Flagellar Motor Biogenesis in Polarly-Flagellated Bacterial Pathogens

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Project Summary Bacterial flagella function as two machines: a type III secretion system (T3SS) that secretes most of the extracytoplasmic components of the flagellum and a reversible rotary motor that promotes swimming motility required for many bacteria pathogens to infect hosts to promote disease. Peritrichous flagellates such as E. coli and Salmonella species have served as models to understand various processes for flagellar biogenesis and function. However, many significant bacterial pathogens, including Campylobacter jejuni, Vibrio cholerae, Helicobacter pylori, and Pseudomonas aeruginosa are polar flagellates that produce flagellar motors in limited numbers only at polar regions. These polar flagellar motors are characteristically more structurally complex than their peritrichous counterparts. By using C. jejuni as a model system to understand polar flagellar motor biogenesis and function in bacterial pathogens, we found that the increased structural complexity in polar flagellar motors is due to both unique structures and conserved substructures formed by a different collection of proteins. Importantly, these structural alterations enhance mechanical functions of both the C. jejuni flagellar T3SS and the rotary motor. We discovered that the C. jejuni flagellar T3SS has an enhanced ability to secrete flagellar proteins and assemble flagella even when lacking components usually essential for other T3SSs to function. Furthermore, we discovered the structural alterations in the C. jejuni flagellum contribute to a common feature of polar flagellar motors of many pathogens – the generation of higher torque for high motility velocities in a range of physiological viscosities. We also identified C. jejuni flagellar T3SS and motor components that promote its ability to multitask in cellular activities beyond motility. The major goal of this proposal is to analyze how these specific structures and substructures form and adapt the C. jejuni flagellum with enhanced mechanics to augment its function as a secretory machine and rotary motor. In Aim 1, we will analyze the composition and arrangement of proteins forming the C. jejuni flagellar T3SS and determine how fueling mechanics are altered relative to other T3SSs to allow it to secrete proteins and assemble flagella even without usually essential parts. In Aim 2, we will analyze how novel disk structures form and how the rotor component of the motor has expanded to affect the number and placement of stator units required to generate high torque for flagellar rotation and a high velocity of motility. In Aim 3, we will explore two C. jejuni proteins that we hypothesize function as a unique molecular brake or clutch to control output of a high-torque polar flagellar motor and regulate optimal motility velocities in different viscosities. Completion of these aims will provide new insights into many bacterial pathogens for: 1) how T3SSs can alter and ensure fueling to enhance secretory activity and organelle assembly; 2) how polar flagellar motors are naturally endowed to generate high torque for propulsion; 3) how novel substructures of polar flagellar motors form; and 4) how motility velocities facilitated by high-torque polar flagellar motors can be modulated in environments with different viscosities.