Putting the brakes on - biomechanics at the nanoscale

Researchers at the Department of Biochemistry and Department of Physics investigated the mechanics of a 45 nm rotary biological motor. Research was carried out to discover how a bacterium, Rhodobacter sphaeroides, can stop and start its motor at the flick of a switch. Work was carried out by D.Phil student Mostyn Brown and Dr Teuta Pilizota from Prof Judy Armitage's lab and Dr Richard Berry's lab respectively; they have recently published their findings in the journal P.N.A.S.1

The project focussed on the bacterial flagellar motor, a complex machine made from several hundred proteins. The motor rotates a propeller at speeds in excess of 10,000 rpm, driven only by the proton motive force (i.e. the flow of ions down their electrochemical gradient through the motor). The cell forms this proton motive force during electron transport e.g. respiration or photosynthesis.

'The bacterial flagellar motor is a remarkably complex rotary machine - we want to learn how it works to aid the design and operation of synthetic rotary nano-motors of our own'

Most bacterial flagellar motor research is carried out on species such as E. coli which use motors that can rotate in two directions, depending on whether the cell wants to swim or change direction. However, the flagellar motor found in R. sphaeroides only rotates in one direction - the cell changes direction by stopping its motor and letting Brownian motion do the rest. Research was conducted to discover how the R. sphaeroides motor is able to start and stop at the cell's discretion. There were two broad possibilities - it either uses a clutch or a brake.

To discriminate between these two hypotheses the researchers applied external torque to the nanoscale motors. Cells were fixed down to microscope slides by their propellers so that the motors pushed the cell body around (this is analogous to watching a ship's hull rotate about its propeller).

External torque was first administered by flowing fluid past the cells. If a stop was achieved using a clutch we expected cell bodies to orientate with the flow at 0° (Fig. 1A). In contrast, they found the cell body was held at particular angles irrespective of the flow direction, indicating that the motor was locked with a brake (Fig. 1B).

Fig. 1 The cell bodies resisted reorientation when external torque was applied showing that motors were stopped using a brake not a clutch

Fig. 1 The cell bodies resisted reorientation when external torque was applied showing that motors were stopped using a brake not a clutch

In order to quantify the strength of the lock, external torque was applied using the cell body as a handle for an optical trap Fig. 1C). More than double the amount of torque produced by the motor (when running) had to be applied in order to move the motor.

The researchers then went on to discover that the motor stops at 27-28 discrete angles (see Fig 2), which most likely reflects the periodicity of the proteins present in the rotor ring. This work demonstrated how one species controls one of its molecular motors. By exposing Nature's motors to additional scrupulous tests the researchers hope to learn enough, so that one day, they can build and exploit nano-machines of their own.

Fig. 2 Stop angles over time. Steps are 13-14° apart corresponding to 27-28 steps per revolution

Fig. 2 Stop angles over time. Steps are 13-14° apart corresponding to 27-28 steps per revolution

1. T. Pilizota*, M.T. Brown*, M.C. Leake, R.W. Branch, R.M. Berry, and J.P. Armitage (2009) A molecular brake, not a clutch, stops the Rhodobacter sphaeroides flagellar motor. Published in the Proceedings of the National Academy of Sciences USA 106, 11582-11587. http://www.pnas.org/content/106/28/11582.full.pdf+html *These authors contributed equally to this work.





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