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Putting randomness to work: unique form of nanoscale random motion may drive key cellular functions

of a phosphate -- releases from the microtubule.
  • ATP hydrolysis makes the switch mechanism irreversible. Though ATP normally provides energy for macromolecular synthesis, Fox argues that in motor proteins ATP performs a switching role, changing the protein conformation and its binding affinity.
  • The unbound head -- just 5-7 nanometers in diameter -- is moved about randomly by Brownian motion in the cellular fluid until it encounters a new site where it can bind. Reported in the early 1800s by biologist Robert Brown, Brownian motion is the irregular activity of tiny particles suspended in a fluid. It results from the thermally driven movement of molecules in a fluid, the velocity of the particles depending on the temperature temperature.
  • Because of structural limits in the kinesin and spacing of binding sites on the microtubules, the moving head can reach only one possible binding site -- 8 nanometers past the bound head, which temporarily remains attached to the microtubule.
  • The head binds to the new site, moving the kinesin and its cargo about 8 nanometers along the microtubule.
  • The process quickly starts anew with the original two heads in interchanged positions.
  • "Normally, Brownian motion cannot do anything concerted or with directionality, because it is random," Fox explained. "But what happens here is a random process in a system that has asymmetric boundary conditions created by the ATP switching. That makes it possible to get a net directed motion along the microtubule."

    The model described by Fox and post-doctoral colleague Mee Hyang Choi depends on two unique properties of structures at the nanometer-scale: thermal energy can be a robust source of power, and random motion occurs very rapidly.

    "Normally, we would t
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    Contact: John Toon
    john.toon@edi.gatech.edu
    404-894-6986
    Georgia Institute of Technology Research News
    3-Jun-2001


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