Dynein: An ancient motor protein involved in multiple modes of transport

Rotavirus viroplasm fusion and perinuclear localization are dynamic processes requiring stabilized microtubules

Rotavirus viroplasms are cytosolic, electron-dense inclusions corresponding to the viral machinery of replication responsible for viral template transcription, dsRNA genome segments replication and assembly of new viral cores. We have previously observed that, over time, those viroplasms increase in size and decrease in number. Therefore, we hypothesized that this process was dependent on the cellular microtubular network and its associated dynamic components. Here, we present evidence demonstrating that viroplasms are dynamic structures, which, in the course of an ongoing infection, move towards the perinuclear region of the cell, where they fuse among each other, thereby gaining considerably in size and, simultaneouly, explaining the decrease in numbers. On the viral side, this process seems to depend on VP2 for movement and on NSP2 for fusion. On the cellular side, both the temporal transition and the maintenance of the viroplasms are dependent on the microtubular network, its stabilization by acetylation, and, surprisingly, on a kinesin motor of the kinesin-5 family, Eg5. Thus, we provide for the first time deeper insights into the dynamics of rotavirus replication, which can explain the behavior of viroplasms in the infected cell.

Structural basis for microtubule binding and release by dynein

Cytoplasmic dynein is a microtubule-based motor required for intracellular transport and cell division. Its movement involves coupling cycles of track binding and release with cycles of force-generating nucleotide hydrolysis. How this is accomplished given the ~25 nanometers separating dynein's track- and nucleotide-binding sites is not understood. Here, we present a subnanometer-resolution structure of dynein's microtubule-binding domain bound to microtubules by cryo-electron microscopy that was used to generate a pseudo-atomic model of the complex with molecular dynamics. We identified large rearrangements triggered by track binding and specific interactions, confirmed by mutagenesis and single-molecule motility assays, which tune dynein's affinity for microtubules. Our results provide a molecular model for how dynein's binding to microtubules is communicated to the rest of the motor.

Coarse-grained modeling of the structural states and transition underlying the powerstroke of dynein motor domain

This study aims to model a minimal dynein motor domain capable of motor function, which consists of the linker domain, six AAA+ modules (AAA1-AAA6), coiled coil stalk, and C-terminus domain. To this end, we have used the newly solved X-ray structures of dynein motor domain to perform a coarse-grained modeling of dynein's post- and pre-powerstroke conformation and the conformational transition between them. First, we have used normal mode analysis to identify a single normal mode that captures the coupled motions of AAA1-AAA2 closing and linker domain rotation, which enables the ATP-driven recovery stroke of dynein. Second, based on the post-powerstroke conformation solved crystallographically, we have modeled dynein's pre-powerstroke conformation by computationally inducing AAA1-AAA2 closing and sliding of coiled coil stalk, and the resulting model features a linker domain near the pre-powerstroke position and a slightly tilted stalk. Third, we have modeled the conformational transition from pre- to post-powerstroke conformation, which predicts a clear sequence of structural events that couple microtubule binding, powerstroke and product release, and supports a signaling path from stalk to AAA1 via AAA3 and AAA4. Finally, we have found that a closed AAA3-AAA4 interface (compatible with nucleotide binding) is essential to the mechano-chemical coupling in dynein. Our modeling not only offers unprecedented structural insights to the motor function of dynein as described by past single-molecule, fluorescence resonance energy transfer, and electron microscopy studies, but also provides new predictions for future experiments to test.

Microtubule-organizing center polarity and the immunological synapse: protein kinase C and beyond

Cytoskeletal polarization is crucial for many aspects of immune function, ranging from neutrophil migration to the sampling of gut flora by intestinal dendritic cells. It also plays a key role during lymphocyte cell–cell interactions, the most conspicuous of which is perhaps the immunological synapse (IS) formed between a T cell and an antigen-presenting cell (APC). IS formation is associated with the reorientation of the T cell’s microtubule-organizing center (MTOC) to a position just beneath the cell–cell interface. This cytoskeletal remodeling event aligns secretory organelles inside the T cell with the IS, enabling the directional release of cytokines and cytolytic factors toward the APC. MTOC polarization is therefore crucial for maintaining the specificity of a T cell’s secretory and cytotoxic responses. It has been known for some time that T cell receptor (TCR) stimulation activates the MTOC polarization response. It has been difficult, however, to identify the machinery that couples early TCR signaling to cytoskeletal remodeling. Over the past few years, considerable progress has been made in this area. This review will present an overview of recent advances, touching on both the mechanisms that drive MTOC polarization and the effector responses that require it. Particular attention will be paid to both novel and atypical members of the protein kinase C family, which are now known to play important roles in both the establishment and the maintenance of the polarized state.

Development and validation of a high-content screening assay to identify inhibitors of cytoplasmic dynein-mediated transport of glucocorticoid receptor to the nucleus

Rapid ligand-induced trafficking of glucocorticoid nuclear hormone receptor (GR) from the cytoplasm to the nucleus is an extensively studied model for intracellular retrograde cargo transport employed in constructive morphogenesis and many other cellular functions. Unfortunately, potent and selective small-molecule disruptors of this process are lacking, which has restricted pharmacological investigations. We describe here the development and validation of a 384-well high-content screening (HCS) assay to identify inhibitors of the rapid ligand-induced retrograde translocation of cytoplasmic glucocorticoid nuclear hormone receptor green fluorescent fusion protein (GR-GFP) into the nuclei of 3617.4 mouse mammary adenocarcinoma cells. We selected 3617.4 cells, because they express GR-GFP under the control of a tetracycline (Tet)-repressible promoter and are exceptionally amenable to image acquisition and analysis procedures. Initially, we investigated the time-dependent expression of GR-GFP in 3617.4 cells under Tet-on and Tet-off control to determine the optimal conditions to measure dexamethasone (Dex)-induced GR-GFP nuclear translocation on the ArrayScan-VTI automated imaging platform. We then miniaturized the assay into a 384-well format and validated the performance of the GR-GFP nuclear translocation HCS assay in our 3-day assay signal window and dimethylsulfoxide validation tests. The molecular chaperone heat shock protein 90 (Hsp90) plays an essential role in the regulation of GR steroid binding affinity and ligand-induced retrograde trafficking to the nucleus. We verified that the GR-GFP HCS assay captured the concentration-dependent inhibition of GR-GFP nuclear translocation by 17-AAG, a benzoquinone ansamycin that selectively blocks the binding and hydrolysis of ATP by Hsp90. We screened the 1280 compound library of pharmacologically active compounds set in the Dex-induced GR-GFP nuclear translocation assay and used the multi-parameter HCS data to eliminate cytotoxic compounds and fluorescent outliers. We identified five qualified hits that inhibited the rapid retrograde trafficking of GR-GFP in a concentration-dependent manner: Bay 11-7085, 4-phenyl-3-furoxancarbonitrile, parthenolide, apomorphine, and 6-nitroso-1,2-benzopyrone. The data presented here demonstrate that the GR-GFP HCS assay provides an effective phenotypic screen and support the proposition that screening a larger library of diversity compounds will yield novel small-molecule probes that will enable the further exploration of intracellular retrograde transport of cargo along microtubules, a process which is essential to the morphogenesis and function of all cells.

Herpesvirus transport to the nervous system and back again

Herpes simplex virus, varicella zoster virus, and pseudorabies virus are neurotropic pathogens of the Alphaherpesvirinae subfamily of the Herpesviridae. These viruses efficiently invade the peripheral nervous system and establish lifelong latency in neurons resident in peripheral ganglia. Primary and recurrent infections cycle virus particles between neurons and the peripheral tissues they innervate. This remarkable cycle of infection is the topic of this review. In addition, some of the distinguishing hallmarks of the infections caused by these viruses are evaluated in terms of their underlying similarities.

Intrinsic disorder in dynein intermediate chain modulates its interactions with NudE and dynactin

The functional diversity of cytoplasmic dynein is in part attributed to multiple interactions between noncatalytic dynein subunits and an array of regulatory proteins. This study focuses on the interaction between the dynein intermediate chain subunit (IC) and a dynein regulator protein (NudE). We use isothermal titration calorimetry and NMR spectroscopy to map their interacting sections to their respective N-terminal domains, which are predicted to form dimeric coiled-coils. Interestingly, the specific residues within IC that interact with NudE are a subset of the bi-segmental binding region reported for p150(Glued), a subunit of the dynein activator protein dynactin. Although the IC binding domains of both NudE and p150(Glued) form dimeric coiled-coils and bind IC at a common site, we observe distinct binding modes for each regulatory protein: 1) NudE binds region 1 of the bi-segmental binding footprint of p150(Glued), whereas p150(Glued) requires regions 1 and 2 to match the binding affinity of NudE with region 1 alone. 2) Compared with unbound IC, NudE-bound IC shows a slight increase in flexibility in region 2, in contrast to the increase in ordered structure observed for p150(Glued)-bound IC (Morgan, J. L., Song, Y., and Barbar, E. (2011) J. Biol. Chem. 286, 39349-39359). 3) Although NudE has a higher affinity for the common binding segment on IC, when all three proteins are in solution, IC preferentially binds p150(Glued). These results underscore the importance of a bi-segmental binding region of IC and disorder in region 2 and flanking linkers in selecting which regulatory protein binds IC.

Subpixel colocalization reveals amyloid precursor protein-dependent kinesin-1 and dynein association with axonal vesicles

Intracellular transport of vesicles and organelles along microtubules is powered by kinesin and cytoplasmic dynein molecular motors. Both motors can attach to the same cargo and thus must be coordinated to ensure proper distribution of intracellular materials. Although a number of hypotheses have been proposed to explain how these motors are coordinated, considerable uncertainty remains, in part because of the absence of methods for assessing motor subunit composition on individual vesicular cargos. We developed a robust quantitative immunofluorescence method based on subpixel colocalization to elucidate relative kinesin-1 and cytoplasmic dynein motor subunit composition of individual, endogenous amyloid precursor protein (APP) vesicles in mouse hippocampal cells. The resulting method and data allow us to test a key in vivo prediction of the hypothesis that APP can recruit kinesin-1 to APP vesicles in neuronal axons. We found that APP levels are well-correlated with the amount of the light chain of kinesin-1 (KLC1) and the heavy chain of cytoplasmic dynein (DHC1) on vesicles. In addition, genetic reduction of APP diminishes KLC1 and DHC1 levels on APP cargos. Finally, our data reveal that reduction of KLC1 leads to decreased levels of DHC1 on APP vesicles, suggesting that KLC1 is necessary for the association of DHC1 to these cargos, and help to explain previously reported retrograde transport defects generated when kinesin-1 is reduced.

A unique ball-shaped Golgi apparatus in the rat pituitary gonadotrope: its functional implications in relation to the arrangement of the microtubule network

In polarized exocrine cells, the Golgi apparatus is cup-shaped and its convex and concave surfaces are designated as cis and trans faces, functionally confronting the rough endoplasmic reticulum and the cell surface, respectively. To clarify the morphological characteristics of the Golgi apparatus in non-polarized endocrine cells, the investigators immunocytochemically examined its precise architecture in pituitary gonadotropes, especially in relation to the arrangement of the intracellular microtubule network. The Golgi apparatus in the gonadotropes was not cup-shaped but ball-shaped or spherical, and its outer and inner surfaces were the cis and trans faces, respectively. Centrioles were situated at the center of the Golgi apparatus, from which radiating microtubules isotropically extended to the cell periphery through the gaps in the spherical wall of the Golgi stack. The shape of the Golgi apparatus and the arrangement of microtubules demonstrated in the present study could explain the microtubule-dependent movements of tubulovesicular carriers and granules within the gonadotropes. Furthermore, the spherical shape of the Golgi apparatus possibly reflects the highly symmetrical arrangement of microtubule arrays, as well as the poor polarity in the cell surface of pituitary gonadotropes.

Unconventional functions of microtubule motors

With the functional characterization of proteins advancing at fast pace, the notion that one protein performs different functions - often with no relation to each other - emerges as a novel principle of how cells work. Molecular motors are no exception to this new development. Here, we provide an account on recent findings revealing that microtubule motors are multifunctional proteins that regulate many cellular processes, in addition to their main function in transport. Some of these functions rely on their motor activity, but others are independent of it. Of the first category, we focus on the role of microtubule motors in organelle biogenesis, and in the remodeling of the cytoskeleton, especially through the regulation of microtubule dynamics. Of the second category, we discuss the function of microtubule motors as static anchors of the cargo at the destination, and their participation in regulating signaling cascades by modulating interactions between signaling proteins, including transcription factors. We also review atypical forms of transport, such as the cytoplasmic streaming in the oocyte, and the movement of cargo by microtubule fluctuations. Our goal is to provide an overview of these unexpected functions of microtubule motors, and to incite future research in this expanding field.

Misfolded Gβ is recruited to cytoplasmic dynein by Nudel for efficient clearance

The Gβγ heterodimer is an important signal transducer. Gβ, however, is prone to misfolding due to its requirement for Gγ and chaperones for proper folding. How cells dispose of misfolded Gβ (mfGβ) is not clear. Here, we showed that mfGβ was able to be polyubiquitinated and subsequently degraded by the proteasome. It was sequestered in aggresomes after the inhibition of the proteasome activity with MG132. Sustained activation of Gβγ signaling further elevated cellular levels of the ubiquitinated Gβ. Moreover, Nudel, a regulator of cytoplasmic dynein, the microtubule minus end-directed motor, directly interacted with both the unubiquitinated and ubiquitinated mfGβ. Increasing the levels of both mfGβ and Nudel promoted the association of Gβ with both Nudel and dynein, resulting in robust aggresome formation in a dynein-dependent manner. Depletion of Nudel by RNAi reduced the dynein-associated mfGβ, impaired the MG132-induced aggresome formation, and markedly prolonged the half-life of nascent Gβ. Therefore, cytosolic mfGβ is recruited to dynein by Nudel and transported to the centrosome for rapid sequestration and degradation. Such a process not only eliminates mfGβ efficiently for the control of protein quality, but may also help to terminate the Gβγ signaling.

Insights into dynein motor domain function from a 3.3-Å crystal structure

Dyneins power the beating of cilia and flagella, transport various intracellular cargos and are important during mitosis. All dyneins have a ~300kDa motor domain consisting of a ring of six AAA+ domains. ATP hydrolysis in the AAA+ ring drives the cyclic relocation of a motile element, the linker domain, to generate the force necessary for movement. How the linker interacts with the ring during the ATP hydrolysis cycle is not known. Here we present a 3.3Å crystal structure of the motor domain of Saccharomyces cerevisiae cytoplasmic dynein, crystallized in the absence of nucleotides. The linker is docked to a conserved site on AAA5, confirmed by mutagenesis as functionally important. Nucleotide soaking experiments show that the main ATP hydrolysis site in dynein (AAA1) is in a low nucleotide affinity conformation and reveal the nucleotide interactions of the other three sites (AAA2-4).

Two independent switches regulate cytoplasmic dynein's processivity and directionality

Cytoplasmic dynein is a microtubule-based molecular motor that participates in a multitude of cell activities, from cell division to organelle transport. Unlike kinesin and myosin, where different tasks are performed by highly specialized members of these superfamilies, a single form of the dynein heavy chain is utilized for different functions. This versatility demands an extensive regulation of motor function. Using an improved application of an optical trap, we were now able to demonstrate that cytoplasmic dynein can generate a discrete power stroke as well as a processive walk in either direction; i.e., towards the plus- or towards the minus-end of a microtubule. Thus, dynein's motor functions can be described by four basic modes of motion: processive and nonprocessive movement, and movement in the forward and reverse directions. Importantly, these four modes of movement can be controlled by two switches. One switch, based on phosphate, determines the directionality of movement. The second switch, depending on magnesium, converts cytoplasmic dynein from a nonprocessive to a processive motor. The two switches can be triggered separately or jointly by changing concentrations of phosphate and magnesium in the local environment. The control of four modes of movement by two switches has major implications for our understanding of the cellular functions and regulation of cytoplasmic dynein. Based on recent studies of dynein's structure we are able to draw new conclusions on cytoplasmic dynein's stepping mechanism.

Molecular motor function in axonal transport in vivo probed by genetic and computational analysis in Drosophila

Amyloid precursor protein (APP) vesicle movement by kinesin-1 and cytoplasmic dynein exhibits kinesin-1–dependent velocity. Our data also suggest that kinesin-1 and cytoplasmic dynein motors assemble in stable mixtures on APP vesicles and that their direction and velocity are controlled at least in part by dynein IC.

Bidirectional axonal transport driven by kinesin and dynein along microtubules is critical to neuronal viability and function. To evaluate axonal transport mechanisms, we developed a high-resolution imaging system to track the movement of amyloid precursor protein (APP) vesicles in Drosophila segmental nerve axons. Computational analyses of a large number of moving vesicles in defined genetic backgrounds with partial reduction or overexpression of motor proteins enabled us to test with high precision existing and new models of motor activity and coordination in vivo. We discovered several previously unknown features of vesicle movement, including a surprising dependence of anterograde APP vesicle movement velocity on the amount of kinesin-1. This finding is largely incompatible with the biophysical properties of kinesin-1 derived from in vitro analyses. Our data also suggest kinesin-1 and cytoplasmic dynein motors assemble in stable mixtures on APP vesicles and their direction and velocity are controlled at least in part by dynein intermediate chain.

The 2.8 Å crystal structure of the dynein motor domain

Dyneins are microtubule-based AAA(+) motor complexes that power ciliary beating, cell division, cell migration and intracellular transport. Here we report the most complete structure obtained so far, to our knowledge, of the 380-kDa motor domain of Dictyostelium discoideum cytoplasmic dynein at 2.8 Å resolution; the data are reliable enough to discuss the structure and mechanism at the level of individual amino acid residues. Features that can be clearly visualized at this resolution include the coordination of ADP in each of four distinct nucleotide-binding sites in the ring-shaped AAA(+) ATPase unit, a newly identified interaction interface between the ring and mechanical linker, and junctional structures between the ring and microtubule-binding stalk, all of which should be critical for the mechanism of dynein motility. We also identify a long-range allosteric communication pathway between the primary ATPase and the microtubule-binding sites. Our work provides a framework for understanding the mechanism of dynein-based motility.

Ndel1-derived peptides modulate bidirectional transport of injected beads in the squid giant axon

Bidirectional transport is a key issue in cellular biology. It requires coordination between microtubule-associated molecular motors that work in opposing directions. The major retrograde and anterograde motors involved in bidirectional transport are cytoplasmic dynein and conventional kinesin, respectively. It is clear that failures in molecular motor activity bear severe consequences, especially in the nervous system. Neuronal migration may be impaired during brain development, and impaired molecular motor activity in the adult is one of the hallmarks of neurodegenerative diseases leading to neuronal cell death. The mechanisms that regulate or coordinate kinesin and dynein activity to generate bidirectional transport of the same cargo are of utmost importance. We examined how Ndel1, a cytoplasmic dynein binding protein, may regulate non-vesicular bidirectional transport. Soluble Ndel1 protein, Ndel1-derived peptides or control proteins were mixed with fluorescent beads, injected into the squid giant axon, and the bead movements were recorded using time-lapse microscopy. Automated tracking allowed for extraction and unbiased analysis of a large data set. Beads moved in both directions with a clear bias to the anterograde direction. Velocities were distributed over a broad range and were typically slower than those associated with fast vesicle transport. Ironically, the main effect of Ndel1 and its derived peptides was an enhancement of anterograde motion. We propose that they may function primarily by inhibition of dynein-dependent resistance, which suggests that both dynein and kinesin motors may remain engaged with microtubules during bidirectional transport.

PATHOLOGIES OF AXONAL TRANSPORT IN NEURODEGENERATIVE DISEASES

Gene products such as organelles, proteins and RNAs are actively transported to synaptic terminals for the remodeling of pre-existing neuronal connections and formation of new ones. Proteins described as molecular motors mediate this transport and utilize specialized cytoskeletal proteins that function as molecular tracks for the motor based transport of cargos. Molecular motors such as kinesins and dynein's move along microtubule tracks formed by tubulins whereas myosin motors utilize tracks formed by actin. Deficits in active transport of gene products have been implicated in a number of neurological disorders. We describe such disorders collectively as "transportopathies". Here we review current knowledge of critical components of active transport and their relevance to neurodegenerative diseases.

Dynein light chain 1 functions in somatic cyst cells regulate spermatogonial divisions in Drosophila

Stem cell progeny often undergo transit amplifying divisions before differentiation. In Drosophila, a spermatogonial precursor divides four times within an enclosure formed by two somatic-origin cyst cells, before differentiating into spermatocytes. Although germline and cyst cell-intrinsic factors are known to regulate these divisions, the mechanistic details are unclear. Here, we show that loss of dynein-light-chain-1 (DDLC1/LC8) in the cyst cells eliminates bag-of-marbles (bam) expression in spermatogonia, causing gonial cell hyperplasia in Drosophila testis. The phenotype is dominantly enhanced by Dhc64C (cytoplasmic Dynein) and didum (Myosin V) loss-of-function alleles. Loss of DDLC1 or Myosin V in the cyst cells also affects their differentiation. Furthermore, cyst cell-specific loss of ddlc1 disrupts Armadillo, DE-cadherin and Integrin-βPS localizations in the cyst. Together, these results suggest that Dynein and Myosin V activities, and independent DDLC1 functions in the cyst cells organize the somatic microenvironment that regulates spermatogonial proliferation and differentiation.

Post-translational regulation of the microtubule cytoskeleton: mechanisms and functions

Half a century of biochemical and biophysical experiments has provided attractive models that may explain the diverse functions of microtubules within cells and organisms. However, the notion of functionally distinct microtubule types has not been explored with similar intensity, mostly because mechanisms for generating divergent microtubule species were not yet known. Cells generate distinct microtubule subtypes through expression of different tubulin isotypes and through post-translational modifications, such as detyrosination and further cleavage to Δ2-tubulin, acetylation, polyglutamylation and polyglycylation. The recent discovery of enzymes responsible for many tubulin post-translational modifications has enabled functional studies demonstrating that these post-translational modifications may regulate microtubule functions through an amazing range of mechanisms.

Structural dynamics and multiregion interactions in dynein-dynactin recognition

Cytoplasmic dynein is a 1.2-MDa multisubunit motor protein complex that, together with its activator dynactin, is responsible for the majority of minus end microtubule-based motility. Dynactin targets dynein to specific cellular locations, links dynein to cargo, and increases dynein processivity. These two macromolecular complexes are connected by a direct interaction between dynactin's largest subunit, p150(Glued), and dynein intermediate chain (IC) subunit. Here, we demonstrate using NMR spectroscopy and isothermal titration calorimetry that the binding footprint of p150(Glued) on IC involves two noncontiguous recognition regions, and both are required for full binding affinity. In apo-IC, the helical structure of region 1, the nascent helix of region 2, and the disorder in the rest of the chain are determined from coupling constants, amide-amide sequential NOEs, secondary chemical shifts, and various dynamics measurements. When bound to p150(Glued), different patterns of spectral exchange broadening suggest that region 1 forms a coiled-coil and region 2 a packed stable helix, with the intervening residues remaining disordered. In the 150-kDa complex of p150(Glued), IC, and two light chains, the noninterface segments remain disordered. The multiregion IC binding interface, the partial disorder of region 2 and its potential for post-translational modification, and the modulation of the length of the longer linker by alternative splicing may provide a basis for elegant and multifaceted regulation of binding between IC and p150(Glued). The long disordered linker between the p150(Glued) binding segments and the dynein light chain consensus sequences could also provide an attractive recognition platform for diverse cargoes.

The mechanism of dynein motility: insight from crystal structures of the motor domain

Dynein is a large cytoskeletal motor protein that belongs to the AAA+ (ATPases associated with diverse cellular activities) superfamily. While dynein has had a rich history of cellular research, its molecular mechanism of motility remains poorly understood. Here we describe recent X-ray crystallographic studies that reveal the architecture of dynein's catalytic ring, mechanical linker element, and microtubule binding domain. This structural information has given rise to new hypotheses on how the dynein motor domain might change its conformation in order to produce motility along microtubules.

A physical model reveals the mechanochemistry responsible for dynein's processive motion

The molecular motor dynein is associated with various cellular activities, such as directed transport along microtubules and the rhythmic beating of the axoneme. Because of the size and complexity of the protein, a detailed understanding of the mechanochemistry that drives dynein's processive motion is lacking. To overcome this deficiency, we developed the first (to our knowledge) computational model for two-headed dynein that couples conformational changes of the motor's subunits to the biochemical steps involved in ATP hydrolysis. Analysis of the model provides what we believe are several novel insights into how the protein functions: 1), structural constraints limit the motion of the free microtubule binding domain to one dimension, increasing the efficiency with which this domain finds a binding site; 2), in addition to the power stroke of the bound head, recovery of the free head to a pre-power-stroke conformation is required for this head to reach a forward binding site; 3), the order in which the power stroke and recovery transitions occur affects the probability of back-stepping; and 4), the existence of multiple equilibria in the motor's bending energy provides a mechanism for processive back-stepping. To the best of our knowledge, our computational model provides the first complete mechanochemical description of the motor protein dynein, and the findings presented here should motivate new experimental investigations to test its predictions.

How does a registry change in dynein's coiled-coil stalk drive binding of dynein to microtubules?

Dynein is a motor protein that transports cellular cargo along the microtubule (MT) by consuming ATP. Dynein's microtubule-binding domain (MTBD) is separated from the ATP-binding core by a ~15 nm stalk that consists of two α-helices forming an antiparallel coiled coil. It was previously suggested that the coiled-coil stalk creates a registry shift to modulate its binding affinity for MT. A crystal structure of the low-affinity form of MTBD was determined, but that of the high-affinity form with the registry shift is not yet available. In this study, we obtained an all-atom model structure for the high-affinity form of MTBD bound to MT by an anisotropic network model, protein-protein docking, and molecular dynamics simulations. We observe that the magnitude of the coiled-coil helix sliding is dramatically reduced near the two prolines that form the stalk-MTBD boundary and subsequently transformed to cyclic movements of MTBD helices, leading to formation of a new salt bridge with MT at the binding interface. The proposed mechanism explains the roles of highly conserved residues such as the two prolines at the stalk-MTBD boundary, the nonpolar tryptophan and proline residues near the binding interface, and the electropositive residues forming salt bridges with MT.

Molecular organization and force-generating mechanism of dynein

Dynein, which is a minus-end-directed microtubule motor, is crucial to a range of cellular processes. The mass of its motor domain is about 10 times that of kinesin, the other microtubule motor. Its large size and the difficulty of expressing and purifying mutants have hampered progress in dynein research. Recently, however, electron microscopy, X-ray crystallography and single-molecule nanometry have shed light on several key unsolved questions concerning how the dynein molecule is organized, what conformational changes in the molecule accompany ATP hydrolysis, and whether two or three motor domains are coordinated in the movements of dynein. This minireview describes our current knowledge of the molecular organization and the force-generating mechanism of dynein, with emphasis on findings from electron microscopy and single-molecule nanometry.

A Nup133-dependent NPC-anchored network tethers centrosomes to the nuclear envelope in prophase

Centrosomes are closely associated with the nuclear envelope (NE) throughout the cell cycle and this association is maintained in prophase when they separate to establish the future mitotic spindle. At this stage, the kinetochore constituents CENP-F, NudE, NudEL, dynein, and dynactin accumulate at the NE. We demonstrate here that the N-terminal domain of the nuclear pore complex (NPC) protein Nup133, although largely dispensable for NPC assembly, is required for efficient anchoring of the dynein/dynactin complex to the NE in prophase. Nup133 exerts this function through an interaction network via CENP-F and NudE/EL. We show that this molecular chain is critical for maintaining centrosome association with the NE at mitotic entry and contributes to this process without interfering with the previously described RanBP2-BICD2-dependent pathway of centrosome anchoring. Finally, our study reveals that tethering of centrosomes to the nuclear surface at the G2/M transition contributes, along with other cellular mechanisms, to early stages of bipolar spindle assembly.

A role for Tctex-1 (DYNLT1) in controlling primary cilium length

The microtubule motor complex cytoplasmic dynein is known to be involved in multiple processes including endomembrane organization and trafficking, mitosis, and microtubule organization. The majority of studies of cytoplasmic dynein have focused on the form of the motor that is built around the dynein-1 heavy chain. A second isoform, dynein heavy chain-2, and its specifically associated light intermediate chain, LIC3 (D2LIC), are known to be involved in the formation and function of primary cilia. We have used RNAi in human epithelial cells to define the cytoplasmic dynein subunits that function with dynein heavy chain 2 in primary cilia. We identify the dynein light chain Tctex-1 as a key modulator of cilia length control; depletion of Tctex-1 results in longer cilia as defined by both acetylated tubulin labeling of the axoneme and Rab8a labeling of the cilia membrane. Suppression of dynein heavy chain-2 causes concomitant loss of Tctex-1 and this correlates with an increase in cilia length. Compared to individual depletions, double siRNA depletion of DHC2 and Tctex-1 causes an even greater increase in cilia length. Our data show that Tctex-1 is a key regulator of cilia length and most likely functions as part of dynein-2.

DYNLT3 is required for chromosome alignment during mouse oocyte meiotic maturation

Dynein light chain, Tctex-type 3 (DYNLT3), is a member of the cytoplasmic dynein DYNLT light chain family and has been reported to have a potential role in chromosome congression in human mitosis. However, its role in mammalian meiosis is unclear. In this study, we examined its localization, expression, and functions in mouse oocyte meiosis. Immunofluorescent staining showed that DYNLT3 was restricted to the germinal vesicle and associated with kinetochores at the germinal vesicle breakdown stage, metaphase I and metaphase II. The expression level of DYNLT3 was similar at all meiotic stages. Depletion of DYNLT3 by antibody injection resulted in chromosome misalignment and decrease of the polar body extrusion rate. We further found that DYNLT3-depleted oocytes displayed kinetochore-microtubule detachments. Chromosome-spread experiments showed that depletion of DYNLT3 inhibited the metaphase-anaphase transition by preventing homologous chromosome segregation in meiosis I. Our data suggest that DYNLT3 is required for chromosome alignment and homologous chromosome segregation during mouse oocyte meiosis.

Functional interaction between dynein light chain and intermediate chain is required for mitotic spindle positioning

Cytoplasmic dynein is a large multisubunit complex involved in retrograde transport and the positioning of various organelles. Dynein light chain (LC) subunits are conserved across species; however, the molecular contribution of LCs to dynein function remains controversial. One model suggests that LCs act as cargo-binding scaffolds. Alternatively, LCs are proposed to stabilize the intermediate chains (ICs) of the dynein complex. To examine the role of LCs in dynein function, we used Saccharomyces cerevisiae, in which the sole function of dynein is to position the spindle during mitosis. We report that the LC8 homologue, Dyn2, localizes with the dynein complex at microtubule ends and interacts directly with the yeast IC, Pac11. We identify two Dyn2-binding sites in Pac11 that exert differential effects on Dyn2-binding and dynein function. Mutations disrupting Dyn2 elicit a partial loss-of-dynein phenotype and impair the recruitment of the dynein activator complex, dynactin. Together these results indicate that the dynein-based function of Dyn2 is via its interaction with the dynein IC and that this interaction is important for the interaction of dynein and dynactin. In addition, these data provide the first direct evidence that LC occupancy in the dynein motor complex is important for function.

X-ray structure of a functional full-length dynein motor domain

Dyneins are large microtubule-based motors that power a wide variety of cellular processes. Here we report a 4.5-Å X-ray crystallographic analysis of the entire functional motor domain of cytoplasmic dynein with ADP from Dictyostelium discoideum, which has revealed the detailed architecture of the functional units required for motor activity, including the ATP-hydrolyzing ring, the long coiled-coil microtubule-binding stalk and the force-generating rod-like linker. We discovered a Y-shaped protrusion composed of two long coiled coils-the stalk and the newly identified 'strut'. This structure supports our model in which the strut coiled coil actively contributes to communication between the primary ATPase site in the ring and the microtubule-binding site at the tip of the stalk coiled coil. Our work also provides insight into how the two motor domains are arranged and how they interact with each other in a functional dimer form of cytoplasmic dynein.

Precursor of brain-derived neurotrophic factor (proBDNF) forms a complex with Huntingtin-associated protein-1 (HAP1) and sortilin that modulates proBDNF trafficking, degradation, and processing

proBDNF, a precursor of brain-derived neurotrophic factor (BDNF), is anterogradely transported and released from nerve terminals, but the mechanism underlying this process remains unclear. In this study, we report that proBDNF forms a complex with Huntingtin associated protein-1 (HAP1) and sortilin, which plays an important role in proBDNF intracellular trafficking and stabilization. The interaction of proBDNF with both HAP1A and sortilin in co-transfected HEK293 cells is confirmed by both fluorescence resonance energy transfer and co-immunoprecipitation. The frequent co-localization (>90%) of endogenous HAP1, sortilin, and proBDNF is also found in cultured cortical neurons. Mapping studies using GST pulldown and competition assays has defined the interacting region of HAP1 with proBDNF within amino acids 371-445 and the binding sequences of proBDNF to HAP1 between amino acids 65 and 90. Fluorescence recovery after photobleaching confirms the defective movement of proBDNF-containing vesicles in neurites of HAP1(-/-) neurons, which can be partially restored by reintroducing HAP1 cDNA into the neurons. However, the effect is significantly increased by simultaneously reintroducing both HAP1 and sortilin. proBDNF and HAP1 are highly co-localized with organelle markers for the Golgi network, microtubules, molecular motor, or endosomes in normal neurons, but this co-localization is reduced in HAP1(-/-) neurons. Co-immunoprecipitation and Western blot showed that sortilin stabilizes the proBDNF·HAP1 complex in co-transfected HEK293 cells, helping to prevent proBDNF degradation. Furthermore, the complex facilitates furin cleavage to release mature BDNF.

Molecular motors: directing traffic during RNA localization

RNA localization, the enrichment of RNA in a specific subcellular region, is a mechanism for the establishment and maintenance of cellular polarity in a variety of systems. Ultimately, this results in a universal method for spatially restricting gene expression. Although the consequences of RNA localization are well-appreciated, many of the mechanisms that are responsible for carrying out polarized transport remain elusive. Several recent studies have illuminated the roles that molecular motor proteins play in the process of RNA localization. These studies have revealed complex mechanisms in which the coordinated action of one or more motor proteins can act at different points in the localization process to direct RNAs to their final destination. In this review, we discuss recent findings from several different systems in an effort to clarify pathways and mechanisms that control the directed movement of RNA.

Molecular recognition of the Tes LIM2-3 domains by the actin-related protein Arp7A

Actin-related proteins (Arps) are a highly conserved family of proteins that have extensive sequence and structural similarity to actin. All characterized Arps are components of large multimeric complexes associated with chromatin or the cytoskeleton. In addition, the human genome encodes five conserved but largely uncharacterized "orphan" Arps, which appear to be mostly testis-specific. Here we show that Arp7A, which has 43% sequence identity with β-actin, forms a complex with the cytoskeletal proteins Tes and Mena in the subacrosomal layer of round spermatids. The N-terminal 65-residue extension to the actin-like fold of Arp7A interacts directly with Tes. The crystal structure of the 1-65(Arp7A)·LIM2-3(Tes)·EVH1(Mena) complex reveals that residues 28-49 of Arp7A contact the LIM2-3 domains of Tes. Two alanine residues from Arp7A that occupy equivalent apolar pockets in both LIM domains as well as an intervening GPAK linker that binds the LIM2-3 junction are critical for the Arp7A-Tes interaction. Equivalent occupied apolar pockets are also seen in the tandem LIM domain structures of LMO4 and Lhx3 bound to unrelated ligands. Our results indicate that apolar pocket interactions are a common feature of tandem LIM domain interactions, but ligand specificity is principally determined by the linker sequence.

Identification of the dynein light chains required for human papillomavirus infection

Human papillomaviruses (HPVs) are a family of small non-enveloped DNA viruses. Some genital HPV types, including HPV type 16 (HPV16), are the causative agent for the development of cancer at the site of infection. HPVs encode two capsid proteins, L1 and L2. After endocytic cell entry and egress from endosomes, L2 accompanies the viral DNA to the nucleus where replication is initiated. For cytoplasmic transport, L2 interacts with the microtubule network via the motor protein complex dynein. We have performed yeast two-hybrid screening and identified the dynein light chain DYNLT1 (previously called Tctex1) as interaction partner of HPV16 L2. Using co-immunoprecipitation and immunofluorescence colocalization studies we confirmed the L2-DYNLT1 interaction in mammalian cells. Further studies revealed that DYNLT3, the second member of the Tctex-light chain family, also interacts with L2 in vitro and in vivo, whereas other constituents of the dynein complex were not found to associate with L2. Depletion of DYNLT1 and DYNLT3 by specific siRNAs or cytosolic delivery of light chain-specific antibodies inhibited infection of HPV16. Therefore, this work identified two host cell proteins involved in HPV16 infection that are most likely required for transport purposes towards the nucleus.

The dynactin complex enhances the speed of microtubule-dependent motions of adenovirus both towards and away from the nucleus

Unlike transport vesicles or organelles, human adenovirus (HAdV) directly binds to the microtubule minus end-directed motor dynein for transport to the nucleus. The dynein cofactor dynactin enhances nuclear transport of HAdV and boosts infection. To determine if dynactin has a specific role in cytoplasmic trafficking of incoming HAdV on microtubules, we used live cell spinning disc confocal microscopy at 25 Hz acquisition frequency and automated tracking of single virus particles at 20–50 nm spatial resolution. Computational dissection by machine-learning algorithms extracted specific motion patterns of viral trajectories. We found that unperturbed cells supported two kinds of microtubule-dependent motions, directed motions (DM) and fast drifts (FD). DM had speeds of 0.2 to 2 μm/s and run lengths of 0.4 up to 7 μm, while FD were slower and less extensive at 0.02 to 0.4 μm/s and 0.05 to 2.5 μm. Dynactin interference by overexpression of p50/dynamitin or a coiled-coil domain of p150/Glued reduced the speeds and amounts of both center- and periphery-directed DM but not FD, and inhibited infection. These results indicate that dynactin enhances adenovirus infection by increasing the speed and efficiency of dynein-mediated virus motion to the nucleus, and, surprisingly, also supports a hereto unknown motor activity for virus transport to the cell periphery.

CK1 activates minus-end-directed transport of membrane organelles along microtubules

This study shows that the signal transduction pathway responsible for the initiation of minus-end–directed movement of membrane-bounded pigment granules in melanophores involves sequential activation of protein phosphatase 2A and casein kinase 1 and that this activation correlates with increased phosphorylation of the dynein intermediate chain.

Microtubule (MT)-based organelle transport is driven by MT motor proteins that move cargoes toward MT minus-ends clustered in the cell center (dyneins) or plus-ends extended to the periphery (kinesins). Cells are able to rapidly switch the direction of transport in response to external cues, but the signaling events that control switching remain poorly understood. Here, we examined the signaling mechanism responsible for the rapid activation of dynein-dependent MT minus-end–directed pigment granule movement in Xenopus melanophores (pigment aggregation). We found that, along with the previously identified protein phosphatase 2A (PP2A), pigment aggregation signaling also involved casein kinase 1ε (CK1ε), that both enzymes were bound to pigment granules, and that their activities were increased during pigment aggregation. Furthermore we found that CK1ε functioned downstream of PP2A in the pigment aggregation signaling pathway. Finally, we discovered that stimulation of pigment aggregation increased phosphorylation of dynein intermediate chain (DIC) and that this increase was partially suppressed by CK1ε inhibition. We propose that signal transduction during pigment aggregation involves successive activation of PP2A and CK1ε and CK1ε-dependent phosphorylation of DIC, which stimulates dynein motor activity and increases minus-end–directed runs of pigment granules.

Controlled and stochastic retention concentrates dynein at microtubule ends to keep endosomes on track

Bidirectional transport of early endosomes (EEs) involves microtubules (MTs) and associated motors. In fungi, the dynein/dynactin motor complex concentrates in a comet-like accumulation at MT plus-ends to receive kinesin-3-delivered EEs for retrograde transport. Here, we analyse the loading of endosomes onto dynein by combining live imaging of photoactivated endosomes and fluorescent dynein with mathematical modelling. Using nuclear pores as an internal calibration standard, we show that the dynein comet consists of ∼55 dynein motors. About half of the motors are slowly turned over (T(1/2): ∼98 s) and they are kept at the plus-ends by an active retention mechanism involving an interaction between dynactin and EB1. The other half is more dynamic (T(1/2): ∼10 s) and mathematical modelling suggests that they concentrate at MT ends because of stochastic motor behaviour. When the active retention is impaired by inhibitory peptides, dynein numbers in the comet are reduced to half and ∼10% of the EEs fall off the MT plus-ends. Thus, a combination of stochastic accumulation and active retention forms the dynein comet to ensure capturing of arriving organelles by retrograde motors.

Light chain-dependent self-association of dynein intermediate chain

Dynein light chains are bivalent dimers that bind two copies of dynein intermediate chain IC to form a cargo attachment subcomplex. The interaction of light chain LC8 with the natively disordered N-terminal domain of IC induces helix formation at distant IC sites in or near a region predicted to form a coiled-coil. This fostered the hypothesis that LC8 binding promotes IC self-association to form a coiled-coil or other interchain helical structure. However, recent studies show that the predicted coiled-coil sequence partially overlaps the light chain LC7 recognition sequence on IC, raising questions about the apparently contradictory effects of LC8 and LC7. Here, we use NMR and fluorescence quenching to localize IC self-association to residues within the predicted coiled-coil that also correspond to helix 1 of the LC7 recognition sequence. LC8 binding promotes IC self-association of helix 1 from each of two IC chains, whereas LC7 binding reverses self-association by incorporating the same residues into two symmetrical, but distant, helices of the LC7-IC complex. Isothermal titration experiments confirm the distinction of LC8 enhancement of IC self-association and LC7 binding effects. When all three light chains are bound, IC self-association is shifted to another region. Such flexibility in association modes may function in maintaining a stable and versatile light chain-intermediate chain assembly under changing cellular conditions.

Probing the force generation and stepping behavior of cytoplasmic Dynein

Cytoplasmic dynein, which is the largest and arguably the most complex cytoskeletal motor protein, plays fundamental roles during cell division, nuclear positioning, and organelle and mRNA transport, by generating force and movement toward the minus ends of microtubules. Consequently, dynein is central to many physiological processes, and its dysfunction is implicated in human diseases. However, the molecular mechanism by which dynein produces force and movement remains poorly understood. Here, we describe the use of optical tweezers to probe the nanometer-scale motion and force generation of individual dynein molecules, and provide a hands-on protocol for how to purify cytoplasmic dynein from budding yeast in amounts sufficient for single-molecule studies.

Hepatic gene networks in morbidly obese patients with nonalcoholic fatty liver disease

BACKGROUND

Genetic factors alter the risk for nonalcoholic fatty liver disease (NAFLD). We sought to identify NAFLD-associated genes and elucidate gene networks and pathways involved in the pathogenesis of NAFLD.

METHODS

Quantitative global hepatic gene expression analysis was performed on 53 morbidly obese Caucasian subjects undergoing bariatric surgery (27 with NAFLD and 26 controls). After standardization of data, gene expression profiles were compared between patients with NAFLD and controls. The set of genes that significantly correlated with NAFLD was further analyzed by hierarchical clustering and ingenuity pathways analyses.

RESULTS

There were 25,643 quantitative transcripts, of which 108 were significantly associated with NAFLD (p < 0.001). Canonical pathway analysis in the NAFLD-associated gene clusters showed that the hepatic fibrosis signaling was the most significant pathway in the up-regulated NAFLD gene cluster containing three (COL1A1, IL10, IGFBP3) significantly altered genes, whereas the endoplasmic reticulum stress and protein ubiquitination pathways were the most significant pathways in the down-regulated NAFLD gene cluster, with the first pathway containing one (HSPA5) and the second containing two (HSPA5, USP25) significantly altered genes. The four primary gene networks associated with NAFLD were involved in cell death, immunological disease, cellular movement, and lipid metabolism with several significantly altered "hub" genes in these networks.

CONCLUSIONS

This study reveals the canonical pathways and gene networks associated with NAFLD in morbidly obese Caucasians. The application of gene network analysis highlights the transcriptional relationships among NAFLD-associated genes and allows identification of hub genes that may represent high-priority candidates for NAFLD.

Mechanism of Ser88 phosphorylation-induced dimer dissociation in dynein light chain LC8

Dynein light chain LC8 is a highly conserved, dimeric protein involved in a variety of essential cellular events. Phosphorylation at Ser88 was found to promote mammalian cell survival and regulate the dimer to monomer transition at physiological pH. Combining molecular dynamics (MD) simulation and free energy calculation methods, we explored the atomistic mechanism of the phosphorylation-induced dimer dissociation. The MD simulation revealed that phosphorylation/phosphomimetic mutation at Ser88 opens an entrance into the dimer interface for water molecules, which disturb the hydrogen bond network around His55 and is expected to raise the pK(a) value and protonation ratio of His55 as well. The free energy calculations showed that the S88E mutation destabilized the dimer by 6.6 kcal/mol, in good agreement with the experimental value of 8.1 kcal/mol. The calculated destabilization upon phosphorylation is 50.8 kcal/mol, showing that phosphorylation definitely prevents dimer formation under physiological conditions. Further analysis of the calculated free energy changes demonstrated that the electrostatic contribution dominates the impact of phosphorylation on dimer dissociation.

Functional dissection of LIS1 and NDEL1 towards understanding the molecular mechanisms of cytoplasmic dynein regulation

LIS1 and NDEL1 are known to be essential for the activity of cytoplasmic dynein in living cells. We previously reported that LIS1 and NDEL1 directly regulated the motility of cytoplasmic dynein in an in vitro motility assay. LIS1 suppressed dynein motility and inhibited the translocation of microtubules (MTs), while NDEL1 dissociated dynein from MTs and restored dynein motility following suppression by LIS1. However, the molecular mechanisms and detailed interactions of dynein, LIS1, and NDEL1 remain unknown. In this study, we dissected the regulatory effects of LIS1 and NDEL1 on dynein motility using full-length or truncated recombinant fragments of LIS1 or NDEL1. The C-terminal fragment of NDEL1 dissociated dynein from MTs, whereas its N-terminal fragment restored dynein motility following suppression by LIS1, demonstrating that the two functions of NDEL1 localize to different parts of the NDEL1 molecule, and that restoration from LIS1 suppression is caused by the binding of NDEL1 to LIS1, rather than to dynein. The truncated monomeric form of LIS1 had little effect on dynein motility, but an artificial dimer of truncated LIS1 suppressed dynein motility, which was restored by the N-terminal fragment of NDEL1. This suggests that LIS1 dimerization is essential for its regulatory function. These results shed light on the molecular interactions between dynein, LIS1, and NDEL1, and the mechanisms of cytoplasmic dynein regulation.

Neuroligin 1 is dynamically exchanged at postsynaptic sites

Neuroligins are postsynaptic cell adhesion molecules that associate with presynaptic neurexins. Both factors form a transsynaptic connection, mediate signaling across the synapse, specify synaptic functions, and play a role in synapse formation. Neuroligin dysfunction impairs synaptic transmission, disrupts neuronal networks, and is thought to participate in cognitive diseases. Here we report that chemical treatment designed to induce long-term potentiation or long-term depression (LTD) induces neuroligin 1/3 turnover, leading to either increased or decreased surface membrane protein levels, respectively. Despite its structural role at a crucial transsynaptic position, GFP-neuroligin 1 leaves synapses in hippocampal neurons over time with chemical LTD-induced neuroligin internalization depending on an intact microtubule cytoskeleton. Accordingly, neuroligin 1 and its binding partner postsynaptic density protein-95 (PSD-95) associate with components of the dynein motor complex and undergo retrograde cotransport with a dynein subunit. Transgenic depletion of dynein function in mice causes postsynaptic NLG1/3 and PSD-95 enrichment. In parallel, PSD lengths and spine head sizes are significantly increased, a phenotype similar to that observed upon transgenic overexpression of NLG1 (Dahlhaus et al., 2010). Moreover, application of a competitive PSD-95 peptide and neuroligin 1 C-terminal mutagenesis each specifically alter neuroligin 1 surface membrane expression and interfere with its internalization. Our data suggest the concept that synaptic plasticity regulates neuroligin turnover through active cytoskeleton transport.

Neurodegenerative mutation in cytoplasmic dynein alters its organization and dynein-dynactin and dynein-kinesin interactions

A single amino acid change, F580Y (Legs at odd angles (Loa), Dync1h1^Loa), in the highly conserved and overlapping homodimerization, intermediate chain, and light intermediate chain binding domain of the cytoplasmic dynein heavy chain can cause severe motor and sensory neuron loss in mice. The mechanism by which the Loa mutation impairs the neuron-specific functions of dynein is not understood. To elucidate the underlying molecular mechanisms of neurodegeneration arising from this mutation, we applied a cohort of biochemical methods combined with in vivo assays to systemically study the effects of the mutation on the assembly of dynein and its interaction with dynactin. We found that the Loa mutation in the heavy chain leads to increased affinity of this subunit of cytoplasmic dynein to light intermediate and a population of intermediate chains and a suppressed association of dynactin to dynein. These data suggest that the Loa mutation drives the assembly of cytoplasmic dynein toward a complex with lower affinity to dynactin and thus impairing transport of cargos that tether to the complex via dynactin. In addition, we detected up-regulation of kinesin light chain 1 (KLC1) and its increased association with dynein but reduced microtubule-associated KLC1 in the Loa samples. We provide a model describing how up-regulation of KLC1 and its interaction with cytoplasmic dynein in Loa could play a regulatory role in restoring the retrograde and anterograde transport in the Loa neurons.

Small peptide inhibitors disrupt a high-affinity interaction between cytoplasmic dynein and a viral cargo protein

Several viruses target the microtubular motor system in early stages of the viral life cycle. African swine fever virus (ASFV) protein p54 hijacks the microtubule-dependent transport by interaction with a dynein light chain (DYNLL1/DLC8). This was shown to be a high-affinity interaction, and the residues gradually disappearing were mapped on DLC8 to define a putative p54 binding surface by nuclear magnetic resonance (NMR) spectroscopy. The potential of short peptides targeting the binding domain to disrupt this high-affinity protein-protein interaction was assayed, and a short peptide sequence was shown to bind and compete with viral protein binding to dynein. Given the complexity and number of proteins involved in cellular transport, the prevention of this viral-DLC8 interaction might not be relevant for successful viral infection. Thus, we tested the capacity of these peptides to interfere with viral infection by disrupting dynein interaction with viral p54. Using this approach, we report on short peptides that inhibit viral growth.

Disease-associated mutations in the p150(Glued) subunit destabilize the CAP-gly domain

Point mutations within the CAP-gly domain of the p150(Glued) subunit of the dynactin complex have been identified in patients with distal spinal bulbar muscular atrophy (dSBMA) and Perry's syndrome. Herein, we show by CD and NMR experiments that each mutated CAP-gly domain is folded but less stable than the wild-type (WT) domain. We also demonstrate that the domains harboring these mutations bind to microtubules but fail to bind to EB1. These data indicate that these disease-associated, point mutations affect the stability of this domain and inhibit their interaction with EB1 but do not inhibit their interaction with microtubules.

Interaction of viruses with host cell molecular motors

Viral particles are generally too large to diffuse freely within the crowded environment of the host cell cytoplasm. They depend on mammalian cell transport systems, in particular the microtubular molecular motor dynein, to deliver their nucleic acids to the vicinity of the nucleus. An understanding of how viruses interact with dynein, and its many accessory proteins, may reveal targets for drug discovery and will unlock the toolbox required to improve the performance of synthetic gene delivery systems.