Transport in cell-biology is mainly mediated by biomolecular motors, the active workhorses in cells. They are complexes of two or more proteins that convert chemical energy, usually in the form of the high-energy phosphate bond of ATP, into directed motion. These motors include relatives of muscle myosin (that also move along actin filaments), as well as members of the kinesin and dynein families. The latter motors move on microtubules (MTs), which are stiff, hollow cylinders (diameter 25 nm) composed of about 13 parallel protofilaments that are made of tubulin dimers (repeat length 8 nm). Since all subunits in a protofilament point in the same direction, the MT has a distinct structural polarity (denoted by the plus and the minus end). This property distinguishes MTs from other filaments like carbon nanotubes and allows the directed motility of motor proteins on them. Kinesin-1 is a motor protein that moves cellular cargo, such as membrane-bounded organelles, processively along MT protofilaments towards the plus end (i.e. mainly towards the cell periphery) with a speed of about 0.8 µm/s. Cytoplasmic dynein carries cargo to the minus-ends of MTs in the cell center with a speed of about 1 µm/s. Actin filaments and MTs form a network of highways within cells, and localized cues are used to target specific cargoes to specific sites in the cell. Using filaments and motors, cells build highly complex and active structures on the molecular (nanometer) scale.

The reason that motors are necessary in cells is that diffusion is too slow to efficiently transport molecules from where they are made, typically near the nucleus, to where they are used, often at the periphery of the cell. For example, the passive diffusion of a small protein to the end of a 1-meter-long neuron would take approximately 1000 years, yet kinesin moves it in week.

Biomolecular motors are unusual machines that do what no man-made machines do: they convert chemical energy to mechanical energy directly rather than via an intermediate such as heat or electrical energy. This is essential because the confinement of heat, for example, on the nanometer scale is not possible because of its high diffusivity in aqueous solutions. As energy converters, biomolecular machines are highly efficient. The chemical energy available from the hydrolysis of ATP is 100 x 10-21 J = 100 pN·nm (under physiological conditions). With this energy, a kinesin molecule is able to perform an 8 nm step against a load of 6 pN. The energy efficiency is therefore nearly 50%.

High efficiency is one feature that makes biomolecular motors attractive for nanotechnological applications. Also, (i) biomolecular motors are small and can therefore operate in a highly parallel manner, (ii) they are easy to produce and can be modified by genetic engineering, and (iii) a wide array of biochemical tools have been developed to manipulate these proteins outside cells.

One might envision that biomolecular motors could be used as molecule-sized robots that work in molecular factories where small, but intricate structures are made on tiny assembly lines; that construct networks of molecular conductors and transistors for use as electrical circuits; or that continually patrol inside "adaptive" materials and repair them when necessary. Thus biomolecular motors could form the basis of bottom-up approaches for constructing, active structuring and maintenance at the nanometer scale.

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