Understanding protein interaction and turnover in the bacterial flagellar motor

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The majority of bacterial species can move, many swimming. They use swimming to reach their best environment for growth and survival. For pathogenic bacteria that can be a wound, or the lung or gut wall, for beneficial bacteria it could be a root, while for many bacteria it might be a surface where they can grow as a slime protected colony. Swimming uses a mechanism unique to bacteria, making it a possible site for controlling bacterial activity such as colonisation. However, to control swimming behaviour, and thus colonisation, we need to know how swimming works. Swimming relies on the rotation of rigid, helical flagellar filaments, the diameter of just over a hundredth of a millimetre. It was initially thought the bacterial flagellar filament was rotated by a structure in the bacterial membrane that operated like a tiny, stable electric motor with a ring of proteins, rotor proteins, anchored to the helical filament, which rotate against a fixed ring of proteins (stators) anchored to the cell wall. As charged particles move through the stator proteins a change in local charge interactions between the stator and rotor proteins drives the rotor ring round, at speeds of up to 1300 rev per sec. While this general mechanism is correct, a few years ago we showed that the proteins within a rotating motor are not in fixed rings, but some of the proteins within the rings are swapping with free proteins even though the motor is still spinning-rather like changing rotating parts of a car engine while driving at full speed down the motorway. Indeed more recently we found that if you removed the driving force, the ion gradient, altogether the supposedly anchored ring of stator proteins simply diffuses away from the rotor into the bacterial membrane, returning to the rotor one at a time only when the driving force is reinstated. The motor can also "feel" outside forces on the motor and adds additional components as the external force increases, as happens when the environment gets more viscous, as in a mucous gut or lung wall or when encountering a surface, allowing rotation to continue. The bacterial motor is therefore continually remodelling to allow swimming under a range of conditions. It has been known for a while that unrelated proteins that change concentration or activity during different growth conditions can alter swimming behaviour. It now seems likely that while remodelling these other proteins can "slip" in to the motor and interact with motor proteins to either slow or stabilise the motor structure and alter activity. In this study we intend to (1) characterise the effect on motor structure of these non-motor proteins, (2) to identify the changes that occur in the motor structure when it rotates under different conditions and (3) compare these changes across very different species with motors showing a common core, but with different rotational behaviours. This will allow the development of a more complete understanding of the dynamics of motor proteins and protein:protein interaction that could lead to the design of molecules that will interfer with that rotation and prevent colonisation.