Control of cytoplasmic dynein force production and processivity by its C-terminal domain

The Power of Three: Dynactin associates with three dyneins under load for greater force production

Cytoplasmic dynein is an essential microtubule motor protein that powers organelle transport and mitotic spindle assembly. Its activity depends on dynein-dynactin-cargo adaptor complexes, such as dynein-dynactin-BicD2 (DDB), which typically function with two dynein motors. We show that mechanical tension recruits a third dynein motor via an auxiliary BicD adaptor binding the light intermediate chain of the third dynein, stabilizing multi-dynein assemblies and enhancing force generation. Lis1 prevents dynein from transitioning into a force-limiting phi-like conformation, allowing single-dynein DDB to sustain forces up to ~4.5 pN, whereas force generation often ends at ~2.5 pN without Lis1. Complexes with two or three dyneins generate ~7 pN and ~9 pN, respectively, consistent with a staggered motor arrangement that enhances collective output. Under load, DDB primarily takes ~8 nm steps, challenging existing dynein coordination models. These findings reveal adaptive mechanisms that enable robust intracellular transport under varying mechanical demands.

DNAHX: a novel, non-motile dynein heavy chain subfamily, identified by cryo-EM endogenously

Ciliogenesis and cilia motility rely on the coordinated actions of diverse dyneins, yet the complexity of these motor proteins in cilia has posed challenges for understanding their specific roles. Traditional evolutionary analyses often overlook key family members due to technical limitations. Here, we present a cryo-EM-based, bottom-up approach for large-scale, de novo protein identification and functional prediction of endogenous axonemal dynein complexes. This approach led to the identification of a novel dynein heavy chain subfamily (XP_041462850), designated as DNAHX, from sea urchin sperm. Phylogenetic analysis indicates that DNAHX branches from the outer-arm dynein alpha chain during evolution and is found in specific animal lineages with external fertilization. DNAHX contains multiple insertions throughout the protein, locking DNAHX permanently in a pre-powerstroke state. The AAA1 site exhibits poor conservation of essential ATPase motifs, consistent with DNAHX's non-motile nature. DNAHX also forms a heterodimeric dynein complex, which we named dynein-X, with another dynein heavy chain and accessory chains. Furthermore, a subset of dynein-X displays an autoinhibited phi particle conformation, potentially facilitating the intraflagellar transport of axonemal dyneins. Our discovery of the novel, non-motile dynein heavy chain and the dynein-X complex provides valuable insights into the evolution of dyneins and potentially their diverse cellular functions.

Intercellular Tunneling Nanotubes as Natural Biophotonic Conveyors

Tunneling nanotubes (TNTs), submicrometer membranous channels that bridge and connect distant cells, play a pivotal role in intercellular communication. Organelle transfer within TNTs is crucial in regulating cell growth, signal transmission, and disease progression. However, precise control over individual organelle transport within TNTs remains elusive. In this study, we introduce an optical technique that harnesses TNTs as biophotonic conveyors for the directional transport of individual organelles between cells. By utilizing near-infrared light propagating along the TNTs, optical forces were exerted on the organelles, enabling their active transport in a predetermined direction and at a controlled velocity. As a potential application, TNT conveyors were employed to inhibit mitochondrial hijacking from immune cells to cancer cells, thereby activating immune cells and suppressing cancer cell growth. Furthermore, neural modulation was achieved by transporting mitochondria and neurotransmitter-containing vesicles between neurons via TNT conveyors and axonal conveyors, respectively. This study presents a robust and precise approach to immune activation and neural regulation through the manipulation of organelle transfer at the subcellular level.

Modeling of Chemomechanical Coupling of Cytoplasmic Dynein Motors

Cytoplasmic dynein homodimer is a motor protein that can step processively on microtubules (MTs) toward the minus end by hydrolyzing ATP molecules. Some dynein motors show a complicated stepping behavior with variable step sizes and having both hand-overhand and inchworm steps, while some mammalian dynein motors show simplistic stepping behavior with a constant step size and having only hand-overhand steps. Here, a model for the chemomechanical coupling of the dynein is presented, based on which an analytical theory is given on the dynamics of the motor. The theoretical results explain consistently and quantitatively the available experimental data on various aspects of the dynamics of dynein with complicated stepping behavior and the dynamics of dynein with simplistic stepping behavior. The very differences in the dynamic behavior between the two motors are due solely to different elastic coefficients of the linkage connecting the two dynein heads, with the dynein motors of the complicated and simplistic stepping behaviors having small and large coefficients, respectively. Moreover, it is analyzed that the ATPase rate of the dynein head with a docked linker being larger than that with an undocked linker is indispensable for the unidirectional motility of the motor, and the small free energy change for the linker docking in the strong MT-binding state facilitates the unidirectional motility.

Mechanics of spindle orientation in human mitotic cells is determined by pulling forces on astral microtubules and clustering of cortical dynein

The forces that orient the spindle in human cells remain poorly understood due to a lack of direct mechanical measurements in mammalian systems. We use magnetic tweezers to measure the force on human mitotic spindles. Combining the spindle's measured resistance to rotation, the speed at which it rotates after laser ablating astral microtubules, and estimates of the number of ablated microtubules reveals that each microtubule contacting the cell cortex is subject to ∼5 pN of pulling force, suggesting that each is pulled on by an individual dynein motor. We find that the concentration of dynein at the cell cortex and extent of dynein clustering are key determinants of the spindle's resistance to rotation, with little contribution from cytoplasmic viscosity, which we explain using a biophysically based mathematical model. This work reveals how pulling forces on astral microtubules determine the mechanics of spindle orientation and demonstrates the central role of cortical dynein clustering.

2-Chloro-3-cyano-4-nitrobenzyl pyridinium bromide as a potent anti-lung cancer molecule prepared using a single-step solvent-free method

Mono-/dimeric-substituted pyridinium and pyrazolium bromides were prepared under conventional and solvent-free silica-supported domestic microwave conditions. The atom economy, environmental product mass intensity and product mass intensity for solvent-free reactions showed significant importance for the synthesis of target molecules. 4-Nitrobenzyl-substituted pyridinium bromide showed potent anticancer properties compared with mono-/dimeric-substituted pyridinium and pyrazolium bromides against a lung cancer cell line (A-549). Molecular simulation studies were carried out for mono-/dimeric-substituted pyridinium and pyrazolium bromide against protein human CDK1/cyclinB1/CKS2 using the AutoDock program.

Coherent light scattering from cellular dynamics in living tissues

This review examines the biological physics of intracellular transport probed by the coherent optics of dynamic light scattering from optically thick living tissues. Cells and their constituents are in constant motion, composed of a broad range of speeds spanning many orders of magnitude that reflect the wide array of functions and mechanisms that maintain cellular health. From the organelle scale of tens of nanometers and upward in size, the motion inside living tissue is actively driven rather than thermal, propelled by the hydrolysis of bioenergetic molecules and the forces of molecular motors. Active transport can mimic the random walks of thermal Brownian motion, but mean-squared displacements are far from thermal equilibrium and can display anomalous diffusion through Lévy or fractional Brownian walks. Despite the average isotropic three-dimensional environment of cells and tissues, active cellular or intracellular transport of single light-scattering objects is often pseudo-one-dimensional, for instance as organelle displacement persists along cytoskeletal tracks or as membranes displace along the normal to cell surfaces, albeit isotropically oriented in three dimensions. Coherent light scattering is a natural tool to characterize such tissue dynamics because persistent directed transport induces Doppler shifts in the scattered light. The many frequency-shifted partial waves from the complex and dynamic media interfere to produce dynamic speckle that reveals tissue-scale processes through speckle contrast imaging and fluctuation spectroscopy. Low-coherence interferometry, dynamic optical coherence tomography, diffusing-wave spectroscopy, diffuse-correlation spectroscopy, differential dynamic microscopy and digital holography offer coherent detection methods that shed light on intracellular processes. In health-care applications, altered states of cellular health and disease display altered cellular motions that imprint on the statistical fluctuations of the scattered light. For instance, the efficacy of medical therapeutics can be monitored by measuring the changes they induce in the Doppler spectra of livingex vivocancer biopsies.

Clustering of cortical dynein regulates the mechanics of spindle orientation in human mitotic cells

The forces which orient the spindle in human cells remain poorly understood due to a lack of direct mechanical measurements in mammalian systems. We use magnetic tweezers to measure the force on human mitotic spindles. Combining the spindle's measured resistance to rotation, the speed it rotates after laser ablating astral microtubules, and estimates of the number of ablated microtubules reveals that each microtubule contacting the cell cortex is subject to ~1 pN of pulling force, suggesting that each is pulled on by an individual dynein motor. We find that the concentration of dynein at the cell cortex and extent of dynein clustering are key determinants of the spindle's resistance to rotation, with little contribution from cytoplasmic viscosity, which we explain using a biophysically based mathematical model. This work reveals how pulling forces on astral microtubules determine the mechanics of spindle orientation and demonstrates the central role of cortical dynein clustering.

Single-Molecule Studies on the Motion and Force Generation of the Kinesin-3 Motor KIF1A

KIF1A is a neuron-specific member of the kinesin-3 family of microtubule (MT) plus-end-directed motor proteins. It powers the migration of nuclei in differentiating brain stem cells and the transport of synaptic precursors and dense core vesicles in axons. Its dysfunction causes severe neurodevelopmental and neurodegenerative diseases termed KIF1A-associated neurological disorders (KAND). KAND mutations span the entirety of the KIF1A protein sequence, of which the majority are located within the motor domain and are thus predicted to affect the motor's motility and force-generating properties. Unfortunately, the molecular etiologies of KAND remain poorly understood, in part because KIF1A's molecular mechanism remains unclear. Here, we describe detailed methods for how to express a tail-truncated dimeric KIF1A in E. coli cells and provide step-by-step protocols for performing single-molecule studies with total internal reflection fluorescence microscopy and optical tweezers assays, which, when combined with structure-function studies, help to decipher KIF1A's molecular mechanism.

Cytoplasmic factories for axonemal dynein assembly

Axonemal dyneins power the beating of motile cilia and flagella. These massive multimeric motor complexes are assembled in the cytoplasm, and subsequently trafficked to cilia and incorporated into the axonemal superstructure. Numerous cytoplasmic factors are required for the dynein assembly process, and, in mammals, defects lead to primary ciliary dyskinesia, which results in infertility, bronchial problems and failure to set up the left-right body axis correctly. Liquid-liquid phase separation (LLPS) has been proposed to underlie the formation of numerous membrane-less intracellular assemblies or condensates. In multiciliated cells, cytoplasmic assembly of axonemal dyneins also occurs in condensates that exhibit liquid-like properties, including fusion, fission and rapid exchange of components both within condensates and with bulk cytoplasm. However, a recent extensive meta-analysis suggests that the general methods used to define LLPS systems in vivo may not readily distinguish LLPS from other mechanisms. Here, I consider the time and length scales of axonemal dynein heavy chain synthesis, and the possibility that during translation of dynein heavy chain mRNAs, polysomes are crosslinked via partially assembled proteins. I propose that axonemal dynein factory formation in the cytoplasm may be a direct consequence of the sheer scale and complexity of the assembly process itself.

Flexible microtubule anchoring modulates the bi-directional motility of the kinesin-5 Cin8

Two modes of motility have been reported for bi-directional kinesin-5 motors: (a) context-dependent directionality reversal, a mode in which motors undergo persistent minus-end directed motility at the single-molecule level and switch to plus-end directed motility in different assays or under different conditions, such as during MT gliding or antiparallel sliding or as a function of motor clustering; and (b) bi-directional motility, defined as movement in two directions in the same assay, without persistent unidirectional motility. Here, we examine how modulation of motor-microtubule (MT) interactions affects these two modes of motility for the bi-directional kinesin-5, Cin8. We report that the large insert in loop 8 (L8) within the motor domain of Cin8 increases the MT affinity of Cin8 in vivo and in vitro and is required for Cin8 intracellular functions. We consistently found that recombinant purified L8 directly binds MTs and L8 induces single Cin8 motors to behave according to context-dependent directionality reversal and bi-directional motility modes at intermediate ionic strength and according to a bi-directional motility mode in an MT surface-gliding assay under low motor density conditions. We propose that the largely unstructured L8 facilitates flexible anchoring of Cin8 to the MTs. This flexible anchoring enables the direct observation of bi-directional motility in motility assays. Remarkably, although L8-deleted Cin8 variants exhibit a strong minus-end directed bias at the single-molecule level, they also exhibit plus-end directed motility in an MT-gliding assay. Thus, L8-induced flexible MT anchoring is required for bi-directional motility of single Cin8 molecules but is not necessary for context-dependent directionality reversal of Cin8 in an MT-gliding assay.

[Effect of dexamethasone on the expression of Dynein heavy chain and Dynactin in the cytoplasm of fetal rat cerebral cortical neurons cultured in vitro]

OBJECTIVE

To study the effect of dexamethasone (DEX) on the expression of Dynein heavy chain (DHC) and Dynactin in the cytoplasm of fetal rat cerebral cortical neurons cultured in vitro.

METHODS

Primary cerebral cortical neurons of fetal rats were cultured in vitro and were used to establish a cellular model of DEX intervention. According to the final concentration of DEX, the neurons were divided into three groups:control (without DEX), 0.1 μmol/L DEX, and 1.0 μmol/L DEX. On days 1, 3, and 7 after intervention, the quantitative PCR was used to observe the effect of DEX on the mRNA expression of DHC and Dynactin. The Western blot was used to observe the effect of DEX on the protein expression of DHC and Dynactin.

RESULTS

There was no significant difference in the mRNA expression levels of DHC and Dynactin among the three groups at all time points (P > 0.05). On day 7 after DEX intervention, the protein expression of DHC in the 1.0 μmol/L DEX group gradually increased and reached the peak over time, which was significantly higher than that in the control and 0.1 μmol/L DEX groups (P < 0.05). The control and 0.1 μmol/L DEX groups had a significant increase in the protein expression of Dynactin from day 1 to days 3 and 7 after DEX intervention (P < 0.05). The control group had a significant increase in the protein expression of Dynactin from day 3 to day 7 after intervention (P < 0.05), while the 0.1 μmol/L DEX group had a significant reduction in the protein expression of Dynactin from day 3 to day 7 after intervention (P < 0.05). On days 3 and 7 after DEX intervention, the 0.1 μmol/L DEX and 1.0 μmol/L DEX groups had a significantly lower protein expression level of Dynactin in the cerebral cortical neurons than the control group (P < 0.05). On day 7 after DEX intervention, the 1.0 μmol/L DEX group had a significantly lower protein expression level of Dynactin than the 0.1 μmol/L DEX group (P < 0.05).

CONCLUSIONS

DEX affects the protein expression of DHC and Dynactin in the fetal rat cerebral cortical neurons cultured in vitro, possibly in a concentration- and time-dependent manner.

Structure and Mechanics of Dynein Motors

Dyneins make up a family of AAA+ motors that move toward the minus end of microtubules. Cytoplasmic dynein is responsible for transporting intracellular cargos in interphase cells and mediating spindle assembly and chromosome positioning during cell division. Other dynein isoforms transport cargos in cilia and power ciliary beating. Dyneins were the least studied of the cytoskeletal motors due to challenges in the reconstitution of active dynein complexes in vitro and the scarcity of high-resolution methods for in-depth structural and biophysical characterization of these motors. These challenges have been recently addressed, and there have been major advances in our understanding of the activation, mechanism, and regulation of dyneins. This review synthesizes the results of structural and biophysical studies for each class of dynein motors. We highlight several outstanding questions about the regulation of bidirectional transport along microtubules and the mechanisms that sustain self-coordinated oscillations within motile cilia.

Cutting-Edge Single-Molecule Technologies Unveil New Mechanics in Cellular Biochemistry

Single-molecule technologies have expanded our ability to detect biological events individually, in contrast to ensemble biophysical technologies, where the result provides averaged information. Recent developments in atomic force microscopy have not only enabled us to distinguish the heterogeneous phenomena of individual molecules, but also allowed us to view up to the resolution of a single covalent bond. Similarly, optical tweezers, due to their versatility and precision, have emerged as a potent technique to dissect a diverse range of complex biological processes, from the nanomechanics of ClpXP protease-dependent degradation to force-dependent processivity of motor proteins. Despite the advantages of optical tweezers, the time scales used in this technology were inconsistent with physiological scenarios, which led to the development of magnetic tweezers, where proteins are covalently linked with the glass surface, which in turn increases the observation window of a single biomolecule from minutes to weeks. Unlike optical tweezers, magnetic tweezers use magnetic fields to impose torque, which makes them convenient for studying DNA topology and topoisomerase functioning. Using modified magnetic tweezers, researchers were able to discover the mechanical role of chaperones, which support their substrate proteinsby pulling them during translocation and assist their native folding as a mechanical foldase. In this article, we provide a focused review of many of these new roles of single-molecule technologies, ranging from single bond breaking to complex chaperone machinery, along with the potential to design mechanomedicine, which would be a breakthrough in pharmacological interventions against many diseases.

Stable tug-of-war between kinesin-1 and cytoplasmic dynein upon different ATP and roadblock concentrations

The maintenance of intracellular processes, like organelle transport and cell division, depend on bidirectional movement along microtubules. These processes typically require kinesin and dynein motor proteins, which move with opposite directionality. Because both types of motors are often simultaneously bound to the cargo, regulatory mechanisms are required to ensure controlled directional transport. Recently, it has been shown that parameters like mechanical motor activation, ATP concentration and roadblocks on the microtubule surface differentially influence the activity of kinesin and dynein motors in distinct manners. However, how these parameters affect bidirectional transport systems has not been studied. Here, we investigate the regulatory influence of these three parameters using in vitro gliding motility assays and stochastic simulations. We find that the number of active kinesin and dynein motors determines the transport direction and velocity, but that variations in ATP concentration and roadblock density have no significant effect. Thus, factors influencing the force balance between opposite motors appear to be important, whereas the detailed stepping kinetics and bypassing capabilities of the motors only have a small effect.

The regulatory function of the AAA4 ATPase domain of cytoplasmic dynein

Cytoplasmic dynein is the primary motor for microtubule minus-end-directed transport and is indispensable to eukaryotic cells. Although each motor domain of dynein contains three active AAA+ ATPases (AAA1, 3, and 4), only the functions of AAA1 and 3 are known. Here, we use single-molecule fluorescence and optical tweezers studies to elucidate the role of AAA4 in dynein's mechanochemical cycle. We demonstrate that AAA4 controls the priming stroke of the motion-generating linker, which connects the dimerizing tail of the motor to the AAA+ ring. Before ATP binds to AAA4, dynein remains incapable of generating motion. However, when AAA4 is bound to ATP, the gating of AAA1 by AAA3 prevails and dynein motion can occur. Thus, AAA1, 3, and 4 work together to regulate dynein function. Our work elucidates an essential role for AAA4 in dynein's stepping cycle and underscores the complexity and crosstalk among the motor's multiple AAA+ domains.

Aggresomal sequestration and STUB1-mediated ubiquitylation during mammalian proteaphagy of inhibited proteasomes

The ubiquitin–proteasome system and autophagy are two major intracellular proteolytic pathways, and both remove misfolded and proteotoxic proteins from eukaryotic cells. This study describes the detailed regulatory pathway of proteasome degradation by autophagy for its own quality control. We discovered that a portion of inhibited proteasomes is actively sequestered into the aggresome, an insoluble fraction of the mammalian cell. The aggresome functions as a triage point for proteasome recovery and autophagic degradation. This mainly distinguishes proteasome quality control in mammals from that in other organisms. STUB1/CHIP E3 Ub ligase has a critical role in targeting inhibited proteasomes into the aggresome. These results provide strong insights into protein catabolism in various pathological conditions originating from impaired proteasomes.

The 26S proteasome, a self-compartmentalized protease complex, plays a crucial role in protein quality control. Multiple levels of regulatory systems modulate proteasomal activity for substrate hydrolysis. However, the destruction mechanism of mammalian proteasomes is poorly understood. We found that inhibited proteasomes are sequestered into the insoluble aggresome via HDAC6- and dynein-mediated transport. These proteasomes colocalized with the autophagic receptor SQSTM1 and cleared through selective macroautophagy, linking aggresomal segregation to autophagic degradation. This proteaphagic pathway was counterbalanced with the recovery of proteasomal activity and was critical for reducing cellular proteasomal stress. Changes in associated proteins and polyubiquitylation on inhibited 26S proteasomes participated in the targeting mechanism to the aggresome and autophagosome. The STUB1 E3 Ub ligase specifically ubiquitylated purified human proteasomes in vitro, mainly via Lys63-linked chains. Genetic and chemical inhibition of STUB1 activity significantly impaired proteasome processing and reduced resistance to proteasomal stress. These data demonstrate that aggresomal sequestration is the crucial upstream event for proteasome quality control and overall protein homeostasis in mammals.

Load-dependent detachment kinetics plays a key role in bidirectional cargo transport by kinesin and dynein

Bidirectional cargo transport along microtubules is carried out by opposing teams of kinesin and dynein motors. Despite considerable study, the factors that determine whether these competing teams achieve net anterograde or retrograde transport in cells remain unclear. The goal of this work is to use stochastic simulations of bidirectional transport to determine the motor properties that most strongly determine overall cargo velocity and directionality. Simulations were carried out based on published optical tweezer characterization of kinesin-1 and kinesin-2, and for available data for cytoplasmic dynein and the dynein-dynactin-BicD2 (DDB) complex. By varying dynein parameters and analyzing cargo trajectories, we find that net cargo transport is predicted to depend minimally on the dynein stall force, but strongly on dynein load-dependent detachment kinetics. In simulations, dynein is dominated by kinesin-1, but DDB and kinesin-1 are evenly matched, recapitulating recent experimental work. Kinesin-2 competes less well against dynein and DDB, and overall, load-dependent motor detachment is the property that most determines a motor's ability to compete in bidirectional transport. It follows that the most effective intracellular regulators of bidirectional transport are predicted to be those that alter motor detachment kinetics rather than motor velocity or stall force.

Dynamic catch-bonding generates the large stall forces of cytoplasmic dynein

Cytoplasmic dynein is an important molecular motor involved in the transport of vesicular and macromolecular cargo along microtubules in cells, often in conjunction with kinesin motors. Dynein is larger and more complex than kinesin and the mechanism and regulation of its movement is currently the subject of intense research. While it was believed for a long time that dynein motors are relatively weak in terms of the force they can generate, recent studies have shown that interactions with regulatory proteins confer large stall forces comparable to those of kinesin. This paper reports on a theoretical study which suggests that these large stall forces may be the result of an emergent, ATP-dependent, bistability resulting in a dynamic catch-bonding behavior that can cause the motor to switch between high and low load-force states.

Force production of human cytoplasmic dynein is limited by its processivity

Cytoplasmic dynein is a highly complex motor protein that generates forces toward the minus end of microtubules. Using optical tweezers, we demonstrate that the low processivity (ability to take multiple steps before dissociating) of human dynein limits its force generation due to premature microtubule dissociation. Using a high trap stiffness whereby the motor achieves greater force per step, we reveal that the motor's true maximal force ("stall force") is ~2 pN. Furthermore, an average force versus trap stiffness plot yields a hyperbolic curve that plateaus at the stall force. We derive an analytical equation that accurately describes this curve, predicting both stall force and zero-load processivity. This theoretical model describes the behavior of a kinesin motor under low-processivity conditions. Our work clarifies the true stall force and processivity of human dynein and provides a new paradigm for understanding and analyzing molecular motor force generation for weakly processive motors.

The Lateral Organization and Mobility of Plasma Membrane Components

Over the last several decades, an impressive array of advanced microscopic and analytical tools, such as single-particle tracking and nanoscopic fluorescence correlation spectroscopy, has been applied to characterize the lateral organization and mobility of components in the plasma membrane. Such analysis can tell researchers about the local dynamic composition and structure of membranes and is important for predicting the outcome of membrane-based reactions. However, owing to the unresolved complexity of the membrane and the structures peripheral to it, identification of the detailed molecular origin of the interactions that regulate the organization and mobility of the membrane has not proceeded quickly. This Perspective presents an overview of how cell-surface structure may give rise to the types of lateral mobility that are observed and some potentially fruitful future directions to elucidate the architecture of these structures in more molecular detail.

The Generation of Dynein Networks by Multi-Layered Regulation and Their Implication in Cell Division

Cytoplasmic dynein-1 (hereafter referred to as dynein) is a major microtubule-based motor critical for cell division. Dynein is essential for the formation and positioning of the mitotic spindle as well as the transport of various cargos in the cell. A striking feature of dynein is that, despite having a wide variety of functions, the catalytic subunit is coded in a single gene. To perform various cellular activities, there seem to be different types of dynein that share a common catalytic subunit. In this review, we will refer to the different kinds of dynein as “dyneins.” This review attempts to classify the mechanisms underlying the emergence of multiple dyneins into four layers. Inside a cell, multiple dyneins generated through the multi-layered regulations interact with each other to form a network of dyneins. These dynein networks may be responsible for the accurate regulation of cellular activities, including cell division. How these networks function inside a cell, with a focus on the early embryogenesis of Caenorhabditis elegans embryos, is discussed, as well as future directions for the integration of our understanding of molecular layering to understand the totality of dynein’s function in living cells.

Collective Force Generation by Molecular Motors Is Determined by Strain-Induced Unbinding

In the living cell, we encounter a large variety of motile processes such as organelle transport and cytoskeleton remodeling. These processes are driven by motor proteins that generate force by transducing chemical free energy into mechanical work. In many cases, the molecular motors work in teams to collectively generate larger forces. Recent optical trapping experiments on small teams of cytoskeletal motors indicated that the collectively generated force increases with the size of the motor team but that this increase depends on the motor type and on whether the motors are studied in vitro or in vivo. Here, we use the theory of stochastic processes to describe the motion of N motors in a stationary optical trap and to compute the N-dependence of the collectively generated forces. We consider six distinct motor types, two kinesins, two dyneins, and two myosins. We show that the force increases always linearly with N but with a prefactor that depends on the performance of the single motor. Surprisingly, this prefactor increases for weaker motors with a lower stall force. This counter-intuitive behavior reflects the increased probability with which stronger motors detach from the filament during strain generation. Our theoretical results are in quantitative agreement with experimental data on small teams of kinesin-1 motors.

Cargo adaptors regulate stepping and force generation of mammalian dynein-dynactin

Cytoplasmic dynein is an ATP-driven motor that transports intracellular cargos along microtubules. Dynein adopts an inactive conformation when not attached to a cargo, and motility is activated when dynein assembles with dynactin and a cargo adaptor. It was unclear how active dynein-dynactin complexes step along microtubules and transport cargos under tension. Using single-molecule imaging, we showed that dynein-dynactin advances by taking 8 to 32-nm steps toward the microtubule minus end with frequent sideways and backward steps. Multiple dyneins collectively bear a large amount of tension because the backward stepping rate of dynein is insensitive to load. Recruitment of two dyneins to dynactin increases the force generation and the likelihood of winning against kinesin in a tug-of-war but does not directly affect velocity. Instead, velocity is determined by cargo adaptors and tail-tail interactions between two closely packed dyneins. Our results show that cargo adaptors modulate dynein motility and force generation for a wide range of cellular functions.

Theory of Nonequilibrium Free Energy Transduction by Molecular Machines

Biomolecular machines are protein complexes that convert between different forms of free energy. They are utilized in nature to accomplish many cellular tasks. As isothermal nonequilibrium stochastic objects at low Reynolds number, they face a distinct set of challenges compared with more familiar human-engineered macroscopic machines. Here we review central questions in their performance as free energy transducers, outline theoretical and modeling approaches to understand these questions, identify both physical limits on their operational characteristics and design principles for improving performance, and discuss emerging areas of research.

Molecular mechanism of cytoplasmic dynein tension sensing

Cytoplasmic dynein is the most complex cytoskeletal motor protein and is responsible for numerous biological functions. Essential to dynein’s function is its capacity to respond anisotropically to tension, so that its microtubule-binding domains bind microtubules more strongly when under backward load than forward load. The structural mechanisms by which dynein senses directional tension, however, are unknown. Using a combination of optical tweezers, mutagenesis, and chemical cross-linking, we show that three structural elements protruding from the motor domain—the linker, buttress, and stalk—together regulate directional tension-sensing. We demonstrate that dynein’s anisotropic response to directional tension is mediated by sliding of the coiled-coils of the stalk, and that coordinated conformational changes of dynein’s linker and buttress control this process. We also demonstrate that the stalk coiled-coils assume a previously undescribed registry during dynein’s stepping cycle. We propose a revised model of dynein’s mechanochemical cycle which accounts for our findings.

The cytoplasmic motor protein dynein senses directional tension; its microtubule-binding domains bind microtubules more strongly when under backward load. Here the authors use optical tweezers to show that the linker, buttress, and stalk domains together regulate directional tension-sensing.

A model for the chemomechanical coupling of the mammalian cytoplasmic dynein molecular motor

Available single-molecule data have shown that some mammalian cytoplasmic dynein dimers move on microtubules with a constant step size of about 8.2 nm. Here, a model is presented for the chemomechanical coupling of these mammalian cytoplasmic dynein dimers. In contrast to the previous models, a peculiar feature of the current model is that the rate constants of ATPase activity are independent of the external force. Based on this model, analytical studies of the motor dynamics are presented. With only four adjustable parameters, the theoretical results reproduce quantitatively diverse available single-molecule data on the force dependence of stepping ratio, velocity, mean dwell time, and dwell-time distribution between two mechanical steps. Predicted results are also provided for the force dependence of the number of ATP molecules consumed per mechanical step, indicating that under no or low force the motors exhibit a tight chemomechanical coupling, and as the force increases the number of ATPs consumed per step increases greatly.

Variation in Cilia Protein Genes and Progression of Lung Disease in Cystic Fibrosis

RATIONALE

Cystic fibrosis, like primary ciliary dyskinesia, is an autosomal recessive disorder characterized by abnormal mucociliary clearance and obstructive lung disease. We hypothesized that genes underlying the development or function of cilia may modify lung disease severity in persons with cystic fibrosis.

OBJECTIVES

To test this hypothesis, we compared variants in 93 candidate genes in both upper and lower tertiles of lung function in a large cohort of children and adults with cystic fibrosis with those of a population control dataset.

METHODS

Variants within candidate genes were tested for association using the SKAT-O test, comparing cystic fibrosis cases defined by poor (n = 127) or preserved (n = 127) lung function with population controls (n = 3,269 or 3,148, respectively). Associated variants were then tested for association with related phenotypes in independent datasets.

RESULTS

Variants in DNAH14 and DNAAF3 were associated with poor lung function in cystic fibrosis, whereas variants in DNAH14 and DNAH6 were associated with preserved lung function in cystic fibrosis. Associations between DNAH14 and lung function were replicated in disease-related phenotypes characterized by obstructive lung disease in adults.

CONCLUSIONS

Genetic variants within DNAH6, DNAH14, and DNAAF3 are associated with variation in lung function among persons with cystic fibrosis.

The cortical force-generating machinery: how cortical spindle-pulling forces are generated

The cortical force-generating machinery pulls on dynamic plus-ends of astral microtubules to control spindle position and orientation, which underlie division type specification and cellular patterning in many eukaryotic cells. A prior work identified cytoplasmic dynein, a minus-end directed microtubule motor, as a key conserved unit of the cortical force-generating machinery. Here, I summarize recent structural, biophysical, and cell-biological studies that advance our understanding of how dynein is activated and organized at the mitotic cell cortex to generate functional spindle-pulling forces. In addition, I introduce recent findings of dynein-independent or parallel mechanisms for achieving oriented cell division.

Force sharing and force generation by two teams of elastically coupled molecular motors

Many active cellular processes such as long-distance cargo transport, spindle organization, as well as flagellar and ciliary beating are driven by molecular motors. These motor proteins act collectively and typically work in small teams. One particularly interesting example is two teams of antagonistic motors that pull a common cargo into opposite directions, thereby generating mutual interaction forces. Important issues regarding such multiple motor systems are whether or not motors from the same team share their load equally, and how the collectively generated forces depend on the single motor properties. Here we address these questions by introducing a stochastic model for cargo transport by an arbitrary number of elastically coupled molecular motors. We determine the state space of this motor system and show that this space has a rather complex and nested structure, consisting of multiple activity states and a large number of elastic substates, even for the relatively small system of two identical motors working against one antagonistic motor. We focus on this latter case because it represents the simplest tug-of-war that involves force sharing between motors from the same team. We show that the most likely motor configuration is characterized by equal force sharing between identical motors and that the most likely separation of these motors corresponds to a single motor step. These likelihoods apply to different types of motors and to different elastic force potentials acting between the motors. Furthermore, these features are observed both in the steady state and during the initial build-up of elastic strains. The latter build-up is non-monotonic and exhibits a maximum at intermediate times, a striking consequence of mutual unbinding of the elastically coupled motors. Mutual strain-induced unbinding also reduces the magnitude of the collectively generated forces. Our computational approach is quite general and can be extended to other motor systems such as motor teams working against an optical trap or mixed teams of motors with different single motor properties.

Mg2+ -free ATP regulates the processivity of native cytoplasmic dynein

Cytoplasmic dynein, a microtubule-based motor protein, is responsible for many cellular functions ranging from cargo transport to cell division. The various functions are carried out by a single isoform of cytoplasmic dynein, thus requiring different forms of motor regulation. A possible pathway to regulate motor function was revealed in optical trap experiments. Switching motor function from single steps to processive runs could be achieved by changing Mg²⁺ and ATP concentrations. Here, we confirm by single molecule total internal reflection fluorescence microscopy that a native cytoplasmic dynein dimer is able to switch to processive runs of more than 680 consecutive steps or 5.5 μm. We also identified the ratio of Mg²⁺ -free ATP to Mg.ATP as the regulating factor and propose a model for dynein processive stepping.

Step Sizes and Rate Constants of Single-headed Cytoplasmic Dynein Measured with Optical Tweezers

A power stroke of dynein is thought to be responsible for the stepping of dimeric dynein. However, the actual size of the displacement driven by a power stroke has not been directly measured. Here, the displacements of single-headed cytoplasmic dynein were measured by optical tweezers. The mean displacement of dynein interacting with microtubule was ~8 nm at 100 µM ATP, and decreased sigmoidally with a decrease in the ATP concentration. The ATP dependence of the mean displacement was explained by a model that some dynein molecules bind to microtubule in pre-stroke conformation and generate 8-nm displacement, while others bind in the post-stroke one and detach without producing a power stroke. Biochemical assays showed that the binding affinity of the post-stroke dynein to a microtubule was ~5 times higher than that of pre-stroke dynein, and the dissociation rate was ~4 times lower. Taking account of these rates, we conclude that the displacement driven by a power stroke is 8.3 nm. A working model of dimeric dynein driven by the 8-nm power stroke was proposed.

A Unified Walking Model for Dimeric Motor Proteins

Dimeric motor proteins, kinesin-1, cytoplasmic dynein-1, and myosin-V, move stepwise along microtubules and actin filaments with a regular step size. The motors take backward as well as forward steps. The step ratio r and dwell time τ, which are the ratio of the number of backward steps to the number of forward steps and the time between consecutive steps, respectively, were observed to change with the load. To understand the movement of motor proteins, we constructed a unified and simple mathematical model to explain the load dependencies of r and of τ measured for the above three types of motors quantitatively. Our model consists of three states, and the forward and backward steps are represented by the cycles of transitions visiting different pairs of states among the three, implying that a backward step is not the reversal of a forward step. Each of r and τ is given by a simple expression containing two exponential functions. The experimental data for r and τ for dynein available in the literature are not sufficient for a quantitative analysis, which is in contrast to those for kinesin and myosin-V. We reanalyze the data to obtain r and τ of native dynein to make up the insufficient data to fit them to the model. Our model successfully describes the behavior of r and τ for all of the motors in a wide range of loads from large assisting loads to superstall loads.

Application of the fluctuation theorem for noninvasive force measurement in living neuronal axons

Although its importance is recently widely accepted, force measurement has been difficult in living biological systems, mainly due to the lack of the versatile noninvasive force measurement methods. The fluctuation theorem, which represents the thermodynamic properties of small fluctuating nonequilibrium systems, has been applied to the analysis of the thermodynamic properties of motor proteins in vitro. Here we extend it to the axonal transport (displacement) of endosomes. The distribution of the displacement fluctuation had three or four distinct peaks around multiples of a unit value, which the fluctuation theorem can convert into the drag force exerted on the endosomes. The results demonstrated that a single cargo vesicle is conveyed by one to three or four units of force production.

Cortical dynein pulling mechanism is regulated by differentially targeted attachment molecule Num1

Cortical dynein generates pulling forces via microtubule (MT) end capture-shrinkage and lateral MT sliding mechanisms. In Saccharomyces cerevisiae, the dynein attachment molecule Num1 interacts with endoplasmic reticulum (ER) and mitochondria to facilitate spindle positioning across the mother-bud neck, but direct evidence for how these cortical contacts regulate dynein-dependent pulling forces is lacking. We show that loss of Scs2/Scs22, ER tethering proteins, resulted in defective Num1 distribution and loss of dynein-dependent MT sliding, the hallmark of dynein function. Cells lacking Scs2/Scs22 performed spindle positioning via MT end capture-shrinkage mechanism, requiring dynein anchorage to an ER- and mitochondria-independent population of Num1, dynein motor activity, and CAP-Gly domain of dynactin Nip100/p150^Glued subunit. Additionally, a CAAX-targeted Num1 rescued loss of lateral patches and MT sliding in the absence of Scs2/Scs22. These results reveal distinct populations of Num1 and underline the importance of their spatial distribution as a critical factor for regulating dynein pulling force.

Cells must divide so that organisms can grow, repair damaged tissues or reproduce. Before dividing, a cell creates two identical copies of its genetic information – one for each daughter. A molecular machine known as the mitotic spindle then moves each set of genetic material to where it will be needed when the daughter cells form. For the process to work properly, however, a motor protein known as dynein must correctly position the spindle by pulling it into place from the outskirts of the cell.

When a baker’s yeast cell divides, it first forms a ‘bump’, which grows into a bud that will ultimately become another yeast. The spindle needs to be precisely placed at the midpoint between the original cell and the bud, so the genetic material can get into the future daughter cell. To do so, dynein travels to the bud, where a protein called Num1 helps it attach to the periphery and pull the filaments of the mitotic spindle (known as microtubules) to the correct position. Num1 also attaches to other cellular structures in the bud, including one known as the endoplasmic reticulum. It was unclear how this connection changes where dynein is located, and how it can pull on the spindle.

To study this, Omer et al. labeled Num1, dynein and microtubules with fluorescent markers so they could be followed in living baker’s yeast using time-lapse microscopy. Mutant yeast strains were also used to disrupt how these proteins associate, which helps to tease out their roles. The experiments show that there are several populations of Num1 in the bud. One associates with the endoplasmic reticulum, and it helps dynein grab the side of a microtubule and make it slide into the bud. The other does not attach to the reticulum, but instead is located at the very tip of the bud. There, it makes dynein capture the end of the microtubule; this destabilizes the filament, which starts to shorten. As the microtubule shrinks, the spindle is pulled closer to the bud’s tip, which aligns it in the right position. The yeast cells thus need Num1 in both locations to fine-tune the pulling activity of dynein, and the spindle’s final positioning.

In the human body, not all divisions create two identical cells; for example, the daughters of stem cells can have different fates. This is due to a precise asymmetric division which dynein partly controls. The results by Omer et al. could help to unravel this mechanism.

Recruitment of two dyneins to an mRNA-dependent Bicaudal D transport complex

We investigated the role of full-length Drosophila Bicaudal D (BicD) binding partners in dynein-dynactin activation for mRNA transport on microtubules. Full-length BicD robustly activated dynein-dynactin motility only when both the mRNA binding protein Egalitarian (Egl) and K10 mRNA cargo were present, and electron microscopy showed that both Egl and mRNA were needed to disrupt a looped, auto-inhibited BicD conformation. BicD can recruit two dimeric dyneins, resulting in faster speeds and longer runs than with one dynein. Moving complexes predominantly contained two Egl molecules and one K10 mRNA. This mRNA-bound configuration makes Egl bivalent, likely enhancing its avidity for BicD and thus its ability to disrupt BicD auto-inhibition. Consistent with this idea, artificially dimerized Egl activates dynein-dynactin-BicD in the absence of mRNA. The ability of mRNA cargo to orchestrate the activation of the mRNP (messenger ribonucleotide protein) complex is an elegant way to ensure that only cargo-bound motors are motile.

Coin Tossing Explains the Activity of Opposing Microtubule Motors on Phagosomes

How the opposing activity of kinesin and dynein motors generates polarized distribution of organelles inside cells is poorly understood and hotly debated [1, 2]. Possible explanations include stochastic mechanical competition [3, 4], coordinated regulation by motor-associated proteins [5-7], mechanical activation of motors [8], and lipid-induced organization [9]. Here, we address this question by using phagocytosed latex beads to generate early phagosomes (EPs) that move bidirectionally along microtubules (MTs) in an in vitro assay [9]. Dynein/kinesin activity on individual EPs is recorded as real-time force generation of the motors against an optical trap. Activity of one class of motors frequently coincides with, or is rapidly followed by opposite motors. This leads to frequent and rapid reversals of EPs in the trap. Remarkably, the choice between dynein and kinesin can be explained by the tossing of a coin. Opposing motors therefore appear to function stochastically and independently of each other, as also confirmed by observing no effect on kinesin function when dynein is inhibited on the EPs. A simple binomial probability calculation based on the geometry of EP-microtubule contact explains the observed activity of dynein and kinesin on phagosomes. This understanding of intracellular transport in terms of a hypothetical coin, if it holds true for other cargoes, provides a conceptual framework to explain the polarized localization of organelles inside cells.

Uncovering the balance of forces driving microtubule aster migration in C. elegans zygotes

Microtubule asters must be positioned precisely within cells. How forces generated by molecular motors such as dynein are integrated in space and time to enable such positioning remains unclear. In particular, whereas aster movements depend on the drag caused by cytoplasm viscosity, in vivo drag measurements are lacking, precluding a thorough understanding of the mechanisms governing aster positioning. Here, we investigate this fundamental question during the migration of asters and pronuclei in C. elegans zygotes, a process essential for the mixing of parental genomes. Detailed quantification of these movements using the female pronucleus as an in vivo probe establish that the drag coefficient of the male-asters complex is approximately five times that of the female pronucleus. Further analysis of embryos lacking cortical dynein, the connection between asters and male pronucleus, or the male pronucleus altogether, uncovers the balance of dynein-driven forces that accurately position microtubule asters in C. elegans zygotes.

Microtubule asters are positioned precisely within cells by forces generated by molecular motors, but it is unclear how these are integrated in space and time. Here the authors perform in vivo drag measurements and genetic manipulations to determine the balance of forces that position microtubule asters in C. elegans zygotes.

Activation of mammalian cytoplasmic dynein in multi-motor motility assays

Long-range intracellular transport is facilitated by motor proteins like kinesin-1 and cyto-plasmic dynein walking along microtubules (MTs). These motors often work in teams for the transport of various intracellular cargos. While transport by multiple kinesin-1 motors has been studied extensively in the past, collective effects of cytoplasmic dynein are less well understood. On the level of single molecules, mammalian cytoplasmic dynein is not active in the absence of dynactin and adaptor proteins. However, when assembled into a team bound to the same cargo, processive motility has been observed. The underlying mechanism of this activation is not known. Here, we found that in MT gliding motility assays the gliding velocity increased with dynein surface density and MT length. Developing a mathematical model based on single-molecule parameters, we were able to simulate the observed behavior. Integral to our model is the usage of an activation term, which describes a mechanical activation of individual dynein motors when being stretched by the other motors. We hypothesize this activation to be similar to the activation of single dynein motors by dynactin and adaptor proteins.

Multiple Roles, Multiple Adaptors: Dynein During Cell Cycle

Dynein is an essential protein complex present in most eukaryotes that regulate biological processes ranging from ciliary beating, intracellular transport, to cell division. Elucidating the detailed mechanism of dynein function has been a challenging task owing to its large molecular weight and high complexity of the motor. With the advent of technologies in the last two decades, studies have uncovered a wealth of information about the structural, biochemical, and cell biological roles of this motor protein. Cytoplasmic dynein associates with dynactin through adaptor proteins to mediate retrograde transport of vesicles, mRNA, proteins, and organelles on the microtubule tracts. In a mitotic cell, dynein has multiple localizations, such as at the nuclear envelope, kinetochores, mitotic spindle and spindle poles, and cell cortex. In line with this, dynein regulates multiple events during the cell cycle, such as centrosome separation, nuclear envelope breakdown, spindle assembly checkpoint inactivation, chromosome segregation, and spindle positioning. Here, we provide an overview of dynein structure and function with focus on the roles played by this motor during different stages of the cell cycle. Further, we review in detail the role of dynactin and dynein adaptors that regulate both recruitment and activity of dynein during the cell cycle.

Combining Structure-Function and Single-Molecule Studies on Cytoplasmic Dynein

Cytoplasmic dynein is the largest and most intricate cytoskeletal motor protein. It is responsible for a vast array of biological functions, ranging from the transport of organelles and mRNAs to the movement of nuclei during neuronal migration and the formation and positioning of the mitotic spindle during cell division. Despite its megadalton size and its complex design, recent success with the recombinant expression of the dynein heavy chain has advanced our understanding of dynein's molecular mechanism through the combination of structure-function and single-molecule studies. Single-molecule fluorescence assays have provided detailed insights into how dynein advances along its microtubule track in the absence of load, while optical tweezers have yielded insights into the force generation and stalling behavior of dynein. Here, using the S. cerevisiae expression system, we provide improved protocols for the generation of dynein mutants and for the expression and purification of the mutated and/or tagged proteins. To facilitate single-molecule fluorescence and optical trapping assays, we further describe updated, easy-to-use protocols for attaching microtubules to coverslip surfaces. The presented protocols together with the recently solved crystal structures of the dynein motor domain will further simplify and accelerate hypothesis-driven mutagenesis and structure-function studies on dynein.

Minimum requirements for motility of a processive motor protein

Motor proteins generally have a two-way coupling between the ATP hydrolysis site, the lever movement and the binding affinity for their track, which allows them to perform efficient stepping. Here we explore the minimal requirements for directed motility based on simpler schemes in which the binding/unbinding from the track is decoupled from the ATPase cycle. We show that a directed power stroke alone is not sufficient for motility, but combined with an asymmetry in force-induced unbinding rates it can generate stepping. The energetic efficiency of such stepping is limited to approximately 20%. We conclude that the allosteric coupling between the ATP hydrolysis and the track binding is not strictly necessary for motility, but it greatly improves its efficiency.

Allosteric conformational change cascade in cytoplasmic dynein revealed by structure-based molecular simulations

Cytoplasmic dynein is a giant ATP-driven molecular motor that proceeds to the minus end of the microtubule (MT). Dynein hydrolyzes ATP in a ring-like structure, containing 6 AAA+ (ATPases associated with diverse cellular activities) modules, which is ~15 nm away from the MT binding domain (MTBD). This architecture implies that long-distance allosteric couplings exist between the AAA+ ring and the MTBD in order for dynein to move on the MT, although little is known about the mechanisms involved. Here, we have performed comprehensive molecular simulations of the dynein motor domain based on pre- and post- power-stroke structural information and in doing so we address the allosteric conformational changes that occur during the power-stroke and recovery-stroke processes. In the power-stroke process, the N-terminal linker movement was the prerequisite to the nucleotide-dependent AAA1 transition, from which a transition cascade propagated, on average, in a circular manner on the AAA+ ring until it reached the AAA6/C-terminal module. The recovery-stroke process was initiated by the transition of the AAA6/C-terminal, from which the transition cascade split into the two directions of the AAA+ ring, occurring both clockwise and anti-clockwise. In both processes, the MTBD conformational change was regulated by the AAA4 module and the AAA5/Strut module.

The linear molecular motor dynein is an intriguing allosteric model protein. ATP hydrolysis, catalyzed by modules in the AAA+ ring, regulates the binding to the rail molecule, microtubule, which is ~15 nm away from the AAA+ ring. The molecular mechanisms underpinning this long-distance communication are unclear. Based on recently solved pre- and post- power-stroke crystal structure information, we performed, for the first time to our knowledge, molecular simulations of complete conformational changes between the two structures. The simulation revealed that module-by-module allosteric conformational changes occur. Interestingly, the transition cascade from the pre- to the post-power-stroke states propagated in a circular manner around the AAA+ ring, while that of the recovery transitions propagated in a bi-directional manner around the ring.

Two populations of cytoplasmic dynein contribute to spindle positioning in C. elegans embryos

The position of the mitotic spindle is tightly controlled in animal cells as it determines the plane and orientation of cell division. Contacts between cytoplasmic dynein and astral microtubules (MTs) at the cell cortex generate pulling forces that position the spindle. An evolutionarily conserved Gα-GPR-1/2Pins/LGN-LIN-5Mud/NuMA cortical complex interacts with dynein and is required for pulling force generation, but the dynamics of this process remain unclear. In this study, by fluorescently labeling endogenous proteins in Caenorhabditis elegans embryos, we show that dynein exists in two distinct cortical populations. One population directly depends on LIN-5, whereas the other is concentrated at MT plus ends and depends on end-binding (EB) proteins. Knockout mutants lacking all EBs are viable and fertile and display normal pulling forces and spindle positioning. However, EB protein-dependent dynein plus end tracking was found to contribute to force generation in embryos with a partially perturbed dynein function, indicating the existence of two mechanisms that together create a highly robust force-generating system.

The Effect of Temperature on Microtubule-Based Transport by Cytoplasmic Dynein and Kinesin-1 Motors

Cytoplasmic dynein and kinesin are both microtubule-based molecular motors but are structurally and evolutionarily unrelated. Under standard conditions, both move with comparable unloaded velocities toward either the microtubule minus (dynein) or plus (most kinesins) end. This similarity is important because it is often implicitly incorporated into models that examine the balance of cargo fluxes in cells and into models of the bidirectional motility of individual cargos. We examined whether this similarity is a robust feature, and specifically whether it persists across the biologically relevant temperature range. The velocity of mammalian cytoplasmic dynein, but not of mammalian kinesin-1, exhibited a break from simple Arrhenius behavior below 15°C-just above the restrictive temperature of mammalian fast axonal transport. In contrast, the velocity of yeast cytoplasmic dynein showed a break from Arrhenius behavior at a lower temperature (∼8°C). Our studies implicate cytoplasmic dynein as a more thermally tunable motor and therefore a potential thermal regulator of microtubule-based transport. Our theoretical analysis further suggests that motor velocity changes can lead to qualitative changes in individual cargo motion and hence net intracellular cargo fluxes. We propose that temperature can potentially be used as a noninvasive probe of intracellular transport.

Cryo-EM Reveals How Human Cytoplasmic Dynein Is Auto-inhibited and Activated

Cytoplasmic dynein-1 binds dynactin and cargo adaptor proteins to form a transport machine capable of long-distance processive movement along microtubules. However, it is unclear why dynein-1 moves poorly on its own or how it is activated by dynactin. Here, we present a cryoelectron microscopy structure of the complete 1.4-megadalton human dynein-1 complex in an inhibited state known as the phi-particle. We reveal the 3D structure of the cargo binding dynein tail and show how self-dimerization of the motor domains locks them in a conformation with low microtubule affinity. Disrupting motor dimerization with structure-based mutagenesis drives dynein-1 into an open form with higher affinity for both microtubules and dynactin. We find the open form is also inhibited for movement and that dynactin relieves this by reorienting the motor domains to interact correctly with microtubules. Our model explains how dynactin binding to the dynein-1 tail directly stimulates its motor activity.