Fluctuation of actin sliding over myosin thick filaments in vitro

Models of protein linear molecular motors for dynamic nanodevices

Protein molecular motors are natural nano-machines that convert the chemical energy from the hydrolysis of adenosine triphosphate into mechanical work. These efficient machines are central to many biological processes, including cellular motion, muscle contraction and cell division. The remarkable energetic efficiency of the protein molecular motors coupled with their nano-scale has prompted an increasing number of studies focusing on their integration in hybrid micro- and nanodevices, in particular using linear molecular motors. The translation of these tentative devices into technologically and economically feasible ones requires an engineering, design-orientated approach based on a structured formalism, preferably mathematical. This contribution reviews the present state of the art in the modelling of protein linear molecular motors, as relevant to the future design-orientated development of hybrid dynamic nanodevices.

Direction and speed of microtubule movements driven by kinesin motors arranged on catchin thick filaments

Conventional kinesin (Kinesin-1) is a microtubule-based molecular motor that supports intracellular vesicle/organelle transport in various eukaryotic cells. To arrange kinesin motors similarly to myosin motors on thick filaments in muscles, the motor domain of rat conventional kinesin (amino acid residues 1-430) fused to the C-terminal 829 amino acid residues of catchin (KHC430Cat) was bacterially expressed and attached to catchin filaments that can attach to and arrange myosin molecules in a bipolar manner on their surface. Unlike the case of myosin where actin filaments move toward the center much faster than in the opposite direction along the catchin filaments, microtubules moved at the same speed in both directions. In addition, many microtubules moved across the filaments at the same speed with various angles between the axes of the microtubule and catchin filament. Kinesin/catchin chimera proteins with a shorter kinesin neck domain were also prepared. Those without the whole hinge 1 domain and the C-terminal part of the neck helix moved microtubules toward the center of the catchin filaments significantly, but only slightly, faster than in the opposite direction, although the movements in both directions were slower than those of the KHC430Cat construct. The results suggest that kinesin has substantial mechanical flexibility within the motor domain, possibly within the neck linker, enabling its interaction with microtubules having any orientation.

Comparing guiding track requirements for myosin- and kinesin-powered molecular shuttles

The design of nanoscale transport systems utilizing motor proteins as engines has advanced rapidly. Here, actin/myosin- and microtubule/kinesin-based molecular shuttles are compared with respect to their requirements for track designs. To this end, the trajectory persistence length of actin filaments gliding on myosin-coated surfaces has been experimentally determined to be equal to 8.8 +/- 2 microm. This measurement complements an earlier determination of the trajectory persistence length of microtubules gliding on kinesin-coated surfaces and enables a comparison of the accessible track designs for kinesin and myosin motor-powered systems. Despite the 200-fold smaller stiffness of actin filaments compared to that of microtubules, the dimensions of myosin tracks for actin filaments have to be quite similar to the dimensions of kinesin tracks for microtubules (radii larger than 200 nm and widths smaller than 0.9 microm compared to 600 nm and 19 microm). The difference in gliding speed is shown to require additional consideration in the design of track modules.

Anomalous fluctuations in sliding motion of cytoskeletal filaments driven by molecular motors: model simulations

It has been found in in vitro experiments that cytoskeletal filaments driven by molecular motors show finite diffusion in sliding motion even in the long filament limit [Imafuku, Y. et al. Biophys. J. 1996, 70, 878-886. Noda, N. et al. Biophysics 2005, 1, 45-53]. This anomalous fluctuation can be evidence for cooperativity among the motors in action because fluctuation should be averaged out for a long filament if the action of each motor is independent. In order to understand the nature of the fluctuation in molecular motors, we perform numerical simulations and analyze velocity correlation in three existing models that are known to show some kind of cooperativity and/or large diffusion coefficient, i.e., the Sekimoto-Tawada model [Sekimoto, K.; Tawada, K. Phys. Rev. Lett. 1995, 75, 180], the Prost model [Prost, J. et al. Phys. Rev. Lett. 1994, 72, 2652], and the Duke model [Duke, T. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 2770]. It is shown that the Prost model and the Duke model do not give a finite diffusion in the long filament limit, in spite of the collective action of motors. On the other hand, the Sekimoto-Tawada model has been shown to give a diffusion coefficient that is independent of filament length, but it comes from the long time correlation whose time scale is proportional to filament length, and our simulations show that such a long correlation time conflicts with the experimental time scales. We conclude that none of the three models represent experimental findings. In order to explain the observed anomalous diffusion, we have to search for a mechanism that will allow both the amplitude and the time scale of the velocity correlation to be independent of the filament length.