New research into the activity
of a key "motor" protein suggests that a unique
form of random motion powered by thermal energy
may play a vital role in moving enzymes and other
chemicals inside cells.
Beyond providing a better understanding of
subcellular functions, the National Science
Foundation-sponsored work may offer a new
mechanism for generating motion in future
nanometer-scale machines.
Within the cells
of the body, kinesin proteins work like "cellular
tow trucks" to pull tiny sacks of chemicals along
pathways known as microtubules. The accepted
explanation for this motion is that the kinesins
use their two leg-like "heads" to walk along the
microtubule paths in a deliberate way, fueled by
the energy molecule adenosine triphosphate
(ATP).
But in a paper published in the May
issue of the journal Physical Review E,
Georgia Institute of Technology physicist Ronald
Fox argues that what appears to be a walk along
the microtubule is really random motion cleverly
constrained by chemical switching carried out by
ATP.
"These are certainly not motors in
the sense of burning a fuel and having a concerted
effort in a one-directional way," said Fox. "If
you could see them, their walk would appear to be
more like a drunken sailor than a concerted
motion."
Composed of fibrous proteins, the
microtubules include sites approximately 8
nanometers apart where kinesin heads can bind
chemically. To move along this pathway, Fox argues
that the kinesins use "rectified Brownian motion"
in the following steps:
* ATP binds to the
leading head that is initially tightly bound to
the microtubule and switches its conformation so
that it is weakly bound to the microtubule. The
kinesin's trailing head -- to which adenosine
diphosphate (ADP) is still bound after ATP
hydrolysis and release 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.
* 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 postdoctoral 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
think of Brownian motion -- or diffusion -- as a
very slow process," he noted. "But when you are on
the nanometer scale, Brownian motion is a very
rapid way to do things. Even though it is random,
it allows you to explore all the possibilities
very rapidly."
Using optical tweezers and
other sophisticated techniques, biologists have
studied the activity of kinesins, measuring their
speed, pulling power and use of ATP. For instance,
they can move at velocities of up to 1,000
nanometers a second, and exert forces of as much
as 6 piconewtons.
Richard Fishel, professor
of microbiology and immunology at the Kimmel
Cancer Institute in Philadelphia, Pa., studies how
DNA repair genes locate and bind with damaged DNA.
He believes all nucleotide-dependent processes
really involve a switching mechanism, and says
Fox's model explains how that works for a broad
range of systems in the context of thermally
powered Brownian motion.
"Ron Fox has now
set the clear physical dimensions for how that has
to work, and how it has to work is by rectified
Brownian motion," he said. "The rectification of
those Brownian events is done by the small
molecule exchange of ATP for ADP. The energy that
drives that process is what's
important."
The switching operation that
involves acceptance or removal of a phosphate
controls a protein's affinity for binding to other
proteins it encounters as Brownian motion moves it
through cells.
"What Ron has provided here is the physical
reasoning behind how these collisions can work and
the conformational transitions that are
rectified," he explained.
The role of
Brownian motion in cellular activity has been
discussed before, but new experimental results and
high-level mathematics in Fox's model provide the
strongest evidence yet to support it. Though the
experimental results are consistent with the model
of rectified Brownian motion, Fox admits there is
no indisputable evidence supporting his model over
the accepted "power stroke" theory.
"What
we really need now is an experiment that will
clearly be consistent with one of these mechanisms
and not the other," he noted. "That's our
objective for the immediate future."
Fishel
argues that a paper published May 24 in the
journal Nature by researchers at the
University of Tokyo and the University of
California supports Fox's model of ATP
switching.
Beyond the biological
implications, Fox hopes the paper leads
nanotechnology researchers to think about heat and
motion in a new way.
"There are lessons
here for nanotechnology in these biological
nano-systems," Fishel added. "This will help
people to appreciate that thermal motion can
actually be harnessed to do many kinds of useful
work."
And the work may also restore the
original hypothesis of Robert Brown, who first
observed the phenomena bearing his name in pollen
particles moving through water. Brown first
believed that what he saw through his primitive
microscope was "the secret of life." But after
observing the same kind of motion with inorganic
particles, he discarded that belief with
disappointment.
"We're arguing that Brown
really had discovered the secret of life," said
Fox. "When you get into this sub-cellular level on
the nanometer scale, the dynamics and vitality of
protein molecules is really due to thermal
motion." - By John Toon
[Contact: Ron
Fox, Richard
Fishel, John
Toon]
07-Jun-2001