Cellular transit system gets meter reading.

Cellular transit system gets meter reading

Using laser-beam "tweezers" that function like Star Trek tractor beams, scientists have measured the forces generated by tiny biological engines inside living cells. The study -- in which researchers put the brakes on cell components that normally zip around within cells -- provides the first direct measurements of the mechanical energy exerted by this basic machinery of life.

The investigators focused on a miniature monorail system found in many cells. The "rails" consist of protein strands called microtubules. Along this network run specialized cell components called organelles -- including mitochondria, the mobile mini-reactors that generate and deliver energy to the farthest reaches of intracellular space. Mitochondria and other organelles move along the microtubules by latching onto one or more "motor proteins" such as kinesin or dynein. These locomotive proteins pull themselves and their cargo along the microtubule railways.

Until now, researchers had only rough estimates of the forces generated by locomotive proteins. The new work, led by physicist Arthur Ashkin of AT&T Bell Laboratories in Holmdel, N.J., and cell biologist Manfred Schliwa of the University of California, Berkeley, replaces those estimates with direct measurements made within living cells.

The team applied a gradient-force optical trap, or "optical tweezers," to mitochondria cruising along microtubules within an amoeba. The tweezers use beams of photons to push around tiny objects or hold them in place (SN: 3/10/90, p.148). First, the researchers applied enough force to bring individual mitochondria to a screeching halt. Then they noted how much they had to tone down the laser before the organelles started moving again, figuring that the minimum amount of force needed to keep the organelles stationary must equal the force of the motor system itself. That comes to about 2.6 X [10.sup.-7] dynes per motor, they report in the Nov. 22 NATURE.

"On an absolute scale, it's not a lot of force," Ashkin says. "But on the scale of these beasts, it's quite impressive." He calculates that from a mitochondrion's point of view, that's about 1,000 times the force of gravity. Seen another way, it's enough force to propel an average-size bacterium through water at about 1 millimeter per second. With most mitochondria pulled along by two or three motor molecules at once, the forces create a powerful transport system that can maintain constant mitochondrial velocities over the wide range of viscosities encountered within cells, Ashkin says.

Steven M. Block, a motor-molecule specialist at the Rowland Institute for Science in Cambridge, Mass., comments that the work foreshadows a future when scientists will understand the mechanical details of biological motility on a molecular level. "How chemical energy in cells gets transduced into mechanical displacement remains completely obscure," Block says. "Optical tweezers provide an exciting new tool that may at last make that understanding possible."

For now, the research remains very basic. Block points out, however, that molecular motors play critical roles in such diverse processes as cell division and muscle contraction, and that motor defects may underlie a variety of diseases or cellular abnormalities.