2.2 Movement Using Biological Molecules and Methods

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tom of a linear groove that guides microtubule movement. In such a set up, microtubules

can even “climb up” walls up to 286 nm.

If there is no groove, a different guiding system is needed. One example is the at-

tachment of carbon nanotubes that are modified with motor proteins, and thus the track

for the microtubules [18]. The carbon nanotubes can be aligned by applying a voltage.

A nanopatterned surface combined with this specific track allows the microtubules to

move past corners and curves. The speed of the movement is based on the ATP concen-

tration, but the direction is random.

One example used kinesin as the ATP-powered motor and microtubules as the car-

rier (Figure 2.6) [19]. The microtubules carried multilamellar vesicles. Due to the move-

ment of the microtubules several vesicles hit each other with enough force to fuse into a

lipid tube network several hundred micrometers in size. These tubular structures mimic

the cellular endoplasmic reticulum (ER) and can be used to isolate and trap nanoparti-

cles, as the ER does [19].

How can the cargo be attached? Genetically engineering the motor proteins to add

a functional group that reacts easily is one possibility [9]. Equally, a microtubule could

be modified with a functional group to attach cargo to it [20]. Biotin-streptavidin, two

proteins that bind to each other strongly, have also been used for attachments in various

systems.

Now, some sort of traffic control has to be created and the synthesis of the system

must be automated. This is obviously the hardest part. A combined microtubule-actin

system took the first steps toward traffic control [21]. Since there are two types of tracks,

several motor protein shuttles can walk at the same time. The traffic control comes from

using motor proteins that walk in opposite directions. It does not seem to be a major

problem for them to move past each other, as they do in life cells. That mechanism,

though, is not well understood and has limits.

The first reported automatic assembly of a motor protein transport system used a

lab-on-chip [20]. The different components of the system were added to different wells,

and the microfluidic system operated them in order to prepare the track, attach the

cargo onto the motor, and then move the motor onto the track (Figure 2.7). This system

allowed them to sequentially attach two different cargos as well.

Moving cargo around is not the only possible application for molecular motors. An

intriguing example for other applications is the assembly of a nanosized force meter

[22] (Figure 2.8). Here, a microtubule is attached to the end of a bead and fixed there.

Another microtubule walks around on kinesin molecules. When the two meet, the fixed

microtubule is bent and the resulting force can be measured (it is in the range of pico-

newtons).

All of these applications are still on the molecular or nanoscale. The body uses

its nanosized motors and builds across scales to produce muscle tissue. A similar self-

organized system was used to build artificial cilia (Figure 2.9) [23]. Fluorescence-labeled

microtubules were attached to polystyrene beads. Kinesin motors were attached to mi-

crotubules. When ATP was present, the kinesin motors walked along the microtubules,