A model of the interactions between the FtsQLB and the FtsWI peptidoglycan synthase complex in bacterial cell division

Abstract

In Escherichia coli, an important step in the divisome assembly pathway is the recruitment of the essential cell wall synthase complex FtsWI to the division site through interactions with the regulatory FtsQLB complex. Here, we investigate a key aspect of this recruitment by characterizing the structural organization of the FtsL-FtsW interaction. Mutations in the cytoplasmic and transmembrane regions of the two proteins result in cell division defects and loss of FtsW localization to division sites. We use these in vivo results to help validate the predicted interfaces from an AlphaFold2 model for the entire FtsQLBWI complex. Given the consistency between the predicted FtsQLBWI model and our current understanding of the structure and function of the complex, we further remodeled it, seeking insight into the potential structural transitions that may lead to activation of the FtsWI complex and PG synthesis. The model suggests that FtsLB serves as a support for FtsI, placing its periplasmic domain in an extended and possibly active conformation but it is also compatible with a proposed compact and possibly inactive conformation. Additionally, we reconfigure the model into an Fts[QLBWI]2 diprotomeric state, which suggests that FtsLB may act as a central hub during assembly of the PG synthesis machinery. Finally, we propose a possible role for FtsQ in activation of this machinery, potentially by acting as a gatekeeper for the interaction between the FtsL AWI region and FtsI. We propose that this gatekeeping function depends on a hinge next to the FtsLB CCD region, which has implications for the mechanisms behind the FtsLB off/on transition that is central to cell division regulation.

Conformational changes in the essentialE. coliseptal cell wall synthesis complex suggest an activation mechanism

“…S3 ). This mechanism differs in specific details from those proposed in recent work that also drew from structure prediction 21,43,53 . Despite their differences, each of these hypotheses involves allostery linking periplasmic interactions to distal active sites.…”

“…The β-sheet formed between FtsQ A252-A257 (β12) and FtsB T83-P89 (β1) has been previously reported 42 ( Fig. S5B ), but that between FtsL N116-Q120 (β1) and FtsI E575-I578 (β16) has not been observed experimentally and was only recently identified by structure prediction 43 . FtsL indirectly interacts with FtsQ via FtsB within this β-sheet.…”

“…and FtsI E575-I578 (β16) has not been observed experimentally and was only recently identified by structure prediction 43 . FtsL indirectly interacts with FtsQ via FtsB within this β-sheet.…”

“…In predictions, FtsW K370 blocks a putative substrate channel 37 , suggesting that FtsW K370 conformation can regulate substrate or product transport. We also did not address in detail the hypothesis of a diprotomeric Fts[QLBWI] 2 complex or the role of cytoplasmic FtsL-FtsW interactions 43 . Nevertheless, we observed stable FtsL-FtsW cytoplasmic interaction in our simulations even though the N-terminal cytoplasmic tail of FtsW was truncated.…”

A unique cell division protein critical for the assembly of the bacterial divisome

“…Among these, the FtsQLB complex not only serves as a scaffold for recruitment of subsequent divisome proteins (43, 44), but also plays a an important role in regulating the activity of the divisome (34, 37). The constriction control domain (CCD) of FtsL was previously defined as a critical component for switching the conformation of FtsLB from an off to an on state in order to activate FtsWI by enabling the interaction between FtsI peri and the AWI (activation of FtsWI) region of FtsL (34, 37, 45). Division obstruction caused by Aeg1 depletion can be effectively circumvented by suppression mutations (FtsB E65A ) in its CCD domain and by activation mutations (M254I and S274G) of FtsW ( Fig.…”