The cell biology of primary cell walls during salt stress.

Salt stress simultaneously causes ionic toxicity, osmotic stress, and oxidative stress, which directly impact plant growth and development. Plants have developed numerous strategies to adapt to saline environments. Whereas some of these strategies have been investigated and exploited for crop improvement, much remains to be understood, including how salt stress is perceived by plants and how plants coordinate effective responses to the stress. It is, however, clear that the plant cell wall is the first contact point between external salt and the plant. In this context, significant advances in our understanding of halotropism, cell wall synthesis, and integrity surveillance, as well as salt-related cytoskeletal rearrangements, have been achieved. Indeed, molecular mechanisms underpinning some of these processes have recently been elucidated. In this review, we aim to provide insights into how plants respond and adapt to salt stress, with a special focus on primary cell wall biology in the model plant Arabidopsis thaliana.

Microtubule binding of the human augmin complex is directly controlled by importins and Ran-GTP.

Mitotic spindle assembly during cell division is a highly regulated process. Ran-GTP produced around chromosomes controls the activity of a multitude of spindle assembly factors by releasing them from inhibitory interaction with importins. A major consequence of Ran-GTP regulation is the local stimulation of branched microtubule nucleation around chromosomes, which is mediated by the augmin complex (composed of the eight subunits HAUS1-HAUS8), a process that is crucially important for correct spindle assembly. However, augmin is not known to be a direct target of the Ran-GTP pathway, raising the question of how its activity is controlled. Here, we present the in vitro reconstitution of Ran-GTP-regulated microtubule binding of the human augmin complex. We demonstrate that importins directly bind to augmin, which prevents augmin from binding to microtubules. Ran-GTP relieves this inhibition. Therefore, the augmin complex is a direct target of the Ran-GTP pathway, suggesting that branching microtubule nucleation is directly regulated by the Ran-GTP gradient around chromosomes in dividing cells.

Fluorescent cytoskeletal markers reveal associations between the actin and microtubule cytoskeleton in rice cells.

Rice (Oryza sativa) is one of our main food crops, feeding ∼3.5 billion people worldwide. An increasing number of studies note the importance of the cytoskeleton, including actin filaments and microtubules, in rice development and environmental responses. Yet, reliable in vivo cytoskeleton markers are lacking in rice, which limits our knowledge of cytoskeletal functions in living cells. Therefore, we generated bright fluorescent marker lines of the actin and microtubule cytoskeletons in rice, suitable for live-cell imaging in a wide variety of rice tissues. Using these lines, we show that actin bundles and microtubules engage and co-function during pollen grain development, how the cytoskeletal components are coordinated during root cell development, and that the actin cytoskeleton is robust and facilitates microtubule responses during salt stress. Hence, we conclude that our cytoskeletal marker lines, highlighted by our findings of cytoskeletal associations and dynamics, will substantially further future investigations in rice biology.

Directionally biased sidestepping of Kip3/kinesin-8 is regulated by ATP waiting time and motor-microtubule interaction strength.

Kinesin-8 motors, which move in a highly processive manner toward microtubule plus ends where they act as depolymerases, are essential regulators of microtubule dynamics in cells. To understand their navigation strategy on the microtubule lattice, we studied the 3D motion of single yeast kinesin-8 motors, Kip3, on freely suspended microtubules in vitro. We observed short-pitch, left-handed helical trajectories indicating that kinesin-8 motors frequently switch protofilaments in a directionally biased manner. Intriguingly, sidestepping was not directly coupled to forward stepping but rather depended on the average dwell time per forward step under limiting ATP concentrations. Based on our experimental findings and numerical simulations we propose that effective sidestepping toward the left is regulated by a bifurcation in the Kip3 step cycle, involving a transition from a two-head-bound to a one-head-bound conformation in the ATP-waiting state. Results from a kinesin-1 mutant with extended neck linker hint toward a generic sidestepping mechanism for processive kinesins, facilitating the circumvention of intracellular obstacles on the microtubule surface.

Challenges in Estimating the Motility Parameters of Single Processive Motor Proteins.

Cytoskeletal motor proteins are essential to the function of a wide range of intracellular mechano-systems. The biophysical characterization of their movement along their filamentous tracks is therefore of large importance. Toward this end, single-molecule, in vitro stepping-motility assays are commonly used to determine motor velocity and run length. However, comparing results from such experiments has proved difficult due to influences from variations in the experimental conditions and the data analysis methods. Here, we investigate the movement of fluorescently labeled, processive, dimeric motor proteins and propose a unified algorithm to correct the measurements for finite filament length as well as photobleaching. Particular emphasis is put on estimating the statistical errors associated with the proposed evaluation method, as knowledge of these values is crucial when comparing measurements from different experiments. Testing our approach with simulated and experimental data from GFP-labeled kinesin-1 motors stepping along immobilized microtubules, we show 1) that velocity distributions should be fitted by a t location-scale probability density function rather than by a normal distribution; 2) that the impossibility to measure events shorter than the image acquisition time needs to be taken into account; 3) that the interaction time and run length of the motors can be estimated independent of the filament length distribution; and 4) that the dimeric nature of the motors needs to be considered when correcting for photobleaching. Moreover, our analysis reveals that controlling the temperature during the experiments with a precision below 1 K is of importance. We believe our method will not only improve the evaluation of experimental data, but also allow for better statistical comparisons between different populations of motor proteins (e.g., with distinct mutations or linked to different cargos) and filaments (e.g., in distinct nucleotide states or with different posttranslational modifications). Therefore, we include a detailed workflow for image processing and analysis (including MATLAB code), serving as a tutorial for the estimation of motility parameters in stepping-motility assays.

Small Crowders Slow Down Kinesin-1 Stepping by Hindering Motor Domain Diffusion.

The dimeric motor protein kinesin-1 moves processively along microtubules against forces of up to 7 pN. However, the mechanism of force generation is still debated. Here, we point to the crucial importance of diffusion of the tethered motor domain for the stepping of kinesin-1: small crowders stop the motor at a viscosity of 5 mPa·s-corresponding to a hydrodynamic load in the sub-fN (~10^{-4} pN) range-whereas large crowders have no impact even at viscosities above 100 mPa·s. This indicates that the scale-dependent, effective viscosity experienced by the tethered motor domain is a key factor determining kinesin's functionality. Our results emphasize the role of diffusion in the kinesin-1 stepping mechanism and the general importance of the viscosity scaling paradigm in nanomechanics.

Branched microtubule nucleation and dynein transport organize RanGTP asters in Xenopus laevis egg extract.

Chromosome segregation relies on the correct assembly of a bipolar spindle. Spindle pole self-organization requires dynein-dependent microtubule (MT) transport along other MTs. However, during M-phase RanGTP triggers MT nucleation and branching generating polarized arrays with nonastral organization in which MT minus ends are linked to the sides of other MTs. This raises the question of how branched-MT nucleation and dynein-mediated transport cooperate to organize the spindle poles. Here, we used RanGTP-dependent MT aster formation in Xenopus laevis (X. laevis) egg extract to study the interplay between these two seemingly conflicting organizing principles. Using temporally controlled perturbations of MT nucleation and dynein activity, we found that branched MTs are not static but instead dynamically redistribute over time as poles self-organize. Our experimental data together with computer simulations suggest a model where dynein together with dynactin and NuMA directly pulls and move branched MT minus ends toward other MT minus ends.

Optimization of isopolar microtubule arrays.

Isopolar arrays of aligned cytoskeletal filaments are components in a number of designs of hybrid nanodevices incorporating biomolecular motors. For example, a combination of filament arrays and motor arrays can form an actuator or a molecular engine resembling an artificial muscle. Here, isopolar arrays of microtubules are fabricated by flow alignment, and their quality is characterized by their degree of alignment. We find, in agreement with our analytical models, that the degree of alignment is ultimately limited by thermal forces, while the kinetics of the alignment process are influenced by the flow strength, the microtubule stiffness, the gliding velocity, and the tip length. Strong flows remove microtubules from the surface and reduce the filament density, suggesting that there is an optimal flow strength for the fabrication of ordered arrays.

Noninvasive Long-Term Imaging of the Cytoskeleton in Arabidopsis Seedlings.

The preparation of biological samples, especially for live-cell microscopy, remains a major experimental challenge in the lab despite technological advances. In addition, high-resolution microscopy techniques require higher sample quality and uniformity, which is difficult to ensure during manual preparation while maintaining "ideal" growth conditions. In this protocol, we provide a way out by growing Arabidopsis thaliana seedlings directly in an imaging chamber, which eliminates invasive sample preparation directly before imaging. This method hinges on the precise placement of seeds into imaging chambers, which can be grown in conventional climate chambers. We detail three methods to grow hypocotyls, cotyledons, leaves, and roots for high-resolution and long-term imaging of the plant cytoskeleton. Furthermore, we show that the growth and development of seedlings inside the chambers can be externally manipulated by the addition of chemicals.

The highly processive kinesin-8, Kip3, switches microtubule protofilaments with a bias toward the left.

Kinesin-1 motor proteins walk parallel to the protofilament axes of microtubules as they step from one tubulin dimer to the next. Is protofilament tracking an inherent property of processive kinesin motors, like kinesin-1, and what are the structural determinants underlying protofilament tracking? To address these questions, we investigated the tracking properties of the processive kinesin-8, Kip3. Using in vitro gliding motility assays, we found that Kip3 rotates microtubules counterclockwise around their longitudinal axes with periodicities of ∼1 µm. These rotations indicate that the motors switch protofilaments with a bias toward the left. Molecular modeling suggests 1), that the protofilament switching may be due to kinesin-8 having a longer neck linker than kinesin-1, and 2), that the leftward bias is due the asymmetric geometry of the motor neck linker complex.

Tracking single particles and elongated filaments with nanometer precision.

Recent developments in image processing have greatly advanced our understanding of biomolecular processes in vitro and in vivo. In particular, using Gaussian models to fit the intensity profiles of nanometer-sized objects have enabled their two-dimensional localization with a precision in the one-nanometer range. Here, we present an algorithm to precisely localize curved filaments whose structures are characterized by subresolution diameters and micrometer lengths. Using surface-immobilized microtubules, fluorescently labeled with rhodamine, we demonstrate positional precisions of ∼2 nm when determining the filament centerline and ∼9 nm when localizing the filament tips. Combined with state-of-the-art single particle tracking we apply the algorithm 1), to motor-proteins stepping on immobilized microtubules, 2), to depolymerizing microtubules, and 3), to microtubules gliding over motor-coated surfaces.

Current and future advances in fluorescence-based visualization of plant cell wall components and cell wall biosynthetic machineries.

Plant cell wall-derived biomass serves as a renewable source of energy and materials with increasing importance. The cell walls are biomacromolecular assemblies defined by a fine arrangement of different classes of polysaccharides, proteoglycans, and aromatic polymers and are one of the most complex structures in Nature. One of the most challenging tasks of cell biology and biomass biotechnology research is to image the structure and organization of this complex matrix, as well as to visualize the compartmentalized, multiplayer biosynthetic machineries that build the elaborate cell wall architecture. Better knowledge of the plant cells, cell walls, and whole tissue is essential for bioengineering efforts and for designing efficient strategies of industrial deconstruction of the cell wall-derived biomass and its saccharification. Cell wall-directed molecular probes and analysis by light microscopy, which is capable of imaging with a high level of specificity, little sample processing, and often in real time, are important tools to understand cell wall assemblies. This review provides a comprehensive overview about the possibilities for fluorescence label-based imaging techniques and a variety of probing methods, discussing both well-established and emerging tools. Examples of applications of these tools are provided. We also list and discuss the advantages and limitations of the methods. Specifically, we elaborate on what are the most important considerations when applying a particular technique for plants, the potential for future development, and how the plant cell wall field might be inspired by advances in the biomedical and general cell biology fields.

Detection of fractional steps in cargo movement by the collective operation of kinesin-1 motors.

The stepping behavior of single kinesin-1 motor proteins has been studied in great detail. However, in cells, these motors often do not work alone but rather function in small groups when they transport cellular cargo. Until now, the cooperative interactions between motors in such groups were poorly understood. A fundamental question is whether two or more motors that move the same cargo step in synchrony, producing the same step size as a single motor, or whether the step size of the cargo movement varies. To answer this question, we performed in vitro gliding motility assays, where microtubules coated with quantum dots were driven over a glass surface by a known number of kinesin-1 motors. The motion of individual microtubules was then tracked with nanometer precision. In the case of transport by two kinesin-1 motors, we found successive 4-nm steps, corresponding to half the step size of a single motor. Dwell-time analysis did not reveal any coordination, in the sense of alternate stepping, between the motors. When three motors interacted in collective transport, we identified distinct forward and backward jumps on the order of 10 nm. The existence of the fractional steps as well as the distinct jumps illustrate a lack of synchronization and has implications for the analysis of motor-driven organelle movement investigated in vivo.

Kinesin-14 motors drive a right-handed helical motion of antiparallel microtubules around each other.

Within the mitotic spindle, kinesin motors cross-link and slide overlapping microtubules. Some of these motors exhibit off-axis power strokes, but their impact on motility and force generation in microtubule overlaps has not been investigated. Here, we develop and utilize a three-dimensional in vitro motility assay to explore kinesin-14, Ncd, driven sliding of cross-linked microtubules. We observe that free microtubules, sliding on suspended microtubules, not only rotate around their own axis but also move around the suspended microtubules with right-handed helical trajectories. Importantly, the associated torque is large enough to cause microtubule twisting and coiling. Further, our technique allows us to measure the in situ spatial extension of the motors between cross-linked microtubules to be about 20 nm. We argue that the capability of microtubule-crosslinking kinesins to cause helical motion of overlapping microtubules around each other allows for flexible filament organization, roadblock circumvention and torque generation in the mitotic spindle.

Impact-Free Measurement of Microtubule Rotations on Kinesin and Cytoplasmic-Dynein Coated Surfaces.

Knowledge about the three-dimensional stepping of motor proteins on the surface of microtubules (MTs) as well as the torsional components in their power strokes can be inferred from longitudinal MT rotations in gliding motility assays. In previous studies, optical detection of these rotations relied on the tracking of rather large optical probes present on the outer MT surface. However, these probes may act as obstacles for motor stepping and may prevent the unhindered rotation of the gliding MTs. To overcome these limitations, we devised a novel, impact-free method to detect MT rotations based on fluorescent speckles within the MT structure in combination with fluorescence-interference contrast microscopy. We (i) confirmed the rotational pitches of MTs gliding on surfaces coated by kinesin-1 and kinesin-8 motors, (ii) demonstrated the superiority of our method over previous approaches on kinesin-8 coated surfaces at low ATP concentration, and (iii) identified MT rotations driven by mammalian cytoplasmic dynein, indicating that during collective motion cytoplasmic dynein side-steps with a bias in one direction. Our novel method is easy to implement on any state-of-the-art fluorescence microscope and allows for high-throughput experiments.

Adaptive braking by Ase1 prevents overlapping microtubules from sliding completely apart.

Short regions of overlap between ends of antiparallel microtubules are central elements within bipolar microtubule arrays. Although their formation requires motors, recent in vitro studies demonstrated that stable overlaps cannot be generated by molecular motors alone. Motors either slide microtubules along each other until complete separation or, in the presence of opposing motors, generate oscillatory movements. Here, we show that Ase1, a member of the conserved MAP65/PRC1 family of microtubule-bundling proteins, enables the formation of stable antiparallel overlaps through adaptive braking of Kinesin-14-driven microtubule-microtubule sliding. As overlapping microtubules start to slide apart, Ase1 molecules become compacted in the shrinking overlap and the sliding velocity gradually decreases in a dose-dependent manner. Compaction is driven by moving microtubule ends that act as barriers to Ase1 diffusion. Quantitative modelling showed that the molecular off-rate of Ase1 is sufficiently low to enable persistent overlap stabilization over tens of minutes. The finding of adaptive braking demonstrates that sliding can be slowed down locally to stabilize overlaps at the centre of bipolar arrays, whereas sliding proceeds elsewhere to enable network self-organization.

Fluorescence imaging of single Kinesin motors on immobilized microtubules.

Recent developments in optical microscopy and nanometer tracking have greatly improved our understanding of cytoskeletal motor proteins. Using fluorescence microscopy, dynamic interactions are now routinely observed in vitro on the level of single molecules mainly using a geometry, where fluorescently labeled motors move on surface-immobilized filaments. In this chapter, we review recent methods related to single-molecule kinesin motility assays. In particular, we aim to provide practical advice on: how to set up the assays, how to acquire high-precision data from fluorescently labeled kinesin motors and attached quantum dots, and how to analyze data by nanometer tracking.

Single-molecule stepping and structural dynamics of myosin X.

Myosin X is an unconventional myosin with puzzling motility properties. We studied the motility of dimerized myosin X using the single-molecule fluorescence techniques polTIRF, FIONA and Parallax to measure the rotation angles and three-dimensional position of the molecule during its walk. It was found that Myosin X steps processively in a hand-over-hand manner following a left-handed helical path along both single actin filaments and bundles. Its step size and velocity are smaller on actin bundles than individual filaments, suggesting myosin X often steps onto neighboring filaments in a bundle. The data suggest that a previously postulated single alpha-helical domain mechanically extends the lever arm, which has three IQ motifs, and either the neck-tail hinge or the tail is flexible. These structural features, in conjunction with the membrane- and microtubule-binding domains, enable myosin X to perform multiple functions on varied actin structures in cells.

Studying kinesin motors by optical 3D-nanometry in gliding motility assays.

Recent developments in optical microscopy and nanometer tracking have facilitated our understanding of microtubules and their associated proteins. Using fluorescence microscopy, dynamic interactions are now routinely observed in vitro on the level of single molecules, mainly using a geometry in which labeled motors move on surface-immobilized microtubules. Yet, we think that the historically older gliding geometry, in which motor proteins bound to a substrate surface drive the motion microtubules, offers some unique advantages. (1) Motility can be precisely followed by coupling multiple fluorophores and/or single bright labels to the surface of microtubules without disturbing the activity of the motor proteins. (2) The number of motor proteins involved in active transport can be determined by several strategies. (3) Multimotor studies can be performed over a wide range of motor densities. These advantages allow for studying cooperativity of processive as well as nonprocessive motors. Moreover, the gliding geometry has proven to be most promising for nanotechnological applications of motor proteins operating in synthetic environments. In this chapter we review recent methods related to gliding motility assays in conjunction with 3D-nanometry. In particular, we aim to provide practical advice on how to set up gliding assays, how to acquire high-precision data from microtubules and attached quantum dots, and how to analyze data by 3D-nanometer tracking.

Quantum-dot-assisted characterization of microtubule rotations during cargo transport.

Owing to their wide spectrum of in vivo functions, motor proteins, such as kinesin-1, show great potential for application as nanomachines in engineered environments. When attached to a substrate surface, these motors are envisioned to shuttle cargo that is bound to reconstituted microtubules--one component of the cell cytoskeleton--from one location to another. One potentially serious problem for such applications is, however, the rotation of the microtubules around their longitudinal axis. Here we explore this issue by labelling the gliding microtubules with quantum dots to simultaneously follow their sinusoidal side-to-side and up-and-down motion in three dimensions with nanometre accuracy. Microtubule rotation, which originates from the kinesin moving along the individual protofilaments of the microtubule, was not impeded by the quantum dots. However, pick-up of large cargo inhibited the rotation but did not affect the velocity of microtubule gliding. Our data show that kinesin-driven microtubules make flexible, responsive and effective molecular shuttles for nanotransport applications.