Biophysicists at Stanford University have finally answered one of the most fundamental questions in molecular biology: How does the tiny motor molecule, known as kinesin, move across a living cell? According to the researchers, the solution to this longstanding problem will provide new insight into how motor proteins function, and may open new avenues of investigation for the treatment of cancer and various neurodegenerative diseases, such as Alzheimer's and Huntington's.
The study, published in the Dec. 4 online edition of the journal Science, was co-authored by Steven M. Block, a professor of applied physics and of biological sciences at Stanford.
"Motion at the cellular level is a hallmark of being alive," Block said. "A fundamental question is, how did living organisms figure out how to move? The answer is they developed kinesin and several other very efficient protein motors. If kinesin were to fail altogether, you wouldn't even make it to the embryo stage, because your cells wouldn't survive. It's that important."
Discovered in 1984, kinesin is now recognized as the workhorse of the cell, hauling chromosomes, neurotransmitters and other vital cargo along tiny molecular tracks called "microtubules." Many types of kinesin and kinesin-related proteins have been discovered in the past two decades in a wide range of organisms – from yeast to humans.
"Kinesin functions like a locomotive in cells to ferry cargo back and forth," Block said. "In brain cells, for example, it grabs these tiny sacs called vesicles, which are loaded with neurotransmitters that are needed for neurons to function, and moves them very long distances along microtubules."
A mere three-millionths of an inch long, a typical kinesin molecule has a tail on one end that hauls the cargo and two globular heads on the other end that alternately grab the microtubule and pull the cargo forward. One head has to be attached to the microtubule at all times for kinesin to advance against loads. "It's like pulling yourself straight up a ladder with your arms," Block explained. "If you were to let go of both hands, you'd fall off and that would be it."
For more than a decade, Block's lab and others have been trying to figure out the precise details of how kinesin's twin heads move – a process researchers call kinesin walking.
"There are two competing models for how kinesin walks: inchworm and hand-over-hand," Block said. "In the inchworm model, the leading head holds onto the microtubule, and the trailing head moves up to meet it. The second possibility is the hand-over-hand motion, which is more akin to the way we walk down the street, taking alternate strides every step."
In their experiment, Block and his colleagues used a specially designed microscope called the optical force clamp – a nanoscale instrument that allows researchers to watch a single kinesin molecule as it walks along a microtubule at rates up to about 100 steps per second. To their surprise, the researchers discovered that, instead of taking regular steps, kinesin actually walks with a limp.
"By 'limp' we mean that the timing of every alternate step is different than the one in between," Block said. "Suppose you have a sore leg that causes you to limp, and you have to walk across a river on stepping-stones, so you don't have any choice about where to put your feet. You don't want to spend a lot of time on the leg that's hurt, so you put it down and quickly get onto the other leg. As a result, the timing on every other step is short, whereas if you had two perfectly good legs, you'd spend equal time on each leg and your stride would be regular, not jerky."
This discovery provided an immediate solution to one part of the kinesin-walking puzzle.
"As soon as we saw limping, we said, 'Ah-hah!'," recalled Charles L. Asbury, a postdoctoral fellow in the Block lab and lead author of the Science study. "It's impossible for an inchworm to limp, because one head is always in the lead. Therefore kinesin must be using a hand-over-hand movement when it walks."
What's causing it to limp? "We're not sure," Block said, "but we speculate that maybe kinesin's coiled neck is getting over-wound and under-wound a little bit every time it takes a step."
Symmetry vs. asymmetry
The discovery of the hand-over-hand movement led the researchers to try and tackle another unanswered question: Is there symmetry in the way kinesin walks?
"It turns out that there are actually two kinds of the hand-over-hand models – asymmetric and symmetric," Block explained. "A normal human walk is asymmetric, because one leg always ends up in front of the other one, so at the end of each step, I'm in a different – that is, asymmetric – position relative to the axis of my body. In symmetric walking, I'm in the same geometry at the end of every step."
An example is a compass with a pencil on one arm that's used to mark off distances on a map. If you "walk" the compass in a line along a sheet of paper by rotating one arm in front of the other, the compass always ends up in the same symmetry after each step.
"But limping, by definition, means that every other step has to be different," he added. "That means kinesin's movements have to be asymmetric, so we can reject both the inchworm model and the symmetric hand-over-hand model."
The kinesin-microtubule system has been implicated in several serious diseases, which is one reason why medical researchers are particularly interested in understanding the fine details of how kinesins walk across microtubules.
"A number of diseases – such as Huntington's and Alzheimer's - involve proteins glomming together in the cell in ways that they shouldn't," Block said. "The cell tries to get rid of these bad protein aggregates by transporting them elsewhere using motor molecules such as kinesin. If we could understand something about how protein transport is done right, then maybe we could learn something about how it's done wrong. It may be possible to develop therapies that block the kinsein motion and prevent bad things from happening. In the future, it may even be possible to develop less toxic therapies that target specific kinesins in cancer cells, without damaging the healthy ones."
For example, taxol, a drug commonly used to treat breast cancer, works by stabilizing the microtubules that kinesins walk along and preventing them from de-polymerizing, Block noted.
Recent studies also have shown that certain viruses, including smallpox and herpes, infect cells by hitching a ride on kinesin, added Adrian N. Fehr, a graduate student in applied physics at Stanford and co-author of the Science study. "Normally, kinesin moves cargo from one part of the cell to another," he said. "But certain viruses have chemical receptors that kinesin recognizes, which enables them to hijack the kinesin and get a free ride through the cell."
The study was funded by the National Institute of General Medical Sciences and Stanford University.
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