Professor Richard Berry D. Phil.

Abstract:

Swimming Escherichia coli cells are propelled by the rotary motion of their flagellar filaments. In the normal swimming pattern, filaments positioned randomly over the cell form a bundle at the posterior pole. It has long been assumed that the hook functions as a universal joint, transmitting rotation on the motor axis through up to ∼90° to the filament in the bundle. Structural models of the hook have revealed how its flexibility is expected to arise from dynamic changes in the distance between monomers in the helical lattice. In particular, each of the 11 protofilaments that comprise the hook is predicted to cycle between short and long forms, corresponding to the inside and outside of the curved hook, once each revolution of the motor when the hook is acting as a universal joint. To test this, we genetically modified the hook so that it could be stiffened by binding streptavidin to biotinylated monomers, impeding their motion relative to each other. We found that impeding the action of the universal joint resulted in atypical swimming behavior as a consequence of disrupted bundle formation, in agreement with the universal joint model.

Abstract:

The reliable response to weak biological signals requires that they be amplified with fidelity. In E. coli, the flagellar motors that control swimming can switch direction in response to very small changes in the concentration of the signaling protein CheY-P, but how this works is not well understood. A recently proposed allosteric model based on cooperative conformational spread in a ring of identical protomers seems promising as it is able to qualitatively reproduce switching, locked state behavior and Hill coefficient values measured for the rotary motor. In this paper we undertook a comprehensive simulation study to analyze the behavior of this model in detail and made predictions on three experimentally observable quantities: switch time distribution, locked state interval distribution, Hill coefficient of the switch response. We parameterized the model using experimental measurements, finding excellent agreement with published data on motor behavior. Analysis of the simulated switching dynamics revealed a mechanism for chemotactic ultrasensitivity, in which cooperativity is indispensable for realizing both coherent switching and effective amplification. These results showed how cells can combine elements of analog and digital control to produce switches that are simultaneously sensitive and reliable. © 2012 Ma et al.

Abstract:

Bacterial cell envelopes often contain a flagellar motor – a reversible rotary nanomachine with an approximate diameter of 45nm – that allows cells to swim. Power is provided by the movement of H+ or Na+ down the electrochemical gradients across the cytoplasmic membrane, often termed the proton motive force or sodium motive force. A helical filament is rotated by each motor at several hundred revolutions per second. In many species, the motor switches direction stochastically; switching rates are controlled by a network of sensory and signaling proteins. The first direct observation, approximately 40 years ago, of the function of a single molecular motor was of the bacterial flagellar motor. Nevertheless, due to the large size and complexity of the motor, much remains to be discovered about this nanomachine, particularly the many structural details of the torque-generating mechanism. This chapter summarizes what has been learned about the structure and function of the motor with a focus on recent observations, particularly those obtained using single molecule techniques.