| Cell division is intimately coordinated with the cell cycle in the developmental model organism, Caulobacter crescentus. This Gram-negative bacterium has two cell types: a non-replicating, flagellated swarmer cell and a replicatine stalked cell. To gain insights into how cell division is coordinated with the cell cycle, and analyze the roles served by FtsA and FtsQ in cell division, the temporal and spatial dynamics of FtsA and FtsQ were characterized at different stages of development. Western analysis of a synchronous population of each cell type revealed that the concentrations of FtsA and FtsQ peak in pre-divisional cells, where they are required to stimulate deep constriction of the cell. Both FtsA and FtsQ were depleted in specific cell types, despite their expression from a constitutive promoter. Thus, these proteins are regulated in a post-transcriptional manner. Pulse-chase analysis demonstrated that FtsA was rapidly degraded in swarmer cells, but stabilized in stalked and pre-divisional cells, demonstrating that the level of FtsA is regulated by proteolytic control.;FtsA and FtsQ were seen to localize predominantly to the mid-cell by immunofluorescence microscopy, At the mid-cell, they are ideally situated to facilitate their role in cytokinesis and possibly be stabilized by interactions with the division complex. Over-expression of FtsA blocked the assembly of FtsZ into a mature ring (Z-ring) at the mid-cell, while depletion of FtsA, or FtsQ and FtsA, stabilized both FtsZ protein and Z-rings. Thus, appearance of FtsA in pre-divisional cells may induce disassembly of the Z-ring.;In summary, C. crescentus carefully regulates the transcription, abundance, localization and degradation of multiple proteins involved in cell division, in part through their interactions with each other. One advantage of such extensive, and seemingly redundant, regulation may be to install many checkpoints that monitor and adjust the progression of cell division in the context of cell development. |
