Even stationary cells keep idling over.

If Salmon and Small are both correct, microtubules must be delivering different signals to the front and back of the cell. Small doesn’t feel the need to explain this situation because, he says, "we think they’re wrong." But Salmon has a possible solution. He has seen microtubules get swept from the front of the cell by the backwards flow of excess, newly formed actin.

Microtubule bending.

The microtubules are bent and broken by the flow, and the exposed ends may load up with different cargoes to deliver to the back of the cell. Thus the actin controls microtubules, even as microtubules control actin. This self-sustaining cycle is layered on top of Borisy’s basic cycle of reciprocal actin and myosin control.

A cell in idle mode.

The cycles are so strong that the cellular engine never stops – it just idles. In the November 5 1999 issue of Science, Eugene Marcantonio and Gregg Gundersen of Columbia University in New York took the unusual step of looking at stationary fibroblasts to see what, if anything, the cells were doing. To their surprise, the cells were slowly forming and then pulling in their actin anchor points.

The cells are stuck in place "not because their feet are stuck down too hard," says Marcantonio, but because they are waiting for a signal to put them into gear. "We propose," he says, "that there is a molecular clutch that engages the movement."

Identifying the clutch and the messages delivered by microtubules will be important future steps, but not the last. Cataloging parts is only a start in understanding a motor that constantly changes over time and space. "How will you know that you have solved the problem?" asks Mitchison. "It may have to be solved by computer modeling. Success will be when you can put the parameters into a computer, and get your model cell to walk."

Making a Movement Machine