Molecular Motors as transport systems: motility and control

One of the central themes of nanotechnology is the scaling down of electromechanical devices or machines to the molecular scale. Advancement in the fields of microelectronics and microfluidics has influenced the development of miniaturized devices, where all necessary parts and methods to perform a chemical analysis are integrated. These micro total analytical systems (m-TAS) will function as integrated sensing, actuating and synthesizing systems, with various uses. Development of such lab-on-a-chip requires integration of microsensors and microactuators on a single chip. This becomes challenging due to the lack of actuating structures with dimensions less than a micron. We at the NSRG wish to pursue this goal with the help of nanomachine systems that nature has already created: “biomotors” or “molecular motors”.

An artist’s rendition of a biomotor based nano train and conveyor belt

Within every living cell is a complex highway system of tiny motors that move along filamentous tracks. Biomotors and the tracks they move on are ubiquitous in the myriad processes occurring with in the cell. These motor proteins along with associated filaments are responsible for muscle contraction, elongation of nerve cells, separation of chromosomes during cell division and transport of membrane vesicles. They also power bacteria’s flagella and the cilia within our lungs. These systems serve a host of other cellular functions, many of which are only beginning to be understood. They have high energy-efficiency, have load carrying velocities of a few hundred nanometers per second and are capable of generating forces amounting to a few piconewtons They do mechanical work through the hydrolysis of ATP: the energy source of the cell. During hydrolysis of ATP, a shape change occurs within the motor protein (a mechano-chemical process), and mechanical work is done. This mechanical work is used to move the motor along a track in order to perform a load transport function, or to apply forces to the filament for cell motility and cell division. Biomotors are in a sense the ultimate nanomachines

These properties make the biomotors compelling actuating components for m-TAS. Integrating biomotors with nanoscale devices enables the use of ATP as an energy source to yield energy efficient systems capable of operating as pumps, valves and vehicles.  For complete integration of molecular motors into m-TAS, we need to combine molecular motors with silicon electronics including the patterning of their position and the control of motion by external signals.

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An artist’s rendition of a biomotor based nano transportation system integrated with nanotubes and silicon electronics. External electrical signals exert control over motility and aid in uploading and downloading a load

Biomotor systems harnessed for nanotechnology purposes include Kinesin/microtubule system for linear transport and F1-ATPase, which has been used for the construction of rotary biomotor-powered nanodevices. In our systems we use Myosin-V, an actin-based processive biomotor, which plays an important role in many forms of eukaryotic motility. Its properties as a transportation system parallel those of the kinesin/microtubule system. Actin microfilaments are semi-flexible protein polymers of approximately 8nm diameter with a persistence length of 7.4 microns. The filaments have an overall structural polarity. Myosin-V moves processively along actin toward the structurally positive end with a step size of 30-38 nm. Like kinesin, this processive movement allows single motors to carry loads along the filament. Conversely, actin is capable of moving on myosin immobilized on a surface. The actin filament flexibility may allow finer definition of filament pathways and capability to turn corners in the fluidic channels of a m-TAS, while the step size may offer greater ability for motor shuttles to cross from one filament to another.

Our fundamental goals are:

1. Demonstrating the ability to pattern filament raceways as functioning tracks for the motion of motor proteins, and exercise control on their motility by using electrical, mechanical and chemical signals.

·        Pattern the deposition of filaments in microfabricated electrodes and control the motor motility with external electric fields. Both dielectrophoretic and electrophoretic techniques will be used.

·        To use microcontact printing, combined with surface functionalization, to pattern motor protein systems into chemically well-defined regions. This offers a facile route to patterned substrates.

·        Laminar flow inside microfluidic channels can be used to pattern and orient filaments on a substrate. Viscous drag and fluid forces can be used as methods for controlling motility.

2. Integrating molecular motors with silicon electronics and Carbon nanotubes. CNT’s represent an opportunity in molecular scale conductors for the interfacing of motors to electrical signals. They may play an increasing role in nanoscale actuating systems.

• We will attempt to functionalize nanotubes with motors and filaments to use them as mobile loads and as scaffolding for filament patterning.
• Define a processing strategy for integrated nanoscale metal, nanotube and biomotor patterning.

3. Study two of the main classes of proteins actin/myosin and microtubule/kinesin to understand their relative merits towards nanotechnology applications and also the application of single motors and collections of motor proteins. These capabilities and fundamental characterizations will be applied to new force sensing analyzing devices and multiplexing arrays.

4.Apart from the nanotechnology applications, we physical scientists work with the biologists to study these molecular systems from a physiological point of view. The techniques and to date technology available in a nanotechnology lab like ours along with years of experience in handling nanoscale systems offers a whole new perspective to the in vitro assay techniques used in most biology labs.. While attempting to achieve the goals above, we discover ways to improve the current assays.

This project is a collaborative effort by a large multidisciplinary team of scientists. Collaborators outside the NSRG are Dr. Richard Cheney (Cell and Molecular Physiology) and Dr. Edward D. Salmon (Biology). With in the NSRG, The “Biomotor” team comprises of undergraduate students Louise Jawerth (Physics major), Rohit Prakash (Biology major), Graduate students Timothy D. Meehan (UNC Chemistry), Sreeja B. Asokan (UNC Physics and Astronomy) and Dr. R. Lloyd Carroll (UNC Physics and Astronomy).