Humanity is just now beginning to learn how to master construction at the nanometer scale, but biology has been tinkering with this for billions of years. And as we've gotten better at manipulating biological materials, the lines between nanotechnology and synthetic biology are getting rather blurry. Over the weekend, Nature Nanotechnology released a paper that describes tiny biological machinery engineered to construct a transit system and then bring cargo to its hub. Its components? DNA and proteins.

To a certain extent, the British team behind the work wasn't engineering a system so much as re-engineering what life has evolved already. Cells have a transit system that uses structures called microtubules. These tiny fibers, 25 nanometers in diameter, are made of repeating units of two proteins (alpha and beta tubulin, naturally). There are also specialized motor proteins that bind to them and drag cargo along the fiber. This cellular transit system does everything from getting packages of proteins to the right place in the cell to ensuring that the chromosomes are split up evenly when a cell divides.

The motor protein used in these experiments is called kinesin. Each molecule of kinesin has a section where it binds cargo and a "head" that sticks to the microtubules. When two of these molecules stick together, the heads act a bit like feet, taking alternating steps along the microtubule, each stride advancing the kinesin 8nm down its length. Each of these steps uses a little bit of energy in the form of ATP. The process is shown in detail in the video below.

In the new paper, the authors performed a clever trick to get kinesin to assemble a transportation network made of microtubules. Rather than having each pair of kinesin molecules linked to a cargo-carrying domain, they linked them to another pair of kinesin molecules. Typically, this will link two different microtubules together. Given a mix of these kinesins and microtubules, you would get a complicated mesh that's crosslinked by the kinesins.

Now, think of what happens if you add ATP. Suddenly every kinesin molecule would start walking down to the same end of the microtubule (microtubules have distinct plus and minus ends, and kinesin only walks toward the plus end). Over time, this will draw all the plus ends into a single central hub. The rest of the microtubules will radiate out from there. The end result is something like a koosh ball or a dandelion that's gone to seed.

Now, with their transit system assembled, the authors decided to use it to move things to the hub. Again, they relied on a customized version of kinesin, this time linked to a DNA binding protein. As long as the DNA has the right sequence, the modified kinesin can latch on to it and carry it. Anything linked to the DNA will be carried right along. To demonstrate that this works, the authors tagged the DNA with a fluorescent molecule and showed that adding ATP would cause the fluorescent signal to gather in the hub of the transit system.

Of course, a transit system isn't much good if it never lets go of its packages. The authors designed a different DNA molecule, one that could base pair with and disrupt the first (we described this disruption of base pairing when we looked at a study that performed calculations with DNA). Hand this second bit of DNA to kinesin, and it will also be taken to the transit hub, liberating the fluorescent-tagged DNA.

The authors also demonstrated that the same principle could be used to disassemble the transit hub. Instead of linking two sets of kinesin molecules chemically, the authors hooked them together with DNA. The DNA-linked molecules would still assemble the hub when given ATP. But now, if another DNA molecule was delivered that disrupted base pairing, the entire assembly would fall apart.

There's essentially no limit to the number of different DNA sequences that can be used to deliver molecules to the hub, so this system is extremely flexible. The real limit is simply that the area around the hub gets very crowded, which limits kinesin's ability to squeeze more cargo into it. The authors haven't yet figured out how to do something practical with it, but they suggest it should be possible to use it for the step-by-step assembly of molecules or complexes of molecules.

Nature Nanotechnology, 2013. DOI: 10.1038/NNANO.2013.230 (About DOIs).