Actin filament guidance on a chip: toward high-throughput assays and lab-on-a-chip applications.

Biological molecular motors that are constrained so that function is effectively limited to predefined nanosized tracks may be used as molecular shuttles in nanotechnological applications. For these applications and in high-throughput functional assays (e.g., drug screening), it is important that the motors propel their cytoskeletal filaments unidirectionally along the tracks with a minimal number of escape events. We here analyze the requirements for achieving this for actin filaments that are propelled by myosin II motor fragments (heavy meromyosin; HMM). First, we tested the guidance of HMM-propelled actin filaments along chemically defined borders. Here, trimethylchlorosilane (TMCS)-derivatized areas with high-quality HMM function were surrounded by SiO(2) domains where HMM did not bind actin. Guidance along the TMCS-SiO(2) border was almost 100% for filament approach angles between 0 and 20 degrees but only about 10% at approach angles near 90 degrees . A model (Clemmens, J.; Hess, H.; Lipscomb, R.; Hanein, Y.; Bohringer, K. F.; Matzke, C. M.; Bachand, G. D.; Bunker, B. C.; Vogel, V. Langmuir 2003, 19, 10967-10974) accounted for essential aspects of the data and also correctly predicted a more efficient guidance of actin filaments than previously shown for kinesin-propelled microtubules. Despite the efficient guidance at low approach angles, nanosized (<700 nm wide) TMCS tracks surrounded by SiO(2) were not effective in guiding actin filaments. Neither was there complete guidance along nanosized tracks that were surrounded by topographical barriers (walls and roof partially covering the track) unless there was also chemically based selectivity between the tracks and surroundings. In the latter case, with dually defined tracks, there was close to 100% guidance. A combined experimental and theoretical analysis, using tracks of the latter type, suggested that a track width of less than about 200-300 nm is sufficient at a high HMM surface density to achieve unidirectional sliding of actin filaments. In accord with these results, we demonstrate the long-term trapping of actin filaments on a closed-loop track (width < 250 nm). The results are discussed in relation to lab-on-a-chip applications and nanotechnology-assisted assays of actomyosin function.