Biomolecular Machines - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 183

a b

unloading sites loading sites

cargo

microtubule

kinesin
glass glass

Fig. 6.11 ( a ) Loading and unloading sites on a kinesin-coated glass substrate. ( b ) Cargo-carrying
microtubule that glides on kinesin

6.3 Biological Motors

In this section, we deal with machines based on biological motility that can transport
cargo molecules from one position to another. For future bionanotechnologies, it
would be of interest to be able to selectively translocate nanoparticles in parallel by
autonomous on-chip systems. This task can be performed by microtubules labeled
by ssDNA strands that glide on surfaces coated with kinesin, as described in Hiyama
et al. ( 2009 ). Nanoparticle cargoes labeled by ssDNA strings that have a common
sequence with that of the microtubule can then be loaded on the microtubule
from a loading site specified by a certain DNA base sequence and transported
to an unloading site. In the experiment described in Hiyama et al. ( 2009 )and
schematically illustrated in Fig. 6.11 , the cargoes were fluorescent polystyrene
microspheres with a diameter of 200 nm labeled with a 23-base ssDNA, which
become trapped at specific loading sites micropatterned on a glass substrate and
containing 10-base ssDNA that hybridizes with the ssDNA on the cargo. Outside the
loading and unloading sites, the glass substrate is covered with immobilized kinesin,
on which microtubules labeled with 15-base ssDNA can glide with an average speed
of 0:35ms 1 . When a microtubule encounters a cargo, the latter is loaded on the
microtubule following a DNA hybridization/strand exchange mechanism with the
longer complementary strand on the microtubule; the state of cargo on microtubule
is more stable than that of cargo on loading site. Upon loading, the microtubules
continue to glide with similar speeds until they reach the unloading sites on
which 23-base ssDNAs, complementary to that on the cargo, are immobilized.
Complete hybridization of ssDNA on cargo and unloading sites allows cargoes to be
unloaded (to reach more stable states than on microtubules). The operation of this
autonomous loading/unloading system is observed via fluorescence, which shows
that in time (over about 1 h), the cargoes are transported to and accumulate on the
unloading sites.
When active transport of cargoes attached on microtubules that glide on kinesin
is desired, the microtubule/kinesin interaction is preserved if the cargo is confined
to the central region of the microtubule ( Bachand et al. 2004 ). Cargo transport
and self-assembly processes can be performed in the same system when biotiny-
lated microtubules are used as scaffolds for streptavidin-coated nanocrystal CdSe