LINKING THE BACTERIAL CYTOSKELETON TO CELL WALL SYNTHESIS ACTIVATION: DISCOVERY OF THE [FTSZ-FZLA]-FTSWI SIGNALING PATHWAY

Bacterial division requires the coordination of cytoskeleton constriction and insertion of new peptidoglycan (PG) into the existing cell wall by PG synthase enzymes. At the launch of this work, we hypothesized that FtsZ filament curvature promotes cell constriction. We therefore chose to study FzlA, a division protein that stabilizes highly curved FtsZ filaments, as a tool for assessing the contribution of FtsZ filament curvature to constriction. We show that in Caulobacter crescentus, FzlA must bind to FtsZ for division to occur and that FzlA-mediated FtsZ curvature is correlated with efficient division. We observed that FzlA influences constriction rate, and that this activity is mildly correlated with its ability to bind and curve FtsZ polymers. Further, we found that a slowly constricting fzlA mutant strain develops “pointy” poles, suggesting that FzlA influences the relative contributions of radial versus longitudinal PG insertion at the septum. These findings implicate FzlA as a critical coordinator of envelope constriction rate through its interaction with FtsZ, though they did not strongly show that FtsZ curvature is a major driver efficient constriction in C. crescentus. We then asked if FzlA is able to regulate constriction because it signals through the division-specific PG synthesis pathway and discovered a promising genetic link. We found that hyperactive mutants of the PG synthases FtsW and FtsI specifically render fzlA, but not other division genes, non-essential. However, FzlA is still required to maintain proper constriction rate and efficiency in a hyperactive PG synthase background. Intriguingly, loss of fzlA in the presence of hyperactivated FtsWI causes cells to rotate about the division plane during constriction and sensitizes cells to cell-wall-specific antibiotics. We demonstrate that FzlA-dependent signaling to division-specific PG synthesis is conserved in another α-proteobacterium, Agrobacterium tumefaciens. These data establish that FzlA helps link FtsZ to cell wall remodeling and is required for signaling to both activate and spatially orient PG synthesis during division. Overall, our findings support the paradigm that activation of SEDS-PBP PG synthases is a broadly conserved requirement for bacterial morphogenesis