Products incorporating these devices could include biosensors, microfluidic systems, and therapeutic agents.

In a continuing research project, nanoscale actuators based on actin-filament end- tracking motors have been synthesized and characterized. It is envisioned that such actuators will eventually be utilized, variously, as molecular shuttles in biosensor devices or as nanoscale biomotors for effecting selection or separation of target microorganisms or molecules. In addition, this research is expected to enhance the fundamental understanding of molecular motors, both in vitro and in vivo and lead to modification of previously developed biomolecular machines and nanobiostructures to make them perform new functions. Some nanoscale actuators like those developed in this research may prove useful as components of micro- and nanofluidic systems. By contributing to understanding of how living cells convert chemical energy into mechanical work during actin-based and microtubule-based cell motility in cell crawling and cell mitosis, this research may lead to development of new therapeutic agents for combating invasive and metastatic cancers, gouty arthritis, Wiskott Aldrich syndrome, and those neurodegenerative disorders linked to loss of functional synapses.

These Electron Micrographs were taken during experiments in which actin filaments were made to elongate by insertional polymerization from 50-¼ beads.

During division of bacterial cells, actin-filament end-tracking motors are responsible for segregating daughter chromosomes. End-tracking motors also provide cell motility for cell-to-cell propulsion of invasive microorganisms. In an actin-filament end-tracking motor, the propulsive force is associated with the elongation of a protein filament (or a bundle of protein filaments) via polymerization reactions at the ends. The elongation is faster at one end, denoted the plus end; the other, slower-growing end is denoted the minus end. The nature of the physical and chemical interactions at the plus end is such as to cause that end to bind strongly to the object being propelled. The minus end may or may not be anchored, depending on the intra- or extracellular character of the specific propulsion process.

One concept underlying this research is that the properties of actin-filament end-tracking motors that afford propulsion in vivo should be exploitable in vitro to effect analogous nanoscale actuation for such purposes as manipulation of beads (see figure), spores, and deoxyribonucleic acid (DNA) molecules. In a typical envisioned application, actin-filament end-tracking motors would be utilized for transport and/or concentration of such particles against diffusion gradients or opposing force fields. For example, it might be possible to use filament-bound nanoparticles or protein complexes with attached oligonucleotides for hybridization to target and separate specific DNA sequences without need for strong electric or magnetic fields required in some prior separation techniques. Uses include:

  • Optimization of conditions for propulsion of particles in cell extracts;
  • Development of single-filament actuators;
  • Guidance of single-filament elongation on patterned and microfabricated substrata;
  • Development and validation of a mathematical model that predicts particlepropulsion velocity as a function of controllable parameters;
  • Development of novel methods of time-of-flight mass spectrometry for imaging of surfaces; and
  • Development of techniques for direct, real-time observation of protein-protein interactions involved in filament end-tracking in vivo.

This work was done by Richard B. Dickinson, Daniel L. Purich, William Zeile, Joseph Phillips, Colin Sturm, and Kimberly Interliggi of the University of Florida; Suzanne Hens, Gary McGuire, Mark Ray, and Darin Thomas of the International Technology Center; Brian Holliday and William Cooke of the College of William and Mary; and Denis Wirtz and Melissa Thompson of Johns Hopkins University for the Defense Advanced Research Projects Agency (DARPA).


This Brief includes a Technical Support Package (TSP).

Nanodevices Based on Actin-Filament End-Tracking Motors (reference DARPA-0007) is currently available for download from the TSP library.

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