Cyclic di-GMP mediated mechanisms on the “gearbox” of the bacterial flagellum

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Bacteria spend the majority of their time as cellular aggregates called biofilms, sporadically swimming to colonize new environments. To swim to these new habitats most bacteria use flagella, cell wall-embedded rotary motors that spin a helical filament to function as a molecular propeller. Crucially, flagella must be deactivated in the new habitat when forming a biofilm. Preliminary studies suggest that motor deactivation is the result of action of cyclic dimeric guanosine monophosphate (cdG), a central regulator of biofilm formation. In the model organism Caulobacter crescentus, multiple uncharacterized cdG binding proteins have recently been identified that bind the functional core of the flagellar motor, the “C-ring”, a multiprotein cytoplasmic ring that lies at the heart of torque generation, rotational direction switching, and signal response. Although these cdG-binding proteins have been shown to deactivate the motor, the underlying molecular mechanism remains unknown. I propose using electron cryo-tomography to directly visualize the conformation of the C. crescentus C-ring in situ when binding each cdG signaling protein to understand the mechanism of their action. To identify the effects of different signaling proteins, bacterial strains gutted of all but the relevant protein will be imaged; subsequently, the protein in question will be overexpressed in a high cdG background to saturate binding. To interpret observations I will also image C-ring mutants engineered to be locked in key conformational states. Work will be conducted in collaboration with the lab of Urs Jenal, experts in cdG action and discoverers of these cdG-binding proteins. Combined, I anticipate results will provide key insights into how cdG regulates the transition to biofilm formation.