| CHAMPAIGN,
Ill. - Researchers using an extremely fast and accurate
imaging technique have shed light on the tiny movements
of molecular motors that shuttle material within living
cells. The motors cooperate in a delicate choreography
of steps, rather than engaging in the brute-force tug
of war many scientists had imagined.
"We discovered
that two molecular motors - dynein and kinesin - do
not compete for control, even though they want to
move the same cargo in opposite directions,"
said Paul Selvin, a professor of physics at the University
of Illinois at Urbana-Champaign and corresponding
author of a paper to appear in the journal Science,
as part of the Science Express Web site, on April
7. "We also found that multiple motors can work
in concert, producing more than 10 times the speed
of individual motors measured outside the cell."
Dynein and kinesin
are biomolecular motors that haul cargo from one part
of a cell to another. Dynein moves material from the
cell membrane to the nucleus; kinesin moves material
from the cell nucleus to the cell membrane. The little
cargo transporters accomplish their task by stepping
along filaments called microtubules.
To measure such
minuscule motion, Selvin and colleagues at Illinois
developed a technique called Fluorescence Imaging
with One Nanometer Accuracy. The technique can locate
a fluorescent dye to within 1.5 nanometers (one nanometer
is a billionth of a meter, or about 10,000 times smaller
than the width of a human hair). Recent improvements
to FIONA now allow scientists to detect motion with
millisecond time resolution.
Selvin's team
used FIONA to track fluorescently labeled peroxisomes
(organelles that break down toxic substances) inside
specially cultured fruit fly cells. This was the first
time the imaging technique had been used inside a
living cell.
"Our measurements
show that both dynein and kinesin carry the peroxisomes
in a step-by-step fashion, moving about 8 nanometers
per step," said Selvin, who also is a researcher
at the Frederick Seitz Materials Research Laboratory
on the Illinois campus.
"Because
we see a fairly constant step size, we don't believe
a tug of war is occurring," Selvin said. "If
the dynein was fighting the kinesin, we would expect
to see a lot of smaller steps as well."
The researchers
also noted that faster movements occurred with the
same step size, but with greater rapidity. When measured
outside the cell, kinesin moved about 0.5 microns
per second. Inside the cell, the speed increased to
12 microns per second.
"There must
be a mechanism that allows the peroxisomes to move
by multiple motors much faster than independent, uncoupled
kinesins and dyneins," Selvin said. "It
appears that motors are somehow regulated, being turned
on or off in a fashion that prevents them from simultaneously
dragging the peroxisome."
In the future,
Selvin wants to combine FIONA and an optical trap
technique to monitor the speed and direction of a
peroxisome, and the force acting upon it.
"By measuring
force we can determine how many molecular motors are
working together," Selvin said. "This will
help us further understand these marvelous little
machines."
Collaborators
on the study included Illinois graduate students Comert
Kural and Hwajin Kim (lead authors), Illinois professor
of cell and structural biology Vladimir Gelfand (now
at the Northwestern University School of Medicine)
and postdoctoral research associates Sheyum Syed at
Illinois and Gohta Goshima at the University of California
at San Francisco.
The work was funded
by the National Institutes of Health, the National
Science Foundation, and the U.S. Department of Energy.
Editor's note:
To reach Paul Selvin, call 217-417-6101; e-mail:
selvin@uiuc.edu.
University
of Illinois
CONTACT: James E. Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@uiuc.edu
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