Autonomous assembly and disassembly of gliding molecular robots regulated by a DNA-based molecular controller.

In recent years, there has been a growing interest in engineering dynamic and autonomous systems with robotic functionalities using biomolecules. Specifically, the ability of molecular motors to convert chemical energy to mechanical forces and the programmability of DNA are regarded as promising components for these systems. However, current systems rely on the manual addition of external stimuli, limiting the potential for autonomous molecular systems. Here, we show that DNA-based cascade reactions can act as a molecular controller that drives the autonomous assembly and disassembly of DNA-functionalized microtubules propelled by kinesins. The DNA controller is designed to produce two different DNA strands that program the interaction between the microtubules. The gliding microtubules integrated with the controller autonomously assemble to bundle-like structures and disassemble into discrete filaments without external stimuli, which is observable by fluorescence microscopy. We believe this approach to be a starting point toward more autonomous behavior of motor protein-based multicomponent systems with robotic functionalities.

Predictable Synthesis of 3D Polymer Networks Using Crystal Component-Linking.

Controlled synthesis of 3D polymer networks presents a significant challenge because of the complexity of the polymerization reaction in solution. In this study, a polymerization system that facilitates the prediction of a polymer network structure via percolation simulations is realized. The most significant difference between general percolation simulations and experimental polymerization systems is the mobility of the molecules during the reaction. A crystal component-linking method that connects the precisely arranged monomer as a supramolecular crystalline state to imitate the simple percolation theory is adopted. The percolation simulation based on the crystal structure of the arranged monomers is used to accurately calculate the gelation point, gel fraction, degree of swelling, and atomic formula, which correspond with the experimental results. This suggests that the network structures polymerized via the crystal component-linking method can be predicted precisely by a simple percolation simulation. Further, the percolation simulation predicts the structures of the loop, branched polymer, and crosslinking point, which are difficult to measure experimentally. The polymerization of precisely-arranged immobilized monomers in supramolecular structures is promising in synthesizing precisely controlled polymer networks.

Role of tubulin C-terminal tail on mechanical properties of microtubule.

Tubulin C-terminal tail (CTT) is a disordered segment extended from each tubulin monomer of αß tubulin heterodimers, the building blocks of microtubules. The tubulin CTT contributes to the cellular function of microtubules such as intracellular transportation by regulating their interaction with other proteins and cell shape regulation by controlling microtubule polymerization dynamics. Although the mechanical integrity of microtubules is crucial for their functions, the role of tubulin CTT on microtubule mechanical properties has remained elusive. In this work, we investigate the role of tubulin CTTs in regulating the mechanical properties of microtubules by estimating the persistence lengths and investigating the buckling behavior of microtubules with and without CTT. We find that microtubules with intact CTTs exhibit twice the rigidity of microtubules lacking tubulin CTTs. Our study will widen the scope of altering microtubule mechanical properties for its application in nano bio-devices and lead to novel therapeutic approaches for neurodegenerative diseases with altered microtubule properties.

Chromic triboluminescence of self-assembled platinum(II) complexes.

A series of Pt(II) complexes bearing N-heterocyclic carbenes, [Pt(CN)2(Rim-Mepy)] (Rim-MepyH+ = 3-alkyl-1-(4-methyl-(2-pyridinyl))-1H-imidazolium, R = Me, Et, iPr, or tBu), exhibits triboluminescence in the visible range from blue to red, as well as the corresponding intense photoluminescence. Remarkably, among the complexes, the iPr-substituted one exhibits chromic triboluminescence behaviour during the process of rubbing and also vapour exposure.

Reversible Photocontrol of Microtubule Stability by Spiropyran-Conjugated Tau-Derived Peptides.

Spatiotemporal modulation of microtubules by light has become an important aspect of the biological and nanotechnological applications of microtubules. We previously developed a Tau-derived peptide as a binding unit to the inside of microtubules. Here, we conjugated the Tau-derived peptide to spiropyran, which is reversibly converted to merocyanine by light, as a reversible photocontrol system to stabilize microtubules. Among the synthesized peptides with spiropyran/merocyanine at different positions, several peptides were bound to the inside of microtubules and stabilized the structures of microtubules. The peptide with spiropyran at the N-terminus induced polymerization and stabilization of microtubules, whereas the same peptide with the merocyanine form did not exert these effects. Reversible formation of microtubules/tubulin aggregates was achieved using the peptide with spiropyran conjugated at the N-terminus and irradiation with UV and visible light. Spiropyran-conjugated Tau-derived peptides would be useful for spatiotemporal modulation of microtubule stability through reversible photocontrol of binding.

Generation of stable microtubule superstructures by binding of peptide-fused tetrameric proteins to inside and outside.

Microtubules play important roles in biological functions by forming superstructures, such as doublets and branched structures, in vivo. Despite the importance, it is challenging to construct these superstructures in vitro. Here, we designed a tetrameric fluorescent protein Azami-Green (AG) fused with His-tag and Tau-derived peptide (TP), TP-AG, to generate the superstructures. Main binding sites of TP-AG can be controlled to the inside and outside of microtubules by changing the polymerization conditions. The binding of TP-AG to the inside promoted microtubule formation and generated rigid and stable microtubules. The binding of TP-AG to the outside induced various microtubule superstructures, including doublets, multiplets, branched structures, and extremely long microtubules by recruiting tubulins to microtubules. Motile microtubule aster structures were also constructed by TP-AG. The generation of various microtubule superstructures by a single type of exogenous protein is a new concept for understanding the functions of microtubules and constructing microtubule-based nanomaterials.

Light-induced stabilization of microtubules by photo-crosslinking of a Tau-derived peptide.

For light-induced stabilization of microtubules (MTs) to manipulate cells, a photo-reactive diazirine group was conjugated to a Tau-derived peptide, a motif binding on the inside of MTs. Ultraviolet (UV) light irradiation induced significant stabilization of MTs via the formation of a covalent bond of the peptide and showed toxicity.

Fluctuation in the Sliding Movement of Kinesin-Driven Microtubules Is Regulated Using the Deep-Sea Osmolyte Trimethylamine N-Oxide.

Nowadays, biomolecular motor-based miniaturized lab-on-a-chip devices have been attracting much attention for their wide range of nanotechnological applications. Most of the applications are dependent on the motor-driven active transportation of their associated filamentous proteins as shuttles. Fluctuation in the movement of the shuttles is a major contributor to the dispersion in motor-driven active transportation, which limits the efficiency of the miniaturized devices. In this work, by employing the biomolecular motor kinesin and its associated protein filament microtubule as a model active transport system, we demonstrate that the deep-sea osmolyte trimethylamine N-oxide (TMAO) is useful in regulating the fluctuation in the motility of microtubule shuttles. We show that the motional diffusion coefficient, a measure of the fluctuation in the movement of the kinesin-propelled microtubules, gradually decreases upon increasing the concentration of TMAO in the transportation system. We have been able to reduce the motional diffusion coefficient of microtubules more than 200 times by employing TMAO at a concentration of 2 M. We also show that upon elimination of TMAO, the motional diffusion coefficient of microtubules can be restored, which confirms that TMAO can be used as a tool to reversibly regulate the fluctuation in the sliding movement of kinesin-propelled microtubule shuttles. Such reversible regulation of the dynamic behavior of the shuttles does not require sacrificing the concentration of fuel used for transportation. Our results confirm the ability to manipulate the nanoscale motion of biomolecular motor-driven active transporters in an artificial environment. This work is expected to further enhance the tunability of biomolecular motor functions, which, in turn, will foster their nanotechnological applications based on active transportation.

Controlling the Rigidity of Kinesin-Propelled Microtubules in an In Vitro Gliding Assay Using the Deep-Sea Osmolyte Trimethylamine N-Oxide.

The biomolecular motor protein kinesin and its associated filamentous protein microtubule have been finding important nanotechnological applications in the recent years. Rigidity of the microtubules, which are propelled by kinesin motors in an in vitro gliding assay, is an important metric that determines the success of utilization of microtubules and kinesins in various applications, such as transportation, sensing, sorting, molecular robotics, etc. Therefore, regulating the rigidity of kinesin-propelled microtubules has been critical. In this work, we report a simple strategy to regulate the rigidity of kinesin-propelled microtubules in an in vitro gliding assay. We demonstrate that rigidity of the microtubules, propelled by kinesins in an in vitro gliding assay, can be modulated simply by using the natural osmolyte trimethylamine N-oxide (TMAO). By varying the concentration of TMAO in the gliding assay, the rigidity of microtubules can be modulated over a wide range. Based on this strategy, we are able to reduce the persistence length of microtubules, a measure of microtubule rigidity, ∼8 fold by using TMAO at the concentration of 1.5 M. Furthermore, we found that the decreased rigidity of the kinesin-propelled microtubules can be restored upon elimination of TMAO from the in vitro gliding assay. Alteration in the rigidity of microtubules is accounted for by the non-uniformity of the force applied by kinesins along the microtubules in the presence of TMAO. This work offers a facile strategy to reversibly regulate the rigidity of kinesin-propelled microtubules in situ, which would widen the applications of the biomolecular motor kinesin and its associated protein microtubule in various fields.

1,2-Disubstituted 1,2-Dihydro-1,2,4,5-tetrazine-3,6-dione as a Dynamic Covalent Bonding Unit at Room Temperature.

Dynamic covalent bonds are useful tools in a wide range of applications. Although various reversible chemical reactions have been studied for this purpose, the requirement for harsh conditions, such as high temperature and low or high pH, to activate generally stable covalent bonds limits their potential applications involving biomolecules or household utilization. Here, we report the design, synthesis, characterization, and dynamic covalent bonding properties of 1,2-disubstituted 1,2-dihydro-1,2,4,5-tetrazine-3,6-dione (TETRAD). Hetero-Diels-Alder reactions of TETRAD with furan derivatives and their retro-reactions proceeded rapidly at room temperature under neutral conditions, enabling a chemically induced sol-gel transition system.

Deformation of microtubules regulates translocation dynamics of kinesin.

Microtubules, the most rigid components of the cytoskeleton, can be key transduction elements between external forces and the cellular environment. Mechanical forces induce microtubule deformation, which is presumed to be critical for the mechanoregulation of cellular events. However, concrete evidence is lacking. In this work, with high-speed atomic force microscopy, we unravel how microtubule deformation regulates the translocation of the microtubule-associated motor protein kinesin-1, responsible for intracellular transport. Our results show that the microtubule deformation by bending impedes the translocation dynamics of kinesins along them. Molecular dynamics simulation shows that the hindered translocation of kinesins can be attributed to an enhanced affinity of kinesins to the microtubule structural units in microtubules deformed by bending. This study advances our understanding of the role of cytoskeletal components in mechanotransduction.

Monopolar flocking of microtubules in collective motion.

Flocking is a fascinating coordinated behavior of living organisms or self-propelled particles (SPPs). Particularly, monopolar flocking has been attractive due to its potential applications in various fields. However, the underlying mechanism behind flocking and emergence of monopolar motion in flocking of SPPs has remained obscured. Here, we demonstrate monopolar flocking of kinesin-driven microtubules, a self-propelled biomolecular motor system. Microtubules with an intrinsic structural chirality preferentially move towards counter-clockwise direction. At high density, the CCW motion of microtubules facilitates monopolar flocking and formation of a spiral pattern. The monopolar flocking of microtubules is accounted for by a torque generated when the motion of microtubules was obstructed due to collisions. Our results shed light on flocking and emergence of monopolar motion in flocking of chiral active matters. This work will help regulate the polarity in collective motion of SPPs which in turn will widen their applications in nanotechnology, materials science and engineering.

Controlling the length of self-assembled microtubes through mechanical stress-induced scission.

We demonstrate that mechanical stress-induced scission is an effective strategy to control the length of self-assembled microtubes. By applying mechanical stress with variable magnitude and mode, the length of microtubes can be tightly regulated. We have succeeded in reducing the average length of microtubes ∼twenty-fold through stretching and compression. The mechanical stress-induced scission of self-assembled, long microtubes into smaller fragments has no adverse effect on the functionality of the microtubes. This work will foster the applications of length-controlled, self-assembled microtubes in various fields.

Photo-regulated trajectories of gliding microtubules conjugated with DNA.

We regulate the persistency in motion of kinesin-driven microtubules (MTs) simply using a photoresponsive DNA (pDNA) and ultraviolet (UV)-visible light. The path persistence length of MTs, which is a measure of the persistency in their motion, increases and decreases upon illuminating the MTs with UV and visible light respectively. Moreover, pDNA is found to work as a shield for MTs against damage under UV irradiation.

Photoinduced Pyramidal Inversion Behavior of Phosphanes Involved with Aggregation-Induced Emission Behavior.

Invited for the cover of this issue is Kenta Kokado and co-workers at Hokkaido University. The cover picture describes the interesting pyramidal inversion behavior of phosphanes in the excited state, like entering "the Mirror World", which we found in this research. Read the full text of the article at 10.1002/chem.202000264.

Magnetic Force-Induced Alignment of Microtubules by Encapsulation of CoPt Nanoparticles Using a Tau-Derived Peptide.

Construction of magnetotactic materials is a significant challenge in nanotechnology applications such as nanodevices and nanotransportation. Artificial magnetotactic materials can be designed from magnetotactic bacteria because these bacteria use magnetic nanoparticles for aligning with and moving within magnetic fields. Microtubules are attractive scaffolds to construct magnetotactic materials because of their intrinsic motility. Nonetheless, it is challenging to magnetically control their orientation while retaining their motility by conjugating magnetic nanoparticles on their outer surface. Here we solve the issue by encapsulating magnetic cobalt-platinum nanoparticles inside microtubules using our developed Tau-derived peptide that binds to their internal pockets. The in situ growth of cobalt-platinum nanoparticles resulted in the formation of a linear-chain assembly of nanoparticles inside the microtubules. The magnetic microtubules significantly aligned with a high order parameter (0.71) along the weak magnetic field (0.37 T) and showed increased motility. This work provides a new concept for designing magnetotactic materials.

Photoinduced Pyramidal Inversion Behavior of Phosphanes Involved with Aggregation-Induced Emission Behavior.

Aggregation-induced emission (AIE) is a fascinating phenomenon because of the applications of luminescent materials in the aggregated state, which exploit the large structural changes of the molecules in the excited state. Recently, it was reported that triphenylphosphane derivatives show AIE behavior in which they undergo potentially large structural changes in the excited state. Inspired by this report, photoinduced pyramidal inversion behavior of phosphanes was investigated. In photochemical experiments, the prepared P-stereogenic phosphanes exhibited photoracemization in dilute solution, and a negative correlation was observed between the photoracemization and the AIE phenomenon. Theoretical computations revealed that the inversion barrier in the excited state was much smaller than that in the ground state. This is the first report on the photoinduced pyramidal inversion behavior of phosphanes, which will provide new and unexplored applications.

Breaking of buckled microtubules is mediated by kinesins.

Microtubule is the most rigid component of eukaryotic cytoskeleton that plays pivotal roles in many important cellular events. Microtubules are known to undergo bending or buckling in cells which often results in breaking of this cytoskeletal protein filament. Various cellular events such as cell migration, chromosome segregation, etc. are dependent on the buckling induced breaking of microtubules. However, the reason behind the breaking of buckled microtubules in cell has remained obscure yet. In this work, we have demonstrated breaking of microtubules on a 2D elastic medium by applying compressive stress. The applied compressive stress caused buckling of the microtubules which ultimately resulted in their breaking. We show that breaking of the buckled microtubules cannot be accounted for by considering the changes in curvature of the microtubules due to mechanical deformation. Our results confirm that, it is the interaction of kinesin, a microtubule-associated motor protein, with microtubules which plays the key role in breaking of the buckled microtubules on the 2D elastic medium. The breaking of buckled microtubules is ascribed to decrease in rigidity of microtubules upon interaction with kinesins. This work for the first time confirms the involvement of a microtubule-associated motor protein in breaking of microtubules under compressive stress, which will help further clarify the mechanism of breaking of buckled microtubules in cells and its significance in the cellular events.

Controlling the kinetics of interaction between microtubules and kinesins over a wide temperature range using the deep-sea osmolyte trimethylamine N-oxide.

Trimethylamine N-oxide is found to be effective in regulating the interaction between microtubules and kinesins over a wide temperature range. The lifetime of the motility of microtubules on kinesins at high temperatures is prolonged using trimethylamine N-oxide. The activation energy of microtubule motility is increased by trimethylamine N-oxide. Prolonged operation at high temperatures decreased the activation energy of MT motility despite the increase in concentration of trimethylamine N-oxide.

Regulation of Biomolecular-Motor-Driven Cargo Transport by Microtubules under Mechanical Stress.

Mechanical stress on cells has profound influences on biological processes, such as cell shape regulation, the formation of tissue patterns, and development. Recently, mechanosensing properties of the microtubule, an important cytoskeletal component, have drawn attention. In this work, we studied cargo transport by dynein, a microtubule-associated motor protein, along microtubules deformed under mechanical stress. We reveal that the microtubule deformation took place as a response to the applied stress and that the deformation of microtubules facilitated the transport of dynein-driven quantum dots. This finding will provide opportunities to explore the role of microtubules as molecular mechanotransducers in cellular processes.