Proteins or neurotransmitters that are synthesized in one part of a cell yet needed in another usually catch the intracellular train: the molecules snuggle up inside vesicles that are then moved along microtubule tracks by motor proteins. Stefan Diez and colleagues have tapped into this transport system to mechanically manipulate DNA molecules and explore the potential of the cell's molecular transport machinery for engineering complex nanostructures and nanocircuits (Nano Lett. doi:10.1021/nl034504h; 2003).

In this work, biotin molecules at both ends of the DNA strands facilitate binding to streptavidin molecules, which are attached to the microtubules. When solutions of the microtubules are exposed to surfaces coated with kinesin motor proteins, the kinesin motors attach to the DNA-carrying microtubules and slide them along the surface. This sequence of images shows five green-fluorescing DNA molecules, condensed into 1-µm-wide coils, hitching a ride on a gliding red-fluorescing microtubule. When the pH of the solution is lowered, some DNA strands bind at one end to the surface; the other end can attach itself through the biotin–streptavidin linkage to a passing microtubule, and the DNA is stretched until it detaches from the surface or breaks.

Using surfaces with high surface densities of kinesin, up to 100 motor molecules interact with a 5-µm-long microtubule and power it along. But at much lower motor densities, the power output of individual kinesin motors barely suffices to stretch a coiled-up DNA strand: surface-anchored DNA can then act as a leash to stop microtubule movement or force it onto a circular path.

Diez et al. point out that improved control might be achieved by using surfaces that have predefined gliding tracks to transport DNA between contact points in an array. By tailoring the sequence of the DNA molecules and employing restriction enzymes, unwanted connections could be cut out. Once such a system is in place, cargo other than DNA — such as functionalized carbon nanotubes — should be able to join the train.