July 18, 2011 | 3
The biological world is getting smaller. When bacteria were first discovered they were no more than blobs, small intriguing shapes beneath the glass of a microscope. The development of more powerful microscopes and more detailed techniques started to unfold a whole new world of the very tiny.
Nowadays the world explored by microbiologists is more detailed than the first microbiologists could have dreamed of. We can even get inside the bacteria to see the inner workings of the cell and the wonderful machines within. The cell can be viewed as a tiny mechanical factory with proteins forming gated channels, energy-creating rotor motors and shuttling transporters. These little nanomachines work perfectly within the interior of the cell where forces like liquid viscosity and random motion of particles become vitally important.
So what kind of nanomachines can be found inside cells? Two of the most interesting ones are the ATP-synthase protein and transport systems such as myosin (myosin is found in human cells rather than bacterial ones, but is easier to explain. Bacteria use a similar system). ATP-synthase is a very important molecule, as it produces ATP which is the energy currency of the cell. It works like a rotor; flowing protons through the bacterial membrane causes the intermembrane section of the molecule to rotate, grinding the top part of the molecule against a stationary stator causing a conformational change which churns out ATP.
While the ATP synthase is a circular rotor the myosin transporter proteins work as linear motors, carrying proteins around the cell. Myosin is a small globular proteins with two little feet that move in discrete steps along long chains of actin. These actin chains are laid like railtracks all over the cell but unlike railtracks they can be dismantled and reformed at a moments notice to allow the myosin to drag proteins wherever they want. Furthermore the actin chains have directionality, because of their structure the myosin will only move in one direction along them.
As these are both perfectly working little nanomachines an interesting question is whether engineers can use them outside the confines of the cell. Attaching ATP synthase to a solid surface and adding ATP makes the motor run backwards (rather than forming ATP from constituent parts it breaks down the ATP you give it) but still causes the molecule to spin. Attaching a long florescent chain to this creates a turning visible propeller (see ref. 2). Although this experiment was originally used to show how the ATP synthase worked it also has potential for use as a tiny propeller inside a nanomachine.
Myosin is even more exciting as several experiments have been done to explore the use of myosin within human-made nanomachines. Like trucks on a tiny scale, the myosin can be engineered to carry bits of DNA and proteins or even inorganic substances such as gold nanowires. Using motor proteins to manipulate gold nanoparticles and nanowires allows very small and precise electrical circuits to be built (ref 3).
At the moment these experiments using bacterial machines are no more than proof of concept, examining the potential uses of biological nanomachines without developing full working systems. However it’s not just the biological world that’s getting smaller, the physical one is as well and as engineers start designing smaller and smaller devices it may be highly practical and useful to be able to use the machines that bacteria build.
Ref 1 = van den Heuvel, M., & Dekker, C. (2007). Motor Proteins at Work for Nanotechnology Science, 317 (5836), 333-336 DOI: 10.1126/science.1139570
Ref 2 = Itoh, H., Takahashi, A., Adachi, K., Noji, H., Yasuda, R., Yoshida, M., & Kinosita, K. (2004). Mechanically driven ATP synthesis by F1-ATPase Nature, 427 (6973), 465-468 DOI: 10.1038/nature02212
Ref 3 = van den Heuvel MG, Butcher CT, Smeets RM, Diez S, & Dekker C (2005). High rectifying efficiencies of microtubule motility on kinesin-coated gold nanostructures. Nano letters, 5 (6), 1117-22 PMID: 15943453