The coiled-coil domain of Escherichia coli FtsLB is a structurally detuned element critical for modulating its activation in bacterial cell division

Samuel J Craven,Samson G.F Condon,Gladys Díaz Vázquez,Qiang Cui,Alessandro Senes

doi:10.1016/j.jbc.2021.101460

Samuel J Craven, Samson G.F Condon + Show 3 more

Open Access

https://doi.org/10.1016/j.jbc.2021.101460

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Abstract

The FtsLB complex is a key regulator of bacterial cell division, existing in either an off state or an on state, which supports the activation of septal peptidoglycan synthesis. In Escherichia coli, residues known to be critical for this activation are located in a region near the C-terminal end of the periplasmic coiled-coil domain of FtsLB, raising questions about the precise role of this conserved domain in the activation mechanism. Here, we investigate an unusual cluster of polar amino acids found within the core of the FtsLB coiled coil. We hypothesized that these amino acids likely reduce the structural stability of the domain and thus may be important for governing conformational changes. We found that mutating these positions to hydrophobic residues increased the thermal stability of FtsLB but caused cell division defects, suggesting that the coiled-coil domain is a “detuned” structural element. In addition, we identified suppressor mutations within the polar cluster, indicating that the precise identity of the polar amino acids is important for fine-tuning the structural balance between the off and on states. We propose a revised structural model of the tetrameric FtsLB (named the “Y-model”) in which the periplasmic domain splits into a pair of coiled-coil branches. In this configuration, the hydrophilic terminal moieties of the polar amino acids remain more favorably exposed to water than in the original four-helix bundle model (“I-model”). We propose that a shift in this architecture, dependent on its marginal stability, is involved in activating the FtsLB complex and triggering septal cell wall reconstruction.

Highlights

Cell division in bacteria is a complex process involving intricate coordination between numerous cellular components
The polar cluster of FtsLB is an unusual feature for coiled-coil structures “a” and “d” positions of coiled coils tend to be mainly hydrophobic, polar amino acids can occur there
To determine if the coiled coil of FtsLB is unusually rich in interfacial polar amino acids in comparison with other coiled coils, we performed a structural analysis of the 2,662 crystal structures available in the CC+ database[45]

Summary

Introduction

Cell division in bacteria is a complex process involving intricate coordination between numerous cellular components. Journal Pre-proof These proteins mediate the various functions necessary for cell division, including establishing the site of division, coordinating invagination of the inner and outer membranes, and remodeling the cell wall at midcell into a septum to compartmentalize the nascent daughter cells If any of these functions is abrogated through deletion or mutation of essential proteins, the bacteria can continue to elongate and replicate their DNA, but they will be unable to divide and form distinct daughter cells. FtsLB regulates FtsWI septal PG synthesis activity by transitioning from an off state to an on state in response to a signal from FtsN (Fig. 1c) This idea was initially proposed following the identification of a series of gain-of-function mutations within both FtsL and FtsB that enable survival in the absence of the normally essential FtsN21,24. This revised model fits the available evidence as well as the original I-model, while displaying better behavior, and we propose it as the more likely candidate for the structural organization of FtsLB

Results and Discussion

Conclusions

Experimental Procedures Plasmid cloning