Molecular motors – a lesson in nanotechnology from Nature

Roop Mallik

They are small, and there are billions of them inside you. Tiny machines, a thousandth of the thickness of human hair, but robust and designed for an amazing variety of functions. Science fiction? Think again … this is real, as real as flesh and blood !! If you can get your hands on a high school biology text book, flip through to the mandatory schematic of an animal cell. Look closely, what you will see is not a floppy bag with random things thrown in here and there. There is amazing structural organization within the cell, with several compartments (e.g. the nucleus, Golgi bodies, mitochondria) at specific locations. Many of these compartments are specialized “factories”, each with its own assembly line which requires specific raw material as input and generates specific products. A constant give-and-take of materials occurs within these factories, because each is dependent on the other. In the big picture of things this incessant exchange of material keeps the factories of the cell functioning, which in turn is what keeps us alive.

How do things get moved inside the cell? Simplified schematic of the transport network of a typical cell. Molecular motors are shown ferrying different cargo. The Kinesin motors usually move towards the cell periphery, while Dyneins move towards the cell center. More detailed schematics of the motors shows the “legs” on which these motors walk while carrying cargo. For an idea of the size scales involved, kinesin and dynein motors have dimensions of approximately 50 nanometers. A nanometer is one-billionth of a meter (10^-9 meters). These motors exert forces of pico-Newtons. A pico-Newton is one-thousand-billionth of a Newton (10^-12 Newtons). We usually measure forces in Newtons in day-to-day life.

This flow of material occurs in a highly regulated and disciplined manner, so that the right things are present at the right place and time. How does this transport of materials take place? This is where the army of tiny machines called “molecular motors” comes in. The cell has a network of “roadways” (one kind of roadways are called microtubules, see Figure) with heavy traffic of molecular motors on them. These motors can be thought of as porters ferrying all kinds of material within the cell. You will be surprised at how well this analogy of molecular motors with porters works, but don’t forget that these motors are a 10-million times scaled down version of what your idea of a porter is.

So, what exactly is this motor that works on a molecular scale? One example is a protein with two “legs” walking along on the cellular roadway, stepping just like a porter and carrying some cellular material as cargo. During every step that the motor takes, it has to generate force and therefore does some work. The energy required for this comes from chewing up a molecule called ATP, which has energy stored in its chemical bonds. For every step that the molecular porter takes, it needs one little packet of energy in the form of an ATP molecule. So, if you travel inside the cell and need somebody to carry your bags, make sure you give them a constant supply of ATP. Just three meals a day does not work at the molecular scale.

Kinesin, one of the best studied molecular motors walks with precise steps of 8 nanometers. For each step, kinesin uses one molecule of ATP and generates 6 piconewtons of force. A simple calculation shows that this makes kinesin a nano-machine with almost 50% efficiency, which is comparable to many machines of human design. To give you an idea of the magnitude of scales here, if a kinesin motor walked upwards starting at your toenail, it would take about 100 million steps to reach your nose. Approximately 1 million-million kinesins would have to team up together to arm-wrestle with you and have any hope of winning. We are really talking nanotech here.

For anyone impressed with kinesin, there is more to come. Dynein is a second class of motors ferrying cargo within the cell, and is much more complicated than kinesin. There is recent evidence that nature has implemented a nanoscale gear mechanism within this complexity. It appears that dynein normally walks with a step size about thrice the size of kinesin. However, if you pull dynein backward the motor can shorten its stepsize and resultantly produce more force, which is just like shifting gears in your car on an uphill road. Only future research can tell why nature felt the need to implement such complex architecture at these minute size scales.

There are many other classes of “motors” which I have not discussed here for reasons of space, and also to keep the discussion simple. One common theme that has emerged from years of research is that of surprisingly intricate and robust architecture within these molecular motors at a size scale which we are only now beginning to comprehend. It is these little things in life that matter, so let us congratulate nature on a little job very well done !!

Text and figure created by Dr. Roop Mallik, Copyright 2005

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