(Nanowerk Spotlight) Shuttles - whether the space shuttle, an airport shuttle bus, or a loom shuttle - basically do one thing: they transport cargo (astronauts, passengers, thread) from one point to another on a controlled route. Although not always called shuttles, the basic concept is critical to modern transportation systems and is used by nearly every society. The concept of the shuttle has been used for centuries from Egyptian barges to Roman railways and canals. Even before these inventions, however, nature employed molecular shuttles in biological organisms. In molecular shuttles, kinesin proteins propel cargo (such as organelles) along hollow tubes called microtubules. Cells use these motors to transport cargo to highly specific destinations, in order to regulate levels of macromolecules and processes, much like a train along a track. Using biological motors to transport and precisely distribute cargo requires a clear understanding of how molecular shuttles pick up and deliver specific payload. However, scientists are challenged by the need to better control the interactions along the route so that the cargo remains on the line when not needed, but when it is needed, can be picked up and transported to a specific location. Researchers in Switzerland have now built nanoscale cargo loading stations and shuttles, an important step towards assembly lines for nanotechnology.
"Imagine you could build machines by assembling single molecules, and use these machines as tools to create complex materials, repair tiny defects on surfaces or in living cells, or to store and retrieve information" says Dr. Viola Vogel. "These nanoscale machines would be totally different from machines like trains, cranes or presses in their appearance, but they would have to perform related functions like transporting things or changing their shape. We are attempting to build a nanoscale train system, complete with tracks, loading docks and a control system."
Vogel, who is a Professor in the Department of Materials heading the Laboratory for Biologically Oriented Materials at the Swiss Federal Institute of Technology (ETH) in Zürich, Switzerland, and her group's effort is inspired by nature, since cells have evolved a complex transport system.
"In this system specialized motor proteins connect to small containers filled with proteins and transport them along the skeleton of the cell" says Vogel. "Since motor proteins are a thousand times smaller than any man-made motor, we try to utilize them in a synthetic environment as the engines powering our nano-trains. An example for such an application is a surface imaging method based on nanoscale probes, which are propelled by motor proteins."
According to the article, because of the lack of appropriate synthetic motors that perform well if loaded, in the past, biological motors have been integrated into micro-fabricated environments. Feasibility studies have demonstrated proof-of-principle for controlling the unidirectional movement of motor-driven transport along engineered tracks; adjusting the speed of the transport system on user demand by using light; and specifically attaching cargo from a solution. Because of their high bending rigidity, microtubule filaments propelled by surface-anchored kinesin motors have been most frequently used as cargo carrier systems.
(A) Motility assay: in contrast to intracellular transport, where cargo is bound to motor proteins (e.g. kinesins and dyneins) and transported along microtubule filaments, the inverted assay uses surface-adsorbed kinesin motors to propel functionalized mictrotubules (molecular shuttles). Kinesin motors bind to the microtubules, each exerting up to 5 pN of force. Casein passivates the substrate surface to preserve the function of motors and their filaments. (B) Illustration of cargo pick-up from loading stations by molecular shuttles. Functionalized microtubules move into cargo-immobilized areas (loading stations) to (1) pick-up and (2) transport the load into cargo-free areas. Cargo is immobilized via reversible linkers to allow the passing shuttle to bind to the cargo and break the surface tether (1). The scheme is roughly to scale: tethers keep the cargo in a favorable elevated position for pick-up. (Reprinted with permission from Royal Society of Chemistry)
Engineering effective cargo loading stations requires that the cargo is bound sufficiently stable to the surface for the lifetime of the device, yet is picked up efficiently by passing microtubules. To this end, the authors designed several alternative tethering systems for this study. Using gold nanoparticles coated in anti-biotin antibodies as cargo, and compared loading stations made of biotin-tipped DNA with biotin-tipped polyethylene glycol. They then tracked the nanoparticles with scanning electron microscopy.
According to the article, the authors found that the shuttles did, in fact, pick up the nanoparticles and held on to them. Over a 12-minute span, the shuttles lost about 28% of their cargo. The authors also reported that the loading stations made from biotin-tipped DNA were more effective than the ones made from polymer.
"The pick-up of functionalized cargo from defined surface regions is a major milestone on the way to directed assembly on the micro- and nanoscale," say the authors. They say that, to the best of their knowledge, "this research is the first attempt to specifically harvest functional objects from micropatterned loading stations. The pick-up of functionalized cargo from defined surface regions is a major milestone on the way to directed assembly on the micro- and nanoscale."
Further development may result in applications for materials, sensors, information technology and drug delivery. With the help of the controlled shuttles, scientists may be able to assemble those things that do not normally self-assemble or impose a particular order on the assembly process.
While this study used common molecular recognition complexes for surface tethering, the authors believe these methods can be easily expanded to other cargos, including quantum systems, viruses and biomolecules. They also state that numerous other antibodies/antigens can be used to adjust the binding strength to other specifications, depending on the purpose. "While demonstrated here for one cargo, arrays or patterns of stations loaded with different cargos can be fabricated in future developments, which would open the door for sequential assembly of diverse nanoscale objects."
The authors say that future challenges toward functional lab-on-a-chip devices or other integrated systems will be the integration of a transport system so that the cargo is picked up from a defined location, guided along the proper route, and delivered to the final destination with specific precision and control.
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