Tether-scanning the kinesin motor domain reveals a core mechanical action.

Natural kinesin motors are tethered to their cargoes via short C-terminal or N-terminal linkers, whose docking against the core motor domain generates directional force. It remains unclear whether linker docking is the only process contributing directional force or whether linker docking is coupled to and amplifies an underlying, more fundamental force-generating mechanical cycle of the kinesin motor domain. Here, we show that kinesin motor domains tethered via double-stranded DNAs (dsDNAs) attached to surface loops drive robust microtubule (MT) gliding. Tethering using dsDNA attached to surface loops disconnects the C-terminal neck-linker and the N-terminal cover strand so that their dock-undock cycle cannot exert force. The most effective attachment positions for the dsDNA tether are loop 2 or loop 10, which lie closest to the MT plus and minus ends, respectively. In three cases, we observed minus-end-directed motility. Our findings demonstrate an underlying, potentially ancient, force-generating core mechanical action of the kinesin motor domain, which drives, and is amplified by, linker docking.

Fast and Easy Transient Mammalian Cell Expression and Purification of Cytoplasmic Dynein.

Recombinant protein expression has been key to studying dynein's mechanochemistry and structure-function relationship. To gain further insight into the energy-converting mechanisms and interactions with an increasing variety of dynein cargos and regulators, rapid expression and purification of a variety of dynein proteins and fragments are important. Here we describe transient expression of cytoplasmic dynein in HEK293 cells and fast small-scale purification for high-throughput protein engineering. Mammalian cell expression might be generally considered to be a laborious process, but with recent technology and some simple inexpensive custom-built labware, dynein expression and purification from mammalian cells can be fast and easy.

Programmable molecular transport achieved by engineering protein motors to move on DNA nanotubes.

Intracellular transport is the basis of microscale logistics within cells and is powered by biomolecular motors. Mimicking transport for in vitro applications has been widely studied; however, the inflexibility in track design and control has hindered practical applications. Here, we developed protein-based motors that move on DNA nanotubes by combining a biomolecular motor dynein and DNA binding proteins. The new motors and DNA-based nanoarchitectures enabled us to arrange the binding sites on the track, locally control the direction of movement, and achieve multiplexed cargo transport by different motors. The integration of these technologies realized microscale cargo sorters and integrators that automatically transport molecules as programmed in DNA sequences on a branched DNA nanotube. Our system should provide a versatile, controllable platform for future applications.

Collective motility of dynein linear arrays built on DNA nanotubes.

Dynein motor proteins usually work as a group in vesicle transport, mitosis, and ciliary/flagellar beating inside cells. Despite the obvious importance of the functions of dynein, the effect of inter-dynein interactions on collective motility remains poorly understood due to the difficulty in building large dynein ensembles with defined geometry. Here, we describe a method to build dynein ensembles to investigate the collective motility of dynein on microtubules. Using electron microscopy, we show that tens to hundreds of cytoplasmic dynein monomers were anchored along a 4- or 10-helix DNA nanotube with an average periodicity of 19 or 44 nm (a programmed periodicity of 14 or 28 nm, respectively). They drove the sliding movement of DNA nanotubes along microtubules at a velocity of 170-620 nm/s. Reducing the stiffness of DNA nanotubes made the nanotube movement discontinuous and considerably slower. Decreasing the spacing between motors simply slowed down the nanotube movement. This slowdown was independent of the number of motors involved but heavily dependent on motor-motor distance. This suggests that steric hindrance or mechanical coupling between dynein molecules was responsible for the slowdown. Furthermore, we observed cyclical buckling of DNA nanotubes on microtubules, reminiscent of ciliary/flagellar beating. These results highlight the importance of the geometric arrangement of dynein motors on their collective motility.

Re-engineering of protein motors to understand mechanisms biasing random motion and generating collective dynamics.

A considerable amount of insight into the mechanisms of protein-based biomolecular motors has been accumulated over decades of research. However, our knowledge about the design principles of these motors is still limited. Even less is known about the design of multi-motor systems that perform various functions within the cell. Here we focus on constructive (or synthetic) approaches to biomolecular motors that could make a breakthrough in our understanding. Recent achievements include studies at different hierarchical levels of complexity: re-engineering of individual motors, construction of multi-motor systems, and generation of large-scale complex behaviour. We then propose a strategy where the collective behaviour can be repeatedly tested upon modifying individual motors, which may provide important clues about how biomolecular motors and their systems are designed.

Creating biomolecular motors based on dynein and actin-binding proteins.

Biomolecular motors such as myosin, kinesin and dynein are protein machines that can drive directional movement along cytoskeletal tracks and have the potential to be used as molecule-sized actuators. Although control of the velocity and directionality of biomolecular motors has been achieved, the design and construction of novel biomolecular motors remains a challenge. Here we show that naturally occurring protein building blocks from different cytoskeletal systems can be combined to create a new series of biomolecular motors. We show that the hybrid motors-combinations of a motor core derived from the microtubule-based dynein motor and non-motor actin-binding proteins-robustly drive the sliding movement of an actin filament. Furthermore, the direction of actin movement can be reversed by simply changing the geometric arrangement of these building blocks. Our synthetic strategy provides an approach to fabricating biomolecular machines that work along artificial tracks at nanoscale dimensions.

Autoinhibition and cooperative activation mechanisms of cytoplasmic dynein.

Cytoplasmic dynein is a two-headed microtubule-based motor responsible for diverse intracellular movements, including minus-end-directed transport of organelles. The motility of cargo transporters is regulated according to the presence or absence of cargo; however, it remains unclear how cytoplasmic dynein achieves such regulation. Here, using a recombinant and native dynein complex in vitro, we show that lone, single dynein molecules are in an autoinhibited state, in which the two motor heads are stacked together. In this state, dynein moves diffusively along a microtubule with only a small bias towards the minus end of the microtubule. When the two heads were physically separated by a rigid rod, the movement of dynein molecules became directed and processive. Furthermore, assembly of multiple dynein molecules on a single cargo enabled them to move unidirectionally and generate force cooperatively. We thus propose a mechanism of autonomous on-off switching of cargo transport, in which single dynein molecules in the cell are autoinhibited through intramolecular head-head stacking and become active when they assemble as a team on a cargo.

Cooperative binding of the outer arm-docking complex underlies the regular arrangement of outer arm dynein in the axoneme.

Outer arm dynein (OAD) in cilia and flagella is bound to the outer doublet microtubules every 24 nm. Periodic binding of OADs at specific sites is important for efficient cilia/flagella beating; however, the molecular mechanism that specifies OAD arrangement remains elusive. Studies using the green alga Chlamydomonas reinhardtii have shown that the OAD-docking complex (ODA-DC), a heterotrimeric complex present at the OAD base, functions as the OAD docking site on the doublet. We find that the ODA-DC has an ellipsoidal shape ∼24 nm in length. In mutant axonemes that lack OAD but retain the ODA-DC, ODA-DC molecules are aligned in an end-to-end manner along the outer doublets. When flagella of a mutant lacking ODA-DCs are supplied with ODA-DCs upon gamete fusion, ODA-DC molecules first bind to the mutant axonemes in the proximal region, and the occupied region gradually extends toward the tip, followed by binding of OADs. This and other results indicate that a cooperative association of the ODA-DC underlies its function as the OAD-docking site and is the determinant of the 24-nm periodicity.

Measuring collective transport by defined numbers of processive and nonprocessive kinesin motors.

Intracellular transport is thought to be achieved by teams of motor proteins bound to a cargo. However, the coordination within a team remains poorly understood as a result of the experimental difficulty in controlling the number and composition of motors. Here, we developed an experimental system that links together defined numbers of motors with defined spacing on a DNA scaffold. By using this system, we linked multiple molecules of two different types of kinesin motors, processive kinesin-1 or nonprocessive Ncd (kinesin-14), in vitro. Both types of kinesins markedly increased their processivities with motor number. Remarkably, despite the poor processivity of individual Ncd motors, the coupling of two Ncd motors enables processive movement for more than 1 µm along microtubules (MTs). This improvement was further enhanced with decreasing spacing between motors. Force measurements revealed that the force generated by groups of Ncd is additive when two to four Ncd motors work together, which is much larger than that generated by single motors. By contrast, the force of multiple kinesin-1s depends only weakly on motor number. Numerical simulations and single-molecule unbinding measurements suggest that this additive nature of the force exerted by Ncd relies on fast MT binding kinetics and the large drag force of individual Ncd motors. These features would enable small groups of Ncd motors to crosslink MTs while rapidly modulating their force by forming clusters. Thus, our experimental system may provide a platform to study the collective behavior of motor proteins from the bottom up.

Systematic comparison of in vitro motile properties between Chlamydomonas wild-type and mutant outer arm dyneins each lacking one of the three heavy chains.

Outer arm dynein (OAD) of cilia and flagella contains two or three distinct heavy chains, each having a motor function. To elucidate their functional difference, we compared the in vitro motile properties of Chlamydomonas wild-type OAD containing the alpha, beta, and gamma heavy chains and three kinds of mutant OADs, each lacking one of the three heavy chains. For systematic comparison, a method was developed to introduce a biotin tag into a subunit, LC2, which served as the specific anchoring site on an avidin-coated glass surface. Wild-type OAD displayed microtubule gliding in the presence of ATP and ADP, with a maximal velocity of 5.0 mum/s, which is approximately 1/4 of the microtubule sliding velocity in the axoneme. The duty ratio was estimated to be as low as 0.08. The absence of the beta heavy chain lowered both the gliding velocity and ATPase activity, whereas the absence of the gamma heavy chain increased both activities. Strikingly, the absence of the alpha heavy chain lowered the gliding velocity but increased the ATPase activity. Thus, the three heavy chains are likely to play distinct roles and regulate each other to achieve coordinated force production.