A tale of the flagellar motor in Escherichia coli

Many species of bacteria swim by means of flagella attached to rotary motors in the cell membrane. The flagellar motors obtain energy from the membrane ion gradient and are capable of rotation in either clockwise or counterclockwise (CW/CCW) direction. Rotation involves a nonrotating (stator) and a rotating (rotor) part. The stator is formed from two membrane proteins MotA and MotB and the rotor is formed from multiple copies of three soluble proteins FliG, FliM and FliN. Recently, crystallography and biochemical studies have generated a model for the overall arrangement of proteins on the rotor. The molecular mechanisms of flagellar rotation, switching, and assembly remain unclear, however. Using various biochemical and genetic experiments, regions on FliM that are involved in switching and flagellar assembly were identified (Chapter 2). Additional binding and cross-linking experiments allowed development of a structural model for the lower part of the C-ring and a conformation change that occurs upon switching (Chapter 3).;Flagellar assembly involves an export process that had been thought to relay on ATP hydrolysis as an energy source. In a re-examination of this question, we have shown that energy for flagellar export comes from the proton gradient across the membrane and not from the hydrolysis of ATP. Based on our finding we proposed that flagellar export apparatus functions as a proton-driven protein pump (Chapter 6).;Bacterial motility is regulated by several cell-signaling pathway but the mechanism of action of many of these regulators is unknown. In this study, we have shown that how a DNA binding protein (HNS) and the signaling molecules (cyclic diguanylate (c-di-GMP) control bacterial motility by modulating the flagellar motor. The result indicate that HNS positively regulates motility by organizing FliG subunits in the motor (Chapter 4), while the c-di-GMP binding protein YcgR interacts with the flagellar motor to disrupt the organization of the C-terminal domain of FliG. This change in FliG conformation alters the rotor-stator interface and affects both torque generation and motor bias (Chapter 5).