense into Z ring effectively, resulting in a division delay. However, superfission mutations facilitate Z ring condensation and stability by prematurely activating and increasing sPG synthesis, thus compensating for the loss of Zap proteins. PG: peptidoglycan; sPG: septal peptidoglycan; IM: inner membrane. ZapA is depicted as a dark-red dumbbell, ZapC as a red ball and ZapD as pink half-overlay ovals. For simplicity, many division proteins are not shown.