Structural and functional studies on the homeostasis and type-III-secretion of flagellin

The ability to move towards favorable and avoid unfavorable conditions is key to the survival of many bacterial species. Bacterial movement relies on a sophisticated nanomachine, the flagellum. Despite being one of the tiniest motors in the biosphere, the flagellum exhibits a complex architecture and is composed of more than 30 different proteins in diverse stoichiometries. Flagellar architecture can be subdivided into a membrane-embedded basal body, a hook and a long helical filament. The process of flagellar assembly involves a plethora of accessory factors and is organized at different stages on the transcriptional, post-transcriptional and translational level. Furthermore, biogenesis of a flagellum is strictly sequential and requires the completion of a building phase prior to initiating the next one. The most abundant constituent within a flagellum is the protein flagellin that assembles into a helical filament with more than 20.000 monomers. The two proteins CsrA and FliW regulate flagellin homeostasis via a posttranscriptional mechanism only allowing flagellin translation when cytoplasmic levels are low, thereby ensuring that flagellin is directly secreted after production. A third protein, the intrinsic chaperone FliS is essential for the recognition and efficient secretion of flagellin. Together these proteins couple translation to secretion of flagellin and keep cytoplasmic flagellin concentrations around a low and narrow threshold. This work aims at unraveling the molecular mechanisms by which the above-named proteins regulate flagellin homeostasis. In enterobacteria CsrA activity is antagonized by small non-coding RNAs (sRNAs) that act as competitive inhibitors. Conversely, the FliW protein allosterically controls CsrA in a variety of flagellated bacteria, which seems to represent the ancestral state of CsrA regulation. This work furthermore demonstrates that interaction of FliW and flagellin seems to be cotranslational or strongly associated with translating ribosomes, therefore coupling homeostasis and secretion. Another level of regulation elucidated in this study is the influence of bactofilins on the process of flagellar biogenesis. This ubiquitous class of proteins is reminiscent of cytoskeleton factors but seems to rather provide a dynamic scaffold for diverse processes. In B. subtilis the bactofilins BacE and BacF are involved in flagellar assembly at the stage of hook-completion but BacE also directly interacts with FliW. Finally, this work includes a model explaining the coupling of homeostasis and secretion of flagellin at atomic resolution