Nonideal polar residues in the core of the coiled coil of FtsLB are critical for modulating its stability and activation

The FtsLB complex is a critical regulator of bacterial cell division, acting as a switch that modulates cell wall reconstruction. Evidence indicates that FtsLB exists in either an off or on state which supports the corresponding activation state of the peptidoglycan synthase complex FtsWI. In Escherichia coli, residues within FtsLB that are critical for this activation are located in a region near the C-terminal end of the periplasmic coiled coil, raising questions about the precise role of this conserved domain in the mechanism. Here, we investigate an unusual cluster of polar amino acids occurring within the core of the coiled coil. 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 an intentionally "detuned" structural element. In addition, suppressor mutations were identified 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. Based on energetic and sequence propensity considerations, 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 polar amino acids participate in packing within the core, but their hydrophilic terminal moieties remain more favorably exposed to water than in the original four-helix bundle model ("I-model"). The Y-model remains well structured during molecular dynamics simulations, unlike the I-model, and satisfies all known experimental constraints. For this reason, we propose the Y-model as the configuration of the coiled coil of FtsLB and that a shift in this architecture, dependent on its marginal stability, is involved in activating the complex during the process that triggers septal cell wall reconstruction.