a ‘snowplough’ a few millionths of a millimetre wide. Michael Brooks reports
As your eyes move across this text, you are performing a task that remains beyond the understanding of modern science. The simple movement in the muscles that control your eyes is still a mystery; inside the muscles, tiny proteins called myosins are doing the work, but no one knows how.
They are one example of biological motors – your body contains billions of them, pushing, pulling, carrying, lifting and breaking up the chemicals that, as a whole, make you what you are.
Biology could essentially be reduced to a collection of machines: exertion and movement are involved in almost everything going on inside a living organism. The dream of the Foresight Institute, a non- profit making nanotechnology organisation that this month held its annual conference in Santa Clara, California, is to create machines approaching the scale of these biological examples. Then we might be able to build tiny robots that could assemble new materials atom by atom, or carry out microsurgery inside the body.
Steven Block, a molecular biologist at Princeton University, doubts that these goals are entirely realistic. However, he spoke about his research into biological motors at the Foresight conference. Block’s research team has recently published studies on RNA polymerase, an enzyme which rips through DNA, separating its twin strands so it can duplicate itself. This is an essential part of life – every cell in the body is relying on this process to work without a hitch.
RNA polymerase, Block has found, is an extraordinarily powerful motor, somewhat like a biological snowplough. A few millionths of a millimetre in size, it is able to exert force equivalent to the weight of a red blood cell more than a thousand times its size.
Remarkably, the exertion that separates DNA strands is fuelled by the DNA itself. “It’s as if you were laying down an asphalt road using the asphalt itself as a fuel,” he says.
Although Block knows much about RNA polymerase, he still has no idea how it, or any other biological motor, actually moves. “This is a truly fundamental question for which a good answer has not been developed,” he admits. “The more we study, the more mysteries and enigmas we come across.”
There is no shortage of examples: bacteria use a rotary motor to twist their tails for propulsion; life starts with sperm cells using their motors to swim to an egg; cell division after fertilisation involves motors to take chemicals from one place to another, and hairs in the ear use motors to change their shape for optimum performance.
One of the biggest surprises in biological motor research came with the discovery of the “railway within”: a molecular motor, called kinesin, pulls chemicals around a cell using tracks called microtubules. Block has taken a kinesin motor molecule, attached it to a micron- diameter plastic bead, and watched it pull the bead along microtubules fixed to a microscope slide.
By holding the bead using “laser tweezers” (the bead is trapped in the brightest part of a laser beam), Block has had a tug of war with the kinesin motor to measure its capabilities.
Kinesin, it turns out, is similar in strength to the myosin motors in muscles, but five or six times weaker than RNA polymerase. But brute force isn’t everything: kinesin zips around cells at around 800 nanometres per second (that’s 3mm per hour), 200 times faster than RNA polymerase.
Understanding the role of these motors is an important endeavour: research indicates that some diseases – like Huntingdon’s disease, sickle cell anaemia and schizophrenia – are associated with their breakdown or absence. The latest discovery is that a lack of myosins can cause deafness in humans and mice.
“Obviously you need molecular motors to do a lot of things, including building the components of the inner ear,” Block says. “But you don’t have to know exactly how they work to cure the problem; you just need to be able to introduce a working motor. That is where gene therapy can help.”
Block still sees his work as fundamental science: not clinical medical research and not a path to building miniature robots as Foresight might hope. He does concede that Eric Drexler, Foresight’s founder, is probably right to look to biology if that is where he wants to go.
“He believes that, if one is to think about building machines on the scale of proteins, we have a lot to learn from the way that nature builds things,” Block says. None of the ways we now build tiny structures – silicon chips, for example – even remotely resemble nature’s way.
“The route to micro-machines will need a very different approach,” Block says. “Whether we can recapitulate nature’s way of building things in an artificial machine remains to be seen.”