Microtubule cytoskeletons play pivotal roles in various cellular processes, including cell division and locomotion, by dynamically changing their length and distribution in cells through tubulin polymerization/depolymerization. Recent structural studies have revealed the polymorphic lattice structure of microtubules closely correlate with the microtubule dynamics, but the studies were limited to averaged structures. To reveal the transient and localized structures, such as GTP-cap, we developed several non-averaging methods for cryogenic electron tomography to precisely measure the longitudinal spacing and helical twisting of individual microtubule lattices at the tubulin subunit level. Our analysis revealed that polymerizing and depolymerizing ends share a similar structure with regards to lattice spacing. The most distinctive property specific to the polymerizing plus end was left-handed twisting in the inter-dimer interface, suggesting that the twisting might accelerate fast polymerization at the plus ends. Our analysis uncovered the heterogeneity of native microtubules and will be indispensable for the study of microtubules dynamics under physiological contexts or during specific cellular events.### Competing Interest StatementThe authors have declared no competing interest.

The third Japan-U.S. symposium on motor proteins and associated single-molecule biophysics will be held on 15 November 2023, during the 61 st Annual Meeting of the Biophysical Society of Japan.This is a series of motor protein symposia, starting in 2021 [1,2], that will bring together researchers from Japan and the U.S.-two leading countries in the field-to foster the exchange of ideas and promote cutting-edge collaborative research.With a lineup of renowned experts in the field, this symposium provides an exceptional opportunity to present the latest advances in our understanding of motor protein movement and regulation.This year's symposium will focus on microtubule-based motility (Figure 1), where a number of new perspectives on the research topic have recently emerged.Microtubules and its associated motors provide driving forces for various types of movement in eukaryotic cells [3], such as chromosome segregation during cell division, morphological changes of developing cells, and intracellular transport of vesicles, organelles and viruses.The symposium will begin with the application of microtubule-based motility as a computer.The movement of microtubules driven by motors has long been used as a method to measure motor activity [4,5].Later, with the discovery of the property that crowded microtubules can form groups and show peculiar patterns when sliding on a surface [6], this phenomenon has attracted much attention in terms of nonequilibrium statistical mechanics, a discipline of physics that studies the motion patterns of aggregated self-moving objects [7].In this context, the Kakugo lab has developed a technique to manipulate the formation of microtubule swarms using light [8], and has made remarkable achievements in controlling the collective motion of microtubules.As a graduate student in the Kakugo lab, Mr. Yiming Gong (Kyoto University) will give a talk on the development of a method to use the swarming of microtubules as a computer.His computational system also revealed a new aspect of microtubule swarming.The symposium will then turn to the molecular properties and regulatory mechanisms of motors.The microtubule cytoskeleton extends throughout the cytoplasm in eukaryotic cells, and vesicles are transported along the cytoskeletal network by the motors: kinesins and cytoplasmic dynein.While kinesins, which are mainly responsible for movement toward the plus end of microtubules, are subdivided into 45 species in humans [9], only one type of cytoplasmic dynein Figure 1 Microtubule-based motility.In eukaryotic cells, many cargoes, including mitochondria, Golgi and endoplasmic vesicles, and viral particles, are transported along microtubules by kinesin and dynein motors.This symposium will dissect this motility from various aspects, such as its kinetics, role in the cell, regulatory system, and application.

Additional file 5: Table S3. Mean fluorescence intensity of detected aggregates of three experimental replicates for Additional file 1: Fig. S4.

CAMSAPs are proteins that show microtubule minus-end-specific localization, decoration, and stabilization. Although the mechanism for minus-end recognition via their C-terminal CKK domain has been well described in recent studies, it is unclear how CAMSAPs stabilize microtubules. Our several binding assays revealed that the D2 region of CAMSAP3 specifically binds to microtubules with the expanded lattice. To investigate the relationship between this preference and the stabilization effect of CAMSAP3, we precisely measured individual microtubule lengths and found that D2 binding expanded the microtubule lattice by ∼3%. Consistent with the notion that the expanded lattice is a common feature of stable microtubules, the presence of D2 slowed the microtubule depolymerization rate to ∼1/20, suggesting that the D2-triggered lattice expansion stabilizes microtubules. Combining these results, we propose that CAMSAP3 stabilizes microtubules by lattice expansion upon D2 binding, which further accelerates the recruitment of other CAMSAP3 molecules. Because only CAMSAP3 has D2 and the highest microtubule-stabilizing effect among mammalian CAMSAPs, our model also explains the molecular basis for the functional diversity of CAMSAP family members.

BACKGROUND:Argonaute proteins play a central role in RNA silencing by forming protein-small RNA complexes responsible for the silencing process. While most Argonaute proteins have a short N-terminal region, Argonaute2 in Drosophila melanogaster (DmAgo2) harbors a long and unique N-terminal region. Previous in vitro biochemical studies have shown that the loss of this region does not impair the RNA silencing activity of the complex. However, an N-terminal mutant of Drosophila melanogaster has demonstrated abnormal RNA silencing activity. To explore the causes of this discrepancy between in vitro and in vivo studies, we investigated the biophysical properties of the region. The N-terminal region is highly rich in glutamine and glycine residues, which is a well-known property for prion-like domains, a subclass of amyloid-forming peptides. Therefore, the possibility of the N-terminal region functioning as an amyloid was tested.RESULTS:Our in silico and biochemical assays demonstrated that the N-terminal region exhibits amyloid-specific properties. The region indeed formed aggregates that were not dissociated even in the presence of sodium dodecyl sulfate. Also, the aggregates enhanced the fluorescence intensity of thioflavin-T, an amyloid detection reagent. The kinetics of the aggregation followed that of typical amyloid formation exhibiting self-propagating activity. Furthermore, we directly visualized the aggregation process of the N-terminal region under fluorescence microscopy and found that the aggregations took fractal or fibril shapes. Together, the results indicate that the N-terminal region can form amyloid-like aggregates.CONCLUSIONS:Many other amyloid-forming peptides have been reported to modulate the function of proteins through their aggregation. Therefore, our findings raise the possibility that aggregation of the N-terminal region regulates the RNA silencing activity of DmAgo2.

Telomerase reverse transcriptase (TERT) is a protein that catalyzes the reverse transcription of telomere elongation. TERT is also expected to play a non‐canonical role beyond telomere lengthening since it localizes not only in the nucleus but also in mitochondria, where telomeres do not exist. Several studies have reported that mitochondrial TERT regulates apoptosis induced by oxidative stress. However, there is still some controversy as to whether mitochondrial TERT promotes or inhibits apoptosis, mainly due to the lack of information on changes in TERT distribution in individual cells over time. Here, we simultaneously detected apoptosis and TERT localization after oxidative stress in individual HeLa cells by live‐cell tracking. Single‐cell tracking revealed that the stress‐induced accumulation of TERT in mitochondria caused apoptosis, but that accumulation increased over time until cell death. The results suggest a new model in which mitochondrial TERT has two opposing effects at different stages of apoptosis: it predetermines apoptosis at the first stage of cell‐fate determination, but also delays apoptosis at the second stage. As such, our data support a model that integrates the two opposing hypotheses on mitochondrial TERT's effect on apoptosis. Furthermore, detailed statistical analysis of TERT mutations, which have been predicted to inhibit TERT transport to mitochondria, revealed that these mutations suppress apoptosis independent of mitochondrial localization of TERT. Together, these results imply that the non‐canonical functions of TERT affect a wide range of mitochondria‐dependent and mitochondria‐independent apoptosis pathways.

Cytoplasmic dynein 1 is almost exclusively responsible for intracellular transport toward the minus-end of microtubules in animal cells. One of the key factors for the unidirectional movement of dynein is the asymmetry of the unbinding of the motor from the microtubule when an external load is applied; it dissociates more easily from microtubules with minus-end directed loading than with plus-end directed loading. To elucidate the molecular basis for this property, we performed molecular dynamics simulations to identify the key residues responsible for asymmetry, which were then examined experimentally. First, we reproduced asymmetry in the unbinding behavior of dynein using coarse-grained simulations. Then, data analysis together with mutational analysis in silico predicted the specific residues that may be responsible for the asymmetry in unbinding. To examine this prediction, we expressed and purified recombinant dynein with mutations in either of the identified key residues. Consistent with the simulations, one of the mutants did not exhibit asymmetry in the in vitro unbinding assay. Moreover, the mutant dynein was able to bind and move diffusely along a microtubule but was unable to restrict its movement to the minus-end direction. Our results demonstrate both experimentally and theoretically how the key residue on the microtubule-binding domain generates asymmetry in unbinding, which is a critical mechanism for the unidirectional movement of dynein along a microtubule track. Significance Statement Cytoplasmic dynein moves to the minus end of microtubules. This unidirectional dynein motility provides the driving force for various cellular activities including vesicle transport, organelle positioning and cell division. One of the key factors for dynein to exhibit unidirectional movement is the asymmetry of unbinding of dynein from the microtubule depending on the direction of external load. By combining computational simulations and in vitro experiments, we identified a residue responsible for the asymmetry. A point mutation at the residue indeed abolished unidirectional motility, highlighting the importance of the asymmetric unbinding property in dynein’s unidirectional movement.

Hidden Markov model (HMM) is widely used to analyze biophysical chronological data with discrete states, such as protein/nucleotide conformational changes. Despite its usefulness, the classical HMM fitting has practical drawbacks that it requires predefinition of the number of hidden states and fine initialization of many parameters. To overcome the drawbacks, several HMM pre-analyses have been reported, but do not provide enough accuracy when data have unknown kinetics and/or low signal-to-noise ratio. Therefore, in many cases, HMM fitting needs trial-and-error manual process that can impair objectivity of the analysis. In particular, for data composed of numerous hidden states, such as stepping data of cytoskeletal motors, there has been difficulty in HMM analysis because the large number of parameters were almost unable to properly initialized. Here, by combining a statistical step finding method and Gaussian mixture model clustering, we developed a new algorithm for more objective HMM analysis. Our algorithm can execute accurate state number estimation and parameter optimization with fully automated way. Simulation analysis demonstrated that our algorithm accurately fit both fast- and slow-transition trajectories. Compared with the previous method, speed of our algorithm was 10-20 times faster for standard size data. Our algorithm also showed accurate fit of the simulated motor stepping data with more than 10 states, suggesting applicability of the method to the data with numerous states. Furthermore, the algorithm can retain flexibility because some available prior information, such as dwell time of each state, can be integrated into the algorithm via two user-tunable parameters. In summary, our method enables fast, accurate and objective HMM analysis, and broadens the application range of HMM fitting that could provide more accurate interpretation of a wide variety of biophysical data.

The hidden Markov model (HMM) is widely used to analyze biophysical chronological data with discrete states, such as binding/detachment of biomolecules, protein/nucleotide conformational changes and step-like movement of single proteins. Despite its usefulness, classical HMM fitting has practical drawbacks that it requires the determination of the number of hidden states and fine initialization of many parameters before fitting. To overcome these drawbacks, several HMM pre-analyses have been reported, but do not provide enough accuracy when data have unknown kinetics and/or low signal-to-noise ratio. Therefore, in many cases, HMM fitting needs trial-and-error manual process that can impair the objectivity of the analysis. Moreover, for data composed of numerous hidden states, such as stepping data of cytoskeletal motors, there has been difficulty in HMM analysis because the large number of parameters were hardly properly initialized. Here, by combining a statistical step-finding method and the Gaussian mixture model clustering, we developed a new algorithm for more objective HMM analysis. Our algorithm can execute accurate state number estimation and parameter optimization with fully automated way. Simulation analysis demonstrated that our algorithm accurately fit both fast- and slow-transition trajectories. Compared with the previous method, the speed of our algorithm was 10–20 times faster for standard size data. Our algorithm also showed the accurate fit of the simulated motor-stepping data with more than 10 transition states, suggesting the applicability of the method to the data with numerous states. Furthermore, the algorithm is flexible enough to cope with cases where some kinetics are known in advance. Some available prior information, such as the dwell time of each state, can be integrated into the algorithm via two user-tunable parameters. In summary, our method enables fast, accurate and objective HMM analysis, and broadens the application range of HMM fitting that can provide more accurate interpretation of a wide variety of biophysical data.

Cytoplasmic dynein is a dimeric motor protein which processively moves along microtubule. Its motor domain (head) hydrolyzes ATP and induces conformational changes of linker, stalk, and microtubule binding domain (MTBD) to trigger stepping motion. Here we applied scattering imaging of gold nanoparticle (AuNP) to visualize load-free stepping motion of processive dynein. We observed artificially-dimerized chimeric dynein, which has the head, linker, and stalk from Dictyostelium discoideum cytoplasmic dynein and the MTBD from human axonemal dynein, whose structure has been well-studied by cryo-electron microscopy. One head of a dimer was labeled with 30 nm AuNP, and stepping motions were observed with 100 μs time resolution and sub-nanometer localization precision at physiologically-relevant 1 mM ATP. We found 8 nm forward and backward steps and 5 nm side steps, consistent with on- and off-axes pitches of binding cleft between αβ-tubulin dimers on the microtubule. Probability of the forward step was 1.8 times higher than that of the backward step, and similar to those of the side steps. One-head bound states were not clearly observed, and the steps were limited by a single rate constant. Our results indicate dynein mainly moves with biased small stepping motion in which only backward steps are slightly suppressed.

SUMMARY The forces generated by Microtubules (MTs) and their associated motors orchestrate essential cellular processes ranging from vesicular trafficking to centrosome positioning [1, 2]. To date, most studies have focused on force exertion from motors anchored on a static surface, such as the cell cortex in vivo or glass surfaces in vitro [2–4]. However, motors also transport large cargos and endomembrane networks, whose hydrodynamic interactions with the viscous cytoplasm should generate sizable forces in bulk. Such forces may contribute to MT aster centration, organization and orientation [5–14], but have yet to be evidenced and studied in a minimal reconstituted system. By developing a bulk motility assay, based on stabilized MTs and dynein-coated beads freely floating in a viscous medium away from any surface, we demonstrate that the motion of a cargo exerts a pulling force on the MT and propels it in opposite direction. Quantification of resulting MT movements for different motors, motor velocities, over a range of cargo size and medium viscosities, shows that the efficiency of this mechanism is primarily determined by cargo size and MT length. Forces exerted by cargos are additive, allowing us to recapitulate tug-of-war situations, or bi-dimensional motions of minimal asters. These data also reveal unappreciated effects of the nature of viscous crowders and hydrodynamic interactions between cargos and MTs, likely relevant to understand this mode of force exertion in living cells. This study places endomembrane transport as a significant mode of MT force exertion with far-reaching consequences for cellular organization.

SHORT ABSTRACT This paper summarizes how to visualize the flexible inter-domain movements of CRISPR-associated protein Cas9 using single molecule FRET LONG ABSTRACT The CRISPR-associated protein Cas9 is widely used as a genome editing tool because of its ability to be programmed to cleave any DNA sequence that is followed by a protospacer adjacent motif. The continuing expansion of Cas9 technologies has stimulated studies regarding the molecular basis of the Cas9 catalytic process. Here we summarize methods for single molecule FRET (smFRET) to visualize the inter-domain movements of Cas9 protein. Our measurements and analysis demonstrate flexible and reversible movements of the Cas9 domains. Such flexible movements allow Cas9 to adopt transient conformations beyond those solved by crystal structures and play important roles in the Cas9 catalytic process. In addition to the smFRET measurement itself, to obtain precise results, it is necessary to validate Cas9 catalytic activity. Also, fluorescence anisotropy data are required to interpret smFRET data properly. Thus, in this paper, we describe the details of these important additional experiments for smFRET measurements.

Cytoplasmic dynein is a two-headed molecular motor that moves to the minus end of a microtubule by ATP hydrolysis free energy. By employing its two heads (motor domains), cytoplasmic dynein exhibits various bipedal stepping motions: inchworm and hand-over-hand motions, as well as nonalternating steps of one head. However, the molecular basis to achieve such diverse stepping manners remains unclear because of the lack of an experimental method to observe stepping and the ATPase reaction of dynein simultaneously. Here, we propose a kinetic model for bipedal motions of cytoplasmic dynein and perform Gillespie Monte Carlo simulations that qualitatively reproduce most experimental data obtained to date. The model represents the status of each motor domain as five states according to conformation and nucleotide- and microtubule-binding conditions of the domain. In addition, the relative positions of the two domains were approximated by three discrete states. Accompanied by ATP hydrolysis cycles, the model dynein stochastically and processively moved forward in multiple steps via diverse pathways, including inchworm and hand-over-hand motions, similarly to experimental data. The model reproduced key experimental motility-related properties, including velocity and run length, as functions of the ATP concentration and external force, therefore providing a plausible explanation of how dynein achieves various stepping manners with explicit characterization of nucleotide states. Our model highlights the uniqueness of dynein in the coupling of ATPase with its movement during both inchworm and hand-over-hand stepping.

Cytoplasmic dynein is a two-headed molecular motor that moves to the minus end of microtubule (MT) using ATP hydrolysis free energy. By employing its two heads (motor domains), cytoplasmic dynein shows various bipedal stepping motions; the inchworm and hand-over-hand motions, as well as non-alternate steps of one head. However, the molecular basis to achieve such diverse stepping manners remains obscure. Here, we propose a kinetic model for bipedal motions of cytoplasmic dynein and performed Gillespie Monte Carlo simulations that reproduces most experimental data obtained to date. The model represents status of each motor domain as five states according to conformations, nucleotide- and MT-binding conditions of the domain. Also, the relative positions of the two domains were approximated by three discrete states. Accompanied by ATP hydrolysis cycles, the model dynein stochastically and processively moved forward in multiple steps via diverse pathways, including inchworm and hand-over-hand motions, same as experimental data. The model reproduced key experimental motility-related parameters including velocity and run-length as functions of ATP concentration and external force. Our model reveals that, in a typical inchworm motion, the leading domain moves via the ATP-dependent power-stroke of the linker coupled with a small change in the stalk angle, whereas the lagging domain moves via diffusion dragged by the leading domain. Moreover, the hand-over-hand motion in the model dynein clearly differs from that of kinesin by the usage of the power-stroke.Author Summary Cytoplasmic dynein is a two-headed molecular motor, which moves linearly and transports intra-cellar organelles along microtubules driven by ATP hydrolysis free energy. In contrast to other better-known molecular motors, such as kinesin, dynein is known to take various stepping motions including motions akin to human walking and inchworm-like motions. However, molecular mechanisms underpinning the diverse stepping motions are unclear. Here, based on recent high-resolution structure information and single-molecule motility assay data, we designed a kinetic model that explicitly include two heads, each of which makes ATP hydrolysis cycles and moves along the microtubules. Using the model, we performed Monte Carlo simulations. The simulation reproduced most of currently available experimental results. More importantly, the simulation suggested molecular mechanisms of various stepping motions. While stepping motions apparently resemble to those proposed before, once looking into details, we found the resulting mechanisms distinct from previously proposed ones in the usage of ATP and protein conformation changes coupled with stepping motions.

Cryo-electron microscopy and X-ray crystallography have been the major tools of protein structure analysis for decades and will certainly continue to be essential in the future. Moreover, nuclear magnetic resonance or Förster resonance energy transfer can measure structural dynamics. Here, we propose to add optical second-harmonic generation (SHG), which is a nonlinear optical scattering process sensitive to molecular structures in illuminated materials, to the tool-kit of structural analysis methodologies. SHG can be expected to probe the structural changes of proteins in the physiological condition, and thus link protein structure and biological function. We demonstrate that a conformational change as well as its dynamics in protein macromolecular assemblies can be detected by means of SHG polarization measurement. To prove the capability of SHG polarization measurement with regard to protein structure analysis, we developed an SHG polarization microscope to analyze microtubules in solution. The difference in conformation between microtubules with different binding molecules was successfully observed as polarization dependence of SHG intensity. We also succeeded in capturing the temporal variation of structure in a photo-switchable protein crystal in both activation and inactivation processes. These results illustrate the potential of this method for protein structure analysis in physiological solutions at room temperature without any labeling.

Recent advances in single-molecule imaging have resulted in a series of discoveries regarding characteristic behavior and dynamics of individual molecules. Among the single-molecule imaging techniques, fluorescence resonance energy transfer (FRET) measurement is relatively easy to set up, yet is a powerful method; it can visualize substrate binding and dissociation as well as intramolecular structural changes within a single molecule in real time. Here, we first review single-molecule fluorescence imaging techniques that open a way to establish single-molecule FRET (smFRET) measurement. Then, we describe two examples of the characteristic dynamics of individual molecules revealed by smFRET: antibiotic-mediated protein translation inhibition and the intramolecular structural changes in CRISPR-Cas9, a versatile genome-editing tool. Finally, we introduce some of the latest advances in smFRET technique.

Prestin is a member of the solute carrier 26 (SLC26) family of anion transporters and expressed in the outer hair cells in the cochlea. It provides electromechanical feedback to amplify sound signals at specific frequencies. Despite high sequence similarity among the SLC26 family members, only prestin is thought to exhibit motile activity driven by changes in the membrane potential (electromotility). In order to identify the key structural element responsible for the electromotility, we generated molecular models for prestin and another SLC26 protein, pendrin. These models show the presence of an extracellular loop whose net charge is opposite for prestin (net positive) and pendrin (net negative). By employing a mutagenesis approach combined with whole-cell patch clamp measurements, we examined how the electrostatic property of the extracellular loops contribute to the functions of prestin and pendrin. We found that pendrin exhibits a sign of voltage-sensing ability at highly hyperpolarized potentials within experimentally measurable voltage range. Pendrin mutants in which negatively charged residues in the extracellular loop were replaced with noncharged or positively charged ones exhibited robust voltage-sensing ability. These observations suggest that pendrin is also able to respond to the membrane potential, and that the operating voltage range of wild-type pendrin is highly hyperpolarized due to the electrostatic property of the extracellular loop. Consistent with this idea, a chimeric prestin construct that contains the extracellular loop of prendrin showed hyperpolarizing shifts in its operating voltage range. Our results challenge the current view that only prestin possesses voltage-sensing ability among the SLC26 family, and highlight the importance of the extracellular loop for establishing the operating voltage range.

Kinesin-1, the founding member of the kinesin superfamily of proteins, is known to use only a subset of microtubules for transport in living cells. This biased use of microtubules is proposed as the guidance cue for polarized transport in neurons, but the underlying mechanisms are still poorly understood. Here, we report that kinesin-1 binding changes the microtubule lattice and promotes further kinesin-1 binding. This high-affinity state requires the binding of kinesin-1 in the nucleotide-free state. Microtubules return to the initial low-affinity state by washing out the binding kinesin-1 or by the binding of non-hydrolyzable ATP analogue AMPPNP to kinesin-1. X-ray fiber diffraction, fluorescence speckle microscopy, and second-harmonic generation microscopy, as well as cryo-EM, collectively demonstrated that the binding of nucleotide-free kinesin-1 to GDP microtubules changes the conformation of the GDP microtubule to a conformation resembling the GTP microtubule.