Molecular motors cooperate in moving cellular cargo, study shows

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.



Contact: James E. Kloeppel, Physical Sciences Editor

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217-244-1073

University of Illinois at Urbana-Champaign

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