What makes up a motor?

Some cell movement researchers invoke complexities worthy of a jet fighter engine, but Gary Borisy of the University of Wisconsin, Madison, prefers the image of a two-stroke lawnmower. Borisy is the movement minimalist – in his world two proteins called actin and myosin can explain just about everything.

But first he had to show that they were even relevant.

Actin and myosin are logical suspects for a movement motor. These two proteins are at the front of moving cells, and they are already known to produce force: in muscle the glob-like myosin motor protein grabs onto cables of actin and slides them over each other. The increased cable overlap makes a muscle cell collapse in on itself, or contract, and it can help a compartmentalized human body move around. But contraction alone, for a single cell, is useless. It makes a cell shorter and fatter, but it won’t get it from one place to another.

Myosin suffered another blow to its credibility when William Loomis of the University of California, San Diego, and James Spudich of Stanford University (Stanford, California) got rid of it in a crawling amoeba and found that the cells could still move. "Up until those papers the contractile models for cell locomotion were widely discussed," says Tim Mitchison of Harvard Medical School in Boston. "Then it became dogma in the field that movement doesn’t involve myosin II."

If myosin wasn’t the culprit, perhaps actin was.

Actin pushing out the front of the cell.

The extension of actin cables is known to push out the front of the cell, and with a strong enough push the rest of the cell might be dragged along behind. A food-poisoning bug called Listeria uses a similarly simple system: it hijacks the cell’s actin to make a propulsive tail – no myosin necessary. Worm sperm, meanwhile, clearly don’t use myosin. They don’t have any actin either, but they use a functionally similar cable to push out the front of the cell.

But Borisy still had faith in myosin, especially as researchers began to identify other versions of myosin that could have been filling in for the one removed by Loomis and Spudich. Now Borisy just had to find the myosin amongst the tangled cables of actin at the front of the cell, and work out how such an unholy mess was driving cell movement. The labor was hard but worthwhile. "This is absolutely beautiful work," says Mitchison. "It has rehabilitated myosin."

Making a Movement Machine