Organisms have a group of enzymes called motor proteins. For example, protein filaments called microtubules run through nerve axons, and the motor protein called kinesin transports membrane vesicles filled with neurotransmitters along the microtubules. Each molecule of the motor protein works as a motor, which is thus quite small. In addition, motor proteins have various features not found in artificial motors, such as the potential to form large structures by self-assembly, which are general properties of proteins. Applied research has been conducted all over the world to use those motor proteins as nanoactuators.

To allow kinesin to move in vitro after taking it out of the cells, a system with a configuration reversed from the in vivo configuration has been used conventionally. In that system, kinesin is adsorbed onto a glass surface on which fluorescence-labeled microtubules move. However, the system does not allow the microtubules to do useful work to the outside because the microtubules move in random direction on the glass surface. We therefore created tracks on the glass surface by lithography as shown in Figure 1, and thereby succeeded in limiting the microtubule movement to one dimension. Moreover, moving almost all microtubules in one direction was realized by adding arrowhead-shaped "rectifier" patterns along the linear tracks. Microtubules moving in one direction along a track are expected to be used as a minute belt conveyor. To that end, various peripheral technologies must be developed, including a technology to control motor activity locally, a technology for external control of the traveling direction of microtubules at the junction of the track, a technology to bind a load to be carried to a moving microtubule and release the load at a destination, and a technology to sustain the movement for an extended period.

At AIST, specialists from various fields have established the Upbringing of Talent in Nanobiotechnology Course, led by Noboru Yumoto, Director, Research Institute for Cell Engineering, with the support of the Special Coordination Funds for Promoting Science and Technology to further technological development while training interdisciplinary personnel.

For example, Nomura and Tatsu (Biomolecular Engineering Research Group, Research Institute for Cell Engineering), who have skills in the technology for a "caged peptide," which is activated by ultraviolet irradiation, identified a peptide that inhibited the motor activity of kinesin reversibly, and have developed a system that stops microtubular movement reversibly by ultraviolet irradiation that "uncages" the peptide (Figure 2).

In addition, Konishi and Kubo (Molecular Neurophysiology Group, Neuroscience Research Institute), experts in protein engineering, have succeeded in the development of a chimeric kinesin molecule which is switched on by calcium ions.

Kato and Shibakami (Lipid Engineering Research Group, Institute for Biological Resources and Functions), who are skilled in organic synthesis, bound cyclodextrin chemically to microtubules. Moreover, they have succeeded in binding and dissociating azobenzene to and from cyclodextrin-conjugated, moving microtubules by reversible photo-modulation of the affinity between cyclodextrin and azobenzene. Taira and Kodaka (Molecular Recognition Research Group, Institute for Biological Resources and Functions) bound oligonucleotides to moving microtubules, and demonstrated that oligonucleotides with complementary sequences can be transported. Because even a single base-pair mismatch prevented this transport, this system may be helpful in the analysis of single nucleotide polymorphisms (SNPs), which will enable tailoring treatment regimens to individual patients. These and other novel technologies should be combined to realize micro-devices and systems in the future.

Rather than using purified motor proteins as nano-components of micro devices, a more biological approach is to modify motile biological structures and use them as pre-assembled motile units in an artificial environment.

For example, Hiratsuka and Uyeda (Gene Function Research Center), with the cooperation of Miyata (Osaka City University) and Tada (Advanced Semiconductor Research Center), are working at the development to use the gliding bacteria Mycoplasma mobile, which moves at high speed (3 µm/s) continuously on a substrate, as a microactuator.

They developed a technology for unidirectional circling movement of Mycoplasma cells in a minute (20 µm in diameter) circular track, and bound a microrotor created using the MEMS technology to circling cells, resulting in the creation of a rotary micromotor driven by the bacteria (Figure 3). As we relied on cattleand horses for a long time before man-made vehicles such as cars are available, micro-cattle and horses may play an important role for some time in the field of nanobiotechnology.

(Positions cited are as of the time of publication).