Phosphorylation regulates targeting of cytoplasmic dynein to kinetochores during mitosis

Cargo specificity, regulation, and therapeutic potential of cytoplasmic dynein

Intracellular retrograde transport in eukaryotic cells relies exclusively on the molecular motor cytoplasmic dynein 1. Unlike its counterpart, kinesin, dynein has a single isoform, which raises questions about its cargo specificity and regulatory mechanisms. The precision of dynein-mediated cargo transport is governed by a multitude of factors, including temperature, phosphorylation, the microtubule track, and interactions with a family of activating adaptor proteins. Activating adaptors are of particular importance because they not only activate the unidirectional motility of the motor but also connect a diverse array of cargoes with the dynein motor. Therefore, it is unsurprising that dysregulation of the dynein-activating adaptor transport machinery can lead to diseases such as spinal muscular atrophy, lower extremity, and dominant. Here, we discuss dynein motor motility within cells and in in vitro, and we present several methodologies employed to track the motion of the motor. We highlight several newly identified activating adaptors and their roles in regulating dynein. Finally, we explore the potential therapeutic applications of manipulating dynein transport to address diseases linked to dynein malfunction.

Pain sensitivity related to gamma oscillation of parvalbumin interneuron in primary somatosensory cortex in Dync1i1-/- mice

Cytoplasmic dynein is an important intracellular motor protein that plays an important role in neuronal growth, axonal polarity formation, dendritic differentiation, and dendritic spine development among others. The intermediate chain of dynein, encoded by Dync1i1, plays a vital role in the dynein complex. Therefore, we assessed the behavioral and related neuronal activities in mice with dync1i1 gene knockout. Neuronal activities in primary somatosensory cortex were recorded by in vivo electrophysiology and manipulated by optogenetic and chemogenetics. Nociception of mechanical, thermal, and cold pain in Dync1i1^-/- mice were impaired. The activities of parvalbumin (PV) interneurons and gamma oscillation in primary somatosensory were also impaired when exposed to mechanical nociceptive stimulation. This neuronal dysfunction was rescued by optogenetic activation of PV neurons in Dync1i1^-/- mice, and mimicked by suppressing PV neurons using chemogenetics in WT mice. Impaired pain sensations in Dync1i1^-/- mice were correlated with impaired gamma oscillations due to a loss of interneurons, especially the PV type. This genotype-driven approach revealed an association between impaired pain sensation and cytoplasmic dynein complex.

StARD9 is a novel lysosomal kinesin required for membrane tubulation, cholesterol transport and Purkinje cell survival

The pathological accumulation of cholesterol is a signature feature of Niemann-Pick type C (NPC) disease, in which excessive lipid levels induce Purkinje cell death in the cerebellum. NPC1 encodes a lysosomal cholesterol-binding protein, and mutations in NPC1 drive cholesterol accumulation in late endosomes and lysosomes (LE/Ls). However, the fundamental role of NPC proteins in LE/L cholesterol transport remains unclear. Here, we demonstrate that NPC1 mutations impair the projection of cholesterol-containing membrane tubules from the surface of LE/Ls. A proteomic survey of purified LE/Ls identified StARD9 as a novel lysosomal kinesin responsible for LE/L tubulation. StARD9 contains an N-terminal kinesin domain, a C-terminal StART domain, and a dileucine signal shared with other lysosome-associated membrane proteins. Depletion of StARD9 disrupts LE/L tubulation, paralyzes bidirectional LE/L motility and induces accumulation of cholesterol in LE/Ls. Finally, a novel StARD9 knock-out mouse recapitulates the progressive loss of Purkinje cells in the cerebellum. Together, these studies identify StARD9 as a microtubule motor protein responsible for LE/L tubulation and provide support for a novel model of LE/L cholesterol transport that becomes impaired in NPC disease.

Dynein at the kinetochore

The microtubule minus-end-directed motility of cytoplasmic dynein 1 (dynein), arguably the most complex and versatile cytoskeletal motor, is harnessed for diverse functions, such as long-range organelle transport in neuronal axons and spindle assembly in dividing cells. The versatility of dynein raises a number of intriguing questions, including how is dynein recruited to its diverse cargo, how is recruitment coupled to activation of the motor, how is motility regulated to meet different requirements for force production and how does dynein coordinate its activity with that of other microtubule-associated proteins (MAPs) present on the same cargo. Here, these questions will be discussed in the context of dynein at the kinetochore, the supramolecular protein structure that connects segregating chromosomes to spindle microtubules in dividing cells. As the first kinetochore-localized MAP described, dynein has intrigued cell biologists for more than three decades. The first part of this Review summarizes current knowledge about how kinetochore dynein contributes to efficient and accurate spindle assembly, and the second part describes the underlying molecular mechanisms and highlights emerging commonalities with dynein regulation at other subcellular sites.

Polar Chromosomes—Challenges of a Risky Path

The process of chromosome congression and alignment is at the core of mitotic fidelity. In this review, we discuss distinct spatial routes that the chromosomes take to align during prometaphase, which are characterized by distinct biomolecular requirements. Peripheral polar chromosomes are an intriguing case as their alignment depends on the activity of kinetochore motors, polar ejection forces, and a transition from lateral to end-on attachments to microtubules, all of which can result in the delayed alignment of these chromosomes. Due to their undesirable position close to and often behind the spindle pole, these chromosomes may be particularly prone to the formation of erroneous kinetochore-microtubule interactions, such as merotelic attachments. To prevent such errors, the cell employs intricate mechanisms to preposition the spindle poles with respect to chromosomes, ensure the formation of end-on attachments in restricted spindle regions, repair faulty attachments by error correction mechanisms, and delay segregation by the spindle assembly checkpoint. Despite this protective machinery, there are several ways in which polar chromosomes can fail in alignment, mis-segregate, and lead to aneuploidy. In agreement with this, polar chromosomes are present in certain tumors and may even be involved in the process of tumorigenesis.

Axonal transport of late endosomes and amphisomes is selectively modulated by local Ca2+ efflux and disrupted by PSEN1 loss of function

Dysfunction and mistrafficking of organelles in autophagy- and endosomal-lysosomal pathways are implicated in neurodegenerative diseases. Here, we reveal selective vulnerability of maturing degradative organelles (late endosomes/amphisomes) to disease-relevant local calcium dysregulation. These organelles undergo exclusive retrograde transport in axons, with occasional pauses triggered by regulated calcium efflux from agonist-evoked transient receptor potential cation channel mucolipin subfamily member 1 (TRPML1) channels-an effect greatly exaggerated by exogenous agonist mucolipin synthetic agonist 1 (ML-SA1). Deacidification of degradative organelles, as seen after Presenilin 1 (PSEN1) loss of function, induced pathological constitutive "inside-out" TRPML1 hyperactivation, slowing their transport comparably to ML-SA1 and causing accumulation in dystrophic axons. The mechanism involved calcium-mediated c-Jun N-terminal kinase (JNK) activation, which hyperphosphorylated dynein intermediate chain (DIC), reducing dynein activity. Blocking TRPML1 activation, JNK activity, or DIC1B serine-80 phosphorylation reversed transport deficits in PSEN1 knockout neurons. Our results, including features demonstrated in Alzheimer-mutant PSEN1 knockin mice, define a mechanism linking dysfunction and mistrafficking in lysosomal pathways to neuritic dystrophy under neurodegenerative conditions.

The Role of Mitotic Kinases and the RZZ Complex in Kinetochore-Microtubule Attachments: Doing the Right Link

During mitosis, the interaction of kinetochores (KTs) with microtubules (MTs) drives chromosome congression to the spindle equator and supports the segregation of sister chromatids. Faithful genome partition critically relies on the ability of chromosomes to establish and maintain proper amphitelic end-on attachments, a configuration in which sister KTs are connected to robust MT fibers emanating from opposite spindle poles. Because the capture of spindle MTs by KTs is error prone, cells use mechanisms that sense and correct inaccurate KT-MT interactions before committing to segregate sister chromatids in anaphase. If left unresolved, these errors can result in the unequal distribution of chromosomes and lead to aneuploidy, a hallmark of cancer. In this review, we provide an overview of the molecular strategies that monitor the formation and fine-tuning of KT-MT attachments. We describe the complex network of proteins that operates at the KT-MT interface and discuss how AURORA B and PLK1 coordinate several concurrent events so that the stability of KT-MT attachments is precisely modulated throughout mitotic progression. We also outline updated knowledge on how the RZZ complex is regulated to ensure the formation of end-on attachments and the fidelity of mitosis.

Phosphorylation and Pin1 binding to the LIC1 subunit selectively regulate mitotic dynein functions

The dynein motor performs multiple functions in mitosis by engaging with a wide cargo spectrum. One way to regulate dynein's cargo-binding selectivity is through the C-terminal domain (CTD) of its light intermediate chain 1 subunit (LIC1), which binds directly with cargo adaptors. Here we show that mitotic phosphorylation of LIC1-CTD at its three cdk1 sites is required for proper mitotic progression, for dynein loading onto prometaphase kinetochores, and for spindle assembly checkpoint inactivation in human cells. Mitotic LIC1-CTD phosphorylation also engages the prolyl isomerase Pin1 predominantly to Hook2-dynein-Nde1-Lis1 complexes, but not to dynein-spindly-dynactin complexes. LIC1-CTD dephosphorylation abrogates dynein-Pin1 binding, promotes prophase centrosome-nuclear envelope detachment, and impairs metaphase chromosome congression and mitotic Golgi fragmentation, without affecting interphase membrane transport. Phosphomutation of a conserved LIC1-CTD SP site in zebrafish leads to early developmental defects. Our work reveals that LIC1-CTD phosphorylation differentially regulates distinct mitotic dynein pools and suggests the evolutionary conservation of this phosphoregulation.

Dynein light intermediate chains as pivotal determinants of dynein multifunctionality

In animal cells, a single cytoplasmic dynein motor mediates microtubule minus-end-directed transport, counterbalancing dozens of plus-end-directed kinesins. The remarkable ability of dynein to interact with a diverse cargo spectrum stems from its tightly regulated recruitment of cargo-specific adaptor proteins, which engage the dynactin complex to make a tripartite processive motor. Adaptor binding is governed by the homologous dynein light intermediate chain subunits LIC1 (DYNC1LI1) and LIC2 (DYNC1LI2), which exist in mutually exclusive dynein complexes that can perform both unique and overlapping functions. The intrinsically disordered and variable C-terminal domains of the LICs are indispensable for engaging a variety of structurally divergent adaptors. Here, we hypothesize that numerous spatiotemporally regulated permutations of posttranslational modifications of the LICs, as well as of the adaptors and cargoes, exponentially expand the spectrum of dynein-adaptor-cargo complexes. We thematically illustrate the possibilities that could generate a vast set of biochemical variations required to support the wide range of dynein functions.

Spindle Architectural Features Must Be Considered Along With Cell Size to Explain the Timing of Mitotic Checkpoint Silencing

Mitosis proceeds through a defined series of events that is largely conserved, but the amount of time needed for their completion can vary in different cells and organisms. In many systems, mitotic duration depends on the time required to satisfy and silence the spindle assembly checkpoint (SAC), also known as the mitotic checkpoint. Because SAC silencing involves trafficking SAC molecules among kinetochores, spindle, and cytoplasm, the size and geometry of the spindle relative to cell volume are expected to affect mitotic duration by influencing the timing of SAC silencing. However, the relationship between SAC silencing, cell size, and spindle dimensions is unclear. To investigate this issue, we used four DLD-1 tetraploid (4N) clones characterized by small or large nuclear and cell size. We found that the small 4N clones had longer mitotic durations than the parental DLD-1 cells and that this delay was due to differences in their metaphase duration. Leveraging a previous mathematical model for spatiotemporal regulation of SAC silencing, we show that the difference in metaphase duration, i.e., SAC silencing time, can be explained by the distinct spindle microtubule densities and sizes of the cell, spindle, and spindle poles in the 4N clones. Lastly, we demonstrate that manipulating spindle geometry can alter mitotic and metaphase duration, consistent with a model prediction. Our results suggest that spindle size does not always scale with cell size in mammalian cells and cell size is not sufficient to explain the differences in metaphase duration. Only when a number of spindle architectural features are considered along with cell size can the kinetics of SAC silencing, and hence mitotic duration, in the different clones be explained.

AMP-activated protein kinase regulates cytoplasmic dynein behavior and contributes to neuronal migration in the developing neocortex

The microtubule motor cytoplasmic dynein contributes to radial migration of newborn pyramidal neurons in the developing neocortex. Here, we show that AMP-activated protein kinase (AMPK) mediates the nucleus-centrosome coupling, a key process for radial neuronal migration that relies on dynein. Depletion of the catalytic subunit of AMPK in migrating neurons impairs this coupling as well as neuronal migration. AMPK shows overlapping subcellular distribution with cytoplasmic dynein and the two proteins interact with each other. Pharmacological inhibition or activation of AMPK modifies the phosphorylation states of dynein intermediate chain (DIC) and dynein functions. Furthermore, AMPK phosphorylates DIC at Ser81. Expression of a phospho-resistant mutant of DIC retards neuronal migration in a similar way to AMPK depletion. Conversely, expression of the phospho-mimetic mutant of DIC alleviates impaired neuronal migration caused by AMPK depletion. Thus, AMPK-regulated dynein function via Ser81 DIC phosphorylation is crucial for radial neuronal migration.

RZZ-SPINDLY-DYNEIN: you got to keep 'em separated

To maintain genome stability, chromosomes must be equally distributed among daughter cells at the end of mitosis. The accuracy of chromosome segregation requires sister-kinetochores to stably attach to microtubules emanating from opposite spindle poles. However, initial kinetochore-microtubule interactions are able to turnover so that defective attachment configurations that typically arise during early mitosis may be corrected. Growing evidence supports a role for the RZZ complex in preventing the stabilization of erroneous kinetochore-microtubule attachments. This inhibitory function of RZZ toward end-on attachments is relieved by DYNEIN-mediated transport of the complex as chromosomes congress and appropriate interactions with microtubules are established. However, it remains unclear how DYNEIN is antagonized to prevent premature RZZ removal. We recently described a new mechanism that sheds new light on this matter. We found that POLO kinase phosphorylates the DYNEIN adaptor SPINDLY to promote the uncoupling between RZZ and DYNEIN. Elevated POLO activity during prometaphase ensures that RZZ is retained at kinetochores to allow the dynamic turnover of kinetochore-microtubule interactions and prevent the stabilization of erroneous attachments. Here, we discuss additional interpretations to explain a model for POLO-dependent regulation of the RZZ-SPINDLY-DYNEIN module during mitosis.

A Journey through the Cytoskeleton with Protein Kinase CK2

Substrate pleiotropicity, a very acidic phosphorylation consensus sequence, and an apparent uncontrolled activity, are the main features of CK2, a Ser/Thr protein kinase that is required for a plethora of cell functions. Not surprisingly, CK2 appears to affect cytoskeletal structures and correlated functions such as cell shape, mechanical integrity, cell movement and division. This review outlines our current knowledge of how CK2 regulates cytoskeletal structures, and discusses involved pathways and molecular mechanisms.

Requirement of the Dynein-Adaptor Spindly for Mitotic and Post-Mitotic Functions in Drosophila

Spindly was originally identified as a specific regulator of Dynein activity at the kinetochore. In early prometaphase, Spindly recruits the Dynein/Dynactin complex, promoting the establishment of stable kinetochore-microtubule interactions and progression into anaphase. While details of Spindly function in mitosis have been worked out in cultured human cells and in the C. elegans zygote, the function of Spindly within the context of an organism has not yet been addressed. Here, we present loss- and gain-of-function studies of Spindly using transgenic RNAi in Drosophila. Knock-down of Spindly in the female germ line results in mitotic arrest during embryonic cleavage divisions. We investigated the requirements of Spindly protein domains for its localisation and function, and found that the carboxy-terminal region controls Spindly localisation in a cell-type specific manner. Overexpression of Spindly in the female germ line is embryonic lethal and results in altered egg morphology. To determine whether Spindly plays a role in post-mitotic cells, we altered Spindly protein levels in migrating cells and found that ovarian border cell migration is sensitive to the levels of Spindly protein. Our study uncovers novel functions of Spindly and a differential, functional requirement for its carboxy-terminal region in Drosophila.

PKCε Controls Mitotic Progression by Regulating Centrosome Migration and Mitotic Spindle Assembly

To form a proper mitotic spindle, centrosomes must be duplicated and driven poleward in a timely and controlled fashion. Improper timing of centrosome separation and errors in mitotic spindle assembly may lead to chromosome instability, a hallmark of cancer. Protein kinase C epsilon (PKCε) has recently emerged as a regulator of several cell-cycle processes associated with the resolution of mitotic catenation during the metaphase-anaphase transition and in regulating the abscission checkpoint. However, an engagement of PKCε in earlier (pre)mitotic events has not been addressed. Here, we now establish that PKCε controls prophase-to-metaphase progression by coordinating centrosome migration and mitotic spindle assembly in transformed cells. This control is exerted through cytoplasmic dynein function. Importantly, it is also demonstrated that the PKCε dependency of mitotic spindle organization is correlated with the nonfunctionality of the TOPO2A-dependent G2 checkpoint, a characteristic of many transformed cells. Thus, PKCε appears to become specifically engaged in a programme of controls that are required to support cell-cycle progression in transformed cells, advocating for PKCε as a potential cancer therapeutic target.Implications: The close relationship between PKCε dependency for mitotic spindle organization and the nonfunctionality of the TOPO2A-dependent G2 checkpoint, a hallmark of transformed cells, strongly suggests PKCε as a therapeutic target in cancer. Mol Cancer Res; 16(1); 3-15. ©2017 AACR.

Cone myoid elongation involves unidirectional microtubule movement mediated by dynein-1

Using structured illumination microscopy and photoconvertible tubulin in zebrafish photoreceptors, it is shown that microtubules move together during myoid elongation, a dark adaptive process in cone photoreceptors. Additionally, cytoplasmic dynein-1, localized at the base of the elongating myoid, mediates this unidirectional movement of microtubules.

Teleosts and amphibians exhibit retinomotor movements, morphological changes in photoreceptors regulated by light and circadian rhythms. Cone myoid elongation occurs during dark adaptation, leading to the positioning of the cone outer segment closer to the retinal pigment epithelium. Although it has been shown that microtubules are essential for cone myoid elongation, the underlying mechanism has not been established. In this work, we generated a transgenic line of zebrafish expressing a photoconvertible form of α-tubulin (tdEOS-tubulin) specifically in cone photoreceptors. Using superresolution structured illumination microscopy in conjunction with both pharmacological and genetic manipulation, we show that cytoplasmic dynein-1, which localizes to the junction between the ellipsoid and myoid, functions to shuttle microtubules from the ellipsoid into the myoid during the course of myoid elongation. We propose a novel model by which stationary complexes of cytoplasmic dynein-1 are responsible for the shuttling of microtubules between the ellipsoid and myoid is the underlying force for the morphological change of myoid elongation.

Spindle Size Scaling Contributes to Robust Silencing of Mitotic Spindle Assembly Checkpoint

Chromosome segregation during mitosis hinges on proper assembly of the microtubule spindle that establishes bipolar attachment to each chromosome. Experiments demonstrate allometry of mitotic spindles and a universal scaling relationship between spindle size and cell size across metazoans, which indicates a conserved principle of spindle assembly at play during evolution. However, the nature of this principle is currently unknown. Researchers have focused on deriving the mechanistic underpinning of the size scaling from the mechanical aspects of the spindle assembly process. In this work we take a different standpoint and ask: What is the size scaling for? We address this question from the functional perspectives of spindle assembly checkpoint (SAC). SAC is the critical surveillance mechanism that prevents premature chromosome segregation in the presence of unattached or misattached chromosomes. The SAC signal gets silenced after and only after the last chromosome-spindle attachment in mitosis. We previously established a model that explains the robustness of SAC silencing based on spindle-mediated spatiotemporal regulation of SAC proteins. Here, we refine the previous model, and find that robust and timely SAC silencing entails proper size scaling of mitotic spindle. This finding provides, to our knowledge, a novel, function-oriented angle toward understanding the observed spindle allometry, and the universal scaling relationship between spindle size and cell size in metazoans. In a broad sense, the functional requirement of robust SAC silencing could have helped shape the spindle assembly mechanism in evolution.

Mechanisms of Chromosome Congression during Mitosis

Chromosome congression during prometaphase culminates with the establishment of a metaphase plate, a hallmark of mitosis in metazoans. Classical views resulting from more than 100 years of research on this topic have attempted to explain chromosome congression based on the balance between opposing pulling and/or pushing forces that reach an equilibrium near the spindle equator. However, in mammalian cells, chromosome bi-orientation and force balance at kinetochores are not required for chromosome congression, whereas the mechanisms of chromosome congression are not necessarily involved in the maintenance of chromosome alignment after congression. Thus, chromosome congression and maintenance of alignment are determined by different principles. Moreover, it is now clear that not all chromosomes use the same mechanism for congressing to the spindle equator. Those chromosomes that are favorably positioned between both poles when the nuclear envelope breaks down use the so-called "direct congression" pathway in which chromosomes align after bi-orientation and the establishment of end-on kinetochore-microtubule attachments. This favors the balanced action of kinetochore pulling forces and polar ejection forces along chromosome arms that drive chromosome oscillatory movements during and after congression. The other pathway, which we call "peripheral congression", is independent of end-on kinetochore microtubule-attachments and relies on the dominant and coordinated action of the kinetochore motors Dynein and Centromere Protein E (CENP-E) that mediate the lateral transport of peripheral chromosomes along microtubules, first towards the poles and subsequently towards the equator. How the opposite polarities of kinetochore motors are regulated in space and time to drive congression of peripheral chromosomes only now starts to be understood. This appears to be regulated by position-dependent phosphorylation of both Dynein and CENP-E and by spindle microtubule diversity by means of tubulin post-translational modifications. This so-called "tubulin code" might work as a navigation system that selectively guides kinetochore motors with opposite polarities along specific spindle microtubule populations, ultimately leading to the congression of peripheral chromosomes. We propose an integrated model of chromosome congression in mammalian cells that depends essentially on the following parameters: (1) chromosome position relative to the spindle poles after nuclear envelope breakdown; (2) establishment of stable end-on kinetochore-microtubule attachments and bi-orientation; (3) coordination between kinetochore- and arm-associated motors; and (4) spatial signatures associated with post-translational modifications of specific spindle microtubule populations. The physiological consequences of abnormal chromosome congression, as well as the therapeutic potential of inhibiting chromosome congression are also discussed.

Label-free target identification using in-gel fluorescence difference via thermal stability shift

A label-free method for proteome-wide target identification was developed using in-gel fluorescence difference caused by thermal stability shift.

Target engagement is a prerequisite for the therapeutic effects of bioactive small molecules, and unbiased identification of their target proteins can facilitate drug discovery and chemical biology research. Structural modifications of bioactive natural products for target identification exhibit potential limitations such as synthetic difficulties, limited supplies from natural sources, and loss of original efficacy. Herein, we developed a label-free method for proteome-wide target identification using in-gel fluorescence difference caused by thermal stability shift, namely TS-FITGE. Quantitative intra-gel image analysis of each protein spot revealed target proteins with shifted thermal stability upon drug engagement, and plotting of melting curves by inter-gel analysis confirmed the positive targets. We demonstrated the robustness and applicability of the TS-FITGE method by identifying target proteins, including membrane-anchored proteins, of complex bioactive compounds. Furthermore, we identified and functionally validated nucleophosmin as a novel target protein of hordenine, a natural product upregulator of in vitro translation.

Erroneous Silencing of the Mitotic Checkpoint by Aberrant Spindle Pole-Kinetochore Coordination

To segregate chromosomes during cell division, microtubules that form the bipolar spindle attach to and pull on paired chromosome kinetochores. The spindle assembly checkpoint (SAC) is activated at unattached and misattached kinetochores to prevent further mitotic progression. The SAC is silenced after all the kinetochores establish proper and stable attachment to the spindle. Robust timing of SAC silencing after the last kinetochore-spindle attachment herein dictates the fidelity of chromosome segregation. Chromosome missegregation is rare in typical somatic cell mitosis, but frequent in cancer cell mitosis and in meiosis I of mammalian oocytes. In the latter cases, SAC is normally activated in response to disruptions of kinetochore-spindle attachments, suggesting that frequent chromosome missegregation ensues from faulty SAC silencing. In-depth understanding of how SAC silencing malfunctions in these cases is yet missing, but is believed to hold promise for treatment of cancer and prevention of human miscarriage and birth defects. We previously established a spatiotemporal model that, to the best of our knowledge, explained the robustness of SAC silencing in normal mitosis for the first time. In this article, we take advantage of the whole-cell perspective of the spatiotemporal model to identify possible causes of chromosome missegregation out of the distinct features of spindle assembly exhibited by cancer cells and mammalian oocytes. The model results explain why multipolar spindle could inhibit SAC silencing and spindle pole clustering could promote it-albeit accompanied by more kinetochore attachment errors. The model also eliminates geometric factors as the cause for nonrobust SAC silencing in oocyte meiosis, and instead, suggests atypical kinetochore-spindle attachment in meiosis as a potential culprit. Overall, the model shows that abnormal spindle-pole formation and its aberrant coordination with atypical kinetochore-spindle attachments could compromise the robustness of SAC silencing. Our model highlights systems-level coupling between kinetochore-spindle attachment and spindle-pole formation in SAC silencing.

N-Acetyl-D-Glucosamine Kinase Interacts with Dynein-Lis1-NudE1 Complex and Regulates Cell Division

N-acetyl-D-glucosamine kinase (GlcNAc kinase or NAGK) primarily catalyzes phosphoryl transfer to GlcNAc during amino sugar metabolism. Recently, it was shown NAGK interacts with dynein light chain roadblock type 1 (DYNLRB1) and upregulates axo-dendritic growth, which is an enzyme activity-independent, non-canonical structural role. The authors examined the distributions of NAGK and NAGK-dynein complexes during the cell cycle in HEK293T cells. NAGK was expressed throughout different stages of cell division and immunocytochemistry (ICC) showed NAGK was localized at nuclear envelope, spindle microtubules (MTs), and kinetochores (KTs). A proximity ligation assay (PLA) for NAGK and DYNLRB1 revealed NAGK-dynein complex on nuclear envelopes in prophase cells and on chromosomes in metaphase cells. NAGK-DYNLRB1 PLA followed by Lis1/NudE1 immunostaining showed NAGK-dynein complexes were colocalized with Lis1 and NudE1 signals, and PLA for NAGK-Lis1 showed similar signal patterns, suggesting a functional link between NAGK and dynein-Lis1 complex. Subsequently, NAGK-dynein complexes were found in KTs and on nuclear membranes where KTs were marked with CENP-B ICC and nuclear membrane with lamin ICC. Furthermore, knockdown of NAGK by small hairpin (sh) RNA was found to delay cell division. These results indicate that the NAGK-dynein interaction with the involvements of Lis1 and NudE1 plays an important role in prophase nuclear envelope breakdown (NEB) and metaphase MT-KT attachment during eukaryotic cell division.

Dynein prevents erroneous kinetochore-microtubule attachments in mitosis

Equal distribution of the genetic material during cell division relies on efficient congression of chromosomes to the metaphase plate. Prior to their alignment, the Dynein motor recruited to kinetochores transports a fraction of laterally-attached chromosomes along microtubules toward the spindle poles. By doing that, Dynein not only contributes to chromosome movements, but also prevents premature stabilization of end-on kinetochore-microtubule attachments. This is achieved by 2 parallel mechanisms: 1) Dynein-mediated poleward movement of chromosomes counteracts opposite polar-ejection forces (PEFs) on chromosome arms by the microtubule plus-end-directed motors chromokinesins. Otherwise, they could stabilize erroneous syntelic kinetochore-microtubule attachments and lead to the random ejection of chromosomes away from the spindle poles; and 2) By transporting chromosomes to the spindle poles, Dynein brings the former to the zone of highest Aurora A kinase activity, further destabilizing kinetochore-microtubule attachments. Thus, Dynein plays an important role in keeping chromosome segregation error-free by preventing premature stabilization of kinetochore-microtubule attachments near the spindle poles.

Cytoskeleton Molecular Motors: Structures and Their Functions in Neuron

Cells make use of molecular motors to transport small molecules, macromolecules and cellular organelles to target region to execute biological functions, which is utmost important for polarized cells, such as neurons. In particular, cytoskeleton motors play fundamental roles in neuron polarization, extension, shape and neurotransmission. Cytoskeleton motors comprise of myosin, kinesin and cytoplasmic dynein. F-actin filaments act as myosin track, while kinesin and cytoplasmic dynein move on microtubules. Cytoskeleton motors work together to build a highly polarized and regulated system in neuronal cells via different molecular mechanisms and functional regulations. This review discusses the structures and working mechanisms of the cytoskeleton motors in neurons.

A dynein independent role of Tctex-1 at the kinetochore

Dynein light chains are accessory subunits of the cytoplasmic dynein complex, a minus-end directed microtubule motor. Here, we demonstrate that the dynein light chain Tctex-1 associates with unattached kinetochores and is essential for accurate chromosome segregation. Tctex-1 knockdown in cells does not affect the localization and function of dynein at the kinetochore, but produces a prolonged mitotic arrest with a few misaligned chromosomes, which are subsequently missegregated during anaphase. This function is independent of Tctex-1's association with dynein. The kinetochore localization of Tctex-1 is independent of the ZW10-dynein pathway, but requires the Ndc80 complex. Thus, our findings reveal a dynein independent role of Tctex-1 at the kinetochore to enhance the stability of kinetochore-microtubule attachment.

Proteomic Analysis of Dynein-Interacting Proteins in Amyotrophic Lateral Sclerosis Synaptosomes Reveals Alterations in the RNA-Binding Protein Staufen1

Synapse disruption takes place in many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). However, the mechanistic understanding of this process is still limited. We set out to study a possible role for dynein in synapse integrity. Cytoplasmic dynein is a multisubunit intracellular molecule responsible for diverse cellular functions, including long-distance transport of vesicles, organelles, and signaling factors toward the cell center. A less well-characterized role dynein may play is the spatial clustering and anchoring of various factors including mRNAs in distinct cellular domains such as the neuronal synapse. Here, in order to gain insight into dynein functions in synapse integrity and disruption, we performed a screen for novel dynein interactors at the synapse. Dynein immunoprecipitation from synaptic fractions of the ALS model mSOD1(G93A) and wild-type controls, followed by mass spectrometry analysis on synaptic fractions of the ALS model mSOD1(G93A) and wild-type controls, was performed. Using advanced network analysis, we identified Staufen1, an RNA-binding protein required for the transport and localization of neuronal RNAs, as a major mediator of dynein interactions via its interaction with protein phosphatase 1-beta (PP1B). Both in vitro and in vivo validation assays demonstrate the interactions of Staufen1 and PP1B with dynein, and their colocalization with synaptic markers was altered as a result of two separate ALS-linked mutations: mSOD1(G93A) and TDP43(A315T). Taken together, we suggest a model in which dynein's interaction with Staufen1 regulates mRNA localization along the axon and the synapses, and alterations in this process may correlate with synapse disruption and ALS toxicity.

Interactions of Yeast Dynein with Dynein Light Chain and Dynactin: GENERAL IMPLICATIONS FOR INTRINSICALLY DISORDERED DUPLEX SCAFFOLDS IN MULTIPROTEIN ASSEMBLIES

Intrinsically disordered protein (IDP) duplexes composed of two IDP chains cross-linked by bivalent partner proteins form scaffolds for assembly of multiprotein complexes. The N-terminal domain of dynein intermediate chain (N-IC) is one such IDP that forms a bivalent scaffold with multiple dynein light chains including LC8, a hub protein that promotes duplex formation of diverse IDP partners. N-IC also binds a subunit of the dynein regulator, dynactin. Here we characterize interactions of a yeast ortholog of N-IC (N-Pac11) with yeast LC8 (Dyn2) or with the intermediate chain-binding subunit of yeast dynactin (Nip100). Residue level changes in Pac11 structure are monitored by NMR spectroscopy, and binding energetics are monitored by isothermal titration calorimetry (ITC). N-Pac11 is monomeric and primarily disordered except for a single α-helix (SAH) at the N terminus and a short nascent helix, LH, flanked by the two Dyn2 recognition motifs. Upon binding Dyn2, the only Pac11 residues making direct protein-protein interactions are in and immediately flanking the recognition motifs. Dyn2 binding also orders LH residues of Pac11. Upon binding Nip100, only Pac11 SAH residues make direct protein-protein interactions, but LH residues at a distant sequence position and L1 residues in an adjacent linker are also ordered. The long distance, ligand-dependent ordering of residues reveals new elements of dynamic structure within IDP linker regions.

GSK-3β Phosphorylation of Cytoplasmic Dynein Reduces Ndel1 Binding to Intermediate Chains and Alters Dynein Motility

Glycogen synthase kinase 3 (GSK‐3) has been linked to regulation of kinesin‐dependent axonal transport in squid and flies, and to indirect regulation of cytoplasmic dynein. We have now found evidence for direct regulation of dynein by mammalian GSK‐3β in both neurons and non‐neuronal cells. GSK‐3β coprecipitates with and phosphorylates mammalian dynein. Phosphorylation of dynein intermediate chain (IC) reduces its interaction with Ndel1, a protein that contributes to dynein force generation. Two conserved residues, S87/T88 in IC‐1B and S88/T89 in IC‐2C, have been identified as GSK‐3 targets by both mass spectrometry and site‐directed mutagenesis. These sites are within an Ndel1‐binding domain, and mutation of both sites alters the interaction of IC's with Ndel1. Dynein motility is stimulated by (i) pharmacological and genetic inhibition of GSK‐3β, (ii) an insulin‐sensitizing agent (rosiglitazone) and (iii) manipulating an insulin response pathway that leads to GSK‐3β inactivation. Thus, our study connects a well‐characterized insulin‐signaling pathway directly to dynein stimulation via GSK‐3 inhibition.

Cdk1 Activates Pre-mitotic Nuclear Envelope Dynein Recruitment and Apical Nuclear Migration in Neural Stem Cells

Dynein recruitment to the nuclear envelope is required for pre-mitotic nucleus-centrosome interactions in nonneuronal cells and for apical nuclear migration in neural stem cells. In each case, dynein is recruited to the nuclear envelope (NE) specifically during G2 via two nuclear pore-mediated mechanisms involving RanBP2-BicD2 and Nup133-CENP-F. The mechanisms responsible for cell-cycle control of this behavior are unknown. We now find that Cdk1 serves as a direct master controller for NE dynein recruitment in neural stem cells and HeLa cells. Cdk1 phosphorylates conserved sites within RanBP2 and activates BicD2 binding and early dynein recruitment. Late recruitment is triggered by a Cdk1-induced export of CENP-F from the nucleus. Forced NE targeting of BicD2 overrides Cdk1 inhibition, fully rescuing dynein recruitment and nuclear migration in neural stem cells. These results reveal how NE dynein recruitment is cell-cycle regulated and identify the trigger mechanism for apical nuclear migration in the brain.

Mitosis. Microtubule detyrosination guides chromosomes during mitosis

Before chromosomes segregate into daughter cells, they align at the mitotic spindle equator, a process known as chromosome congression. Centromere-associated protein E (CENP-E)/Kinesin-7 is a microtubule plus-end-directed kinetochore motor required for congression of pole-proximal chromosomes. Because the plus-ends of many astral microtubules in the spindle point to the cell cortex, it remains unknown how CENP-E guides pole-proximal chromosomes specifically toward the equator. We found that congression of pole-proximal chromosomes depended on specific posttranslational detyrosination of spindle microtubules that point to the equator. In vitro reconstitution experiments demonstrated that CENP-E-dependent transport was strongly enhanced on detyrosinated microtubules. Blocking tubulin tyrosination in cells caused ubiquitous detyrosination of spindle microtubules, and CENP-E transported chromosomes away from spindle poles in random directions. Thus, CENP-E-driven chromosome congression is guided by microtubule detyrosination.

Distinct functional roles of cytoplasmic dynein defined by the intermediate chain isoforms

The motor protein, cytoplasmic dynein is responsible for the movement of a variety of cargoes toward microtubule minus ends in cells. Little is understood about how dynein is regulated to specifically transport its various cargoes. In vertebrates, the dynein motor domain (DYNC1H) is encoded by a single gene; while there are two genes for the five smaller subunits that comprise the cargo binding domain of the dynein complex. The isoforms of the intermediate chain (DYNC1I) provide a good model system with which to study the roles the different isoforms of the cargo domain subunits have in designating specific dynein functions. The intermediate chains (DYNC1I) play a key scaffold role in the dynein complex. In neurons, dynein complexes with different intermediate chain isoforms have distinct roles, including cargo binding and transport. Some of the phospho-isoforms of the intermediate chain also specify binding to specific cargo. These data support the model that cytoplasmic dynein can be specifically regulated through the different isoforms of the subunits.

Mitotic catenation is monitored and resolved by a PKCε-regulated pathway

Exit from mitosis is controlled by silencing of the spindle assembly checkpoint (SAC). It is important that preceding exit, all sister chromatid pairs are correctly bioriented, and that residual catenation is resolved, permitting complete sister chromatid separation in the ensuing anaphase. Here we determine that the metaphase response to catenation in mammalian cells operates through PKCε. The PKCε-controlled pathway regulates exit from the SAC only when mitotic cells are challenged by retained catenation and this delayed exit is characterized by BubR1-high and Mad2-low kinetochores. In addition, we show that this pathway is necessary to facilitate resolution of retained catenanes in mitosis. When delayed by catenation in mitosis, inhibition of PKCε results in premature entry into anaphase with PICH-positive strands and chromosome bridging. These findings demonstrate the importance of PKCε-mediated regulation in protection from loss of chromosome integrity in cells failing to resolve catenation in G2.

Microtubule plus-end tracking proteins and their roles in cell division

Microtubules are cellular components that are required for a variety of essential processes such as cell motility, mitosis, and intracellular transport. This is possible because of the inherent dynamic properties of microtubules. Many of these properties are tightly regulated by a number of microtubule plus-end-binding proteins or +TIPs. These proteins recognize the distal end of microtubules and are thus in the right context to control microtubule dynamics. In this review, we address how microtubule dynamics are regulated by different +TIP families, focusing on how functionally diverse +TIPs spatially and temporally regulate microtubule dynamics during animal cell division.

Spatial-temporal model for silencing of the mitotic spindle assembly checkpoint

The spindle assembly checkpoint arrests mitotic progression until each kinetochore secures a stable attachment to the spindle. Despite fluctuating noise, this checkpoint remains robust and remarkably sensitive to even a single unattached kinetochore among many attached kinetochores; moreover, the checkpoint is silenced only after the final kinetochore-spindle attachment. Experimental observations have shown that checkpoint components stream from attached kinetochores along microtubules toward spindle poles. Here, we incorporate this streaming behavior into a theoretical model that accounts for the robustness of checkpoint silencing. Poleward streams are integrated at spindle poles, but are diverted by any unattached kinetochore; consequently, accumulation of checkpoint components at spindle poles increases markedly only when every kinetochore is properly attached. This step-change robustly triggers checkpoint silencing after, and only after, the final kinetochore-spindle attachment. Our model offers a conceptual framework that highlights the role of spatiotemporal regulation in mitotic spindle checkpoint signaling and fidelity of chromosome segregation.

The Drosophila MAST kinase Drop out is required to initiate membrane compartmentalisation during cellularisation and regulates dynein-based transport

Cellularisation of the Drosophila syncytial blastoderm embryo into the polarised blastoderm epithelium provides an excellent model with which to determine how cortical plasma membrane asymmetry is generated during development. Many components of the molecular machinery driving cellularisation have been identified, but cell signalling events acting at the onset of membrane asymmetry are poorly understood. Here we show that mutations in drop out (dop) disturb the segregation of membrane cortical compartments and the clustering of E-cadherin into basal adherens junctions in early cellularisation. dop is required for normal furrow formation and controls the tight localisation of furrow canal proteins and the formation of F-actin foci at the incipient furrows. We show that dop encodes the single Drosophila homologue of microtubule-associated Ser/Thr (MAST) kinases. dop interacts genetically with components of the dynein/dynactin complex and promotes dynein-dependent transport in the embryo. Loss of dop function reduces phosphorylation of Dynein intermediate chain, suggesting that dop is involved in regulating cytoplasmic dynein activity through direct or indirect mechanisms. These data suggest that Dop impinges upon the initiation of furrow formation through developmental regulation of cytoplasmic dynein.

Function and regulation of dynein in mitotic chromosome segregation

Cytoplasmic dynein is a large minus-end-directed microtubule motor complex, involved in many different cellular processes including intracellular trafficking, organelle positioning, and microtubule organization. Furthermore, dynein plays essential roles during cell division where it is implicated in multiple processes including centrosome separation, chromosome movements, spindle organization, spindle positioning, and mitotic checkpoint silencing. How is a single motor able to fulfill this large array of functions and how are these activities temporally and spatially regulated? The answer lies in the unique composition of the dynein motor and in the interactions it makes with multiple regulatory proteins that define the time and place where dynein becomes active. Here, we will focus on the different mitotic processes that dynein is involved in, and how its regulatory proteins act to support dynein. Although dynein is highly conserved amongst eukaryotes (with the exception of plants), there is significant variability in the cellular processes that depend on dynein in different species. In this review, we concentrate on the functions of cytoplasmic dynein in mammals but will also refer to data obtained in other model organisms that have contributed to our understanding of dynein function in higher eukaryotes.

Live cell imaging reveals differential modifications to cytoplasmic dynein properties by phospho- and dephosphomimic mutations of the intermediate chain 2C S84

Cytoplasmic dynein is a multisubunit motor protein responsible for intracellular cargo transport toward microtubule minus ends. There are multiple isoforms of the dynein intermediate chain (DYNC1I, IC), which is encoded by two genes. One way to regulate cytoplasmic dynein is by IC phosphorylation. The IC-2C isoform is expressed in all cells, and the functional significance of phosphorylation on IC-2C serine 84 was investigated by using live cell imaging of fluorescent protein-tagged IC-2C wild type (WT) and phospho- and dephosphomimic mutant isoforms in axonal transport model systems. Both mutations modulated dynein functional properties. The dephosphomimic mutant IC-2C S84A had greater colocalization with mitochondria than the IC-2C WT or the phosphomimic mutant IC-2C S84D. The dephosphomimic mutant IC-2C S84A was also more likely to be motile than the phosphomimic mutant IC-2C S84D or the IC-2C WT. In contrast, the phosphomimic mutant IC-2C S84D mutant was more likely to move in the retrograde direction than was the IC-2C S84A mutant. The phosphomimic IC-2C S84D was also as likely as the IC-2C WT to colocalize with mitochondria. Both the S84D phospho- and the S84A dephosphomimic mutants were found to be capable of microtubule minus-end-directed (retrograde) movement in axons. They were also observed to be passively transported in the anterograde direction. These data suggest that the IC-2C S84 has a role in modulating dynein properties.

Insights from an erroneous kinetochore-microtubule attachment state

Faithful distribution of the genome requires that sister kinetochores, which assemble on each chromatid during cell division, interact with dynamic microtubules from opposite spindle poles in a configuration called chromosome biorientation. Biorientation produces tension that increases the affinity of kinetochores for microtubules via ill-defined mechanisms. Non-bioriented kinetochore-microtubule (kt-MT) interactions are prevalent but short-lived due to an error correction pathway that reduces the affinity of kinetochores for microtubules. Interestingly, incorrect kt-MT interactions can be stabilized by experimentally applying force to misoriented chromosomes. Here, a live-cell force assay is utilized to characterize the molecular composition of a common type of improper kt-MT attachment. Our force-related studies are also discussed in the context of current models for tension-dependent stabilization of kt-MT interactions.

Systematic dissection of dynein regulators in mitosis

Cytoplasmic dynein is a large minus end-directed motor complex with multiple functions during cell division. The dynein complex interacts with various adaptor proteins, including the dynactin complex, thought to be critical for most dynein functions. Specific activities have been linked to several subunits and adaptors, but the function of the majority of components has remained elusive. Here, we systematically address the function of each dynein-dynactin subunit and adaptor protein in mitosis. We identify the essential components that are required for all mitotic functions of dynein. Moreover, we find specific dynein recruitment factors, and adaptors, like Nde1/L1, required for activation, but largely dispensable for dynein localization. Most surprisingly, our data show that dynactin is not required for dynein-dependent spindle organization, but acts as a dynein recruitment factor. These results provide a comprehensive overview of the role of dynein subunits and adaptors in mitosis and reveal that dynein forms distinct complexes requiring specific recruiters and activators to promote orderly progression through mitosis.

Dynein and dynactin leverage their bivalent character to form a high-affinity interaction

Cytoplasmic dynein and dynactin participate in retrograde transport of organelles, checkpoint signaling and cell division. The principal subunits that mediate this interaction are the dynein intermediate chain (IC) and the dynactin p150(Glued); however, the interface and mechanism that regulates this interaction remains poorly defined. Herein, we use multiple methods to show the N-terminus of mammalian dynein IC, residues 10-44, is sufficient for binding p150(Glued). Consistent with this mapping, monoclonal antibodies that antagonize the dynein-dynactin interaction also bind to this region of the IC. Furthermore, double and triple alanine point mutations spanning residues 6 to 19 in the yeast IC homolog, Pac11, produce significant defects in spindle positioning. Using the same methods we show residues 381 to 530 of p150(Glued) form a minimal fragment that binds to the dynein IC. Sedimentation equilibrium experiments indicate that these individual fragments are predominantly monomeric, but admixtures of the IC and p150(Glued) fragments produce a 2:2 complex. This tetrameric complex is sensitive to salt, temperature and pH, suggesting that the binding is dominated by electrostatic interactions. Finally, circular dichroism (CD) experiments indicate that the N-terminus of the IC is disordered and becomes ordered upon binding p150(Glued). Taken together, the data indicate that the dynein-dynactin interaction proceeds through a disorder-to-order transition, leveraging its bivalent-bivalent character to form a high affinity, but readily reversible interaction.

Dynactin helps target Polo-like kinase 1 to kinetochores via its left-handed beta-helical p27 subunit

Dynactin is a protein complex required for the in vivo function of cytoplasmic dynein, a microtubule (MT)-based motor. Dynactin binds both dynein and MTs via its p150(Glued) subunit, but little is known about the 'pointed-end complex' that includes the protein subunits Arp11, p62 and the p27/p25 heterodimer. Here, we show that the p27/p25 heterodimer undergoes mitotic phosphorylation by cyclin-dependent kinase 1 (Cdk1) at a single site, p27 Thr186, to generate an anchoring site for polo-like kinase 1 (Plk1) at kinetochores. Removal of p27/p25 from dynactin results in reduced levels of Plk1 and its phosphorylated substrates at kinetochores in prometaphase, which correlates with aberrant kinetochore-MT interactions, improper chromosome alignment and abbreviated mitosis. To investigate the structural implications of p27 phosphorylation, we determined the structure of human p27. This revealed an unusual left-handed β-helix domain, with the phosphorylation site located within a disordered, C-terminal segment. We conclude that dynactin plays a previously undescribed regulatory role in the spindle assembly checkpoint by recruiting Plk1 to kinetochores and facilitating phosphorylation of important downstream targets.

TCTEX1D4, a novel protein phosphatase 1 interactor: connecting the phosphatase to the microtubule network

Reversible phosphorylation plays an important role as a mechanism of intracellular control in eukaryotes. PPP1, a major eukaryotic Ser/Thr-protein phosphatase, acquires its specificity by interacting with different protein regulators, also known as PPP1 interacting proteins (PIPs). In the present work we characterized a physiologically relevant PIP in testis. Using a yeast two-hybrid screen with a human testis cDNA library, we identified a novel PIP of PPP1CC2 isoform, the T-complex testis expressed protein 1 domain containing 4 (TCTEX1D4) that has recently been described as a Tctex1 dynein light chain family member. The overlay assays confirm that TCTEX1D4 interacts with the different spliced isoforms of PPP1CC. Also, the binding domain occurs in the N-terminus, where a consensus PPP1 binding motif (PPP1BM) RVSF is present. The distribution of TCTEX1D4 in testis suggests its involvement in distinct functions, such as TGFβ signaling at the blood–testis barrier and acrosome cap formation. Immunofluorescence in human ejaculated sperm shows that TCTEX1D4 is present in the flagellum and in the acrosome region of the head. Moreover, TCTEX1D4 and PPP1 co-localize in the microtubule organizing center (MTOC) and microtubules in cell cultures. Importantly, the TCTEX1D4 PPP1BM seems to be relevant for complex formation, for PPP1 retention in the MTOC and movement along microtubules.

These novel results open new avenues to possible roles of this dynein, together with PPP1. In essence TCTEX1D4/PPP1C complex appears to be involved in microtubule dynamics, sperm motility, acrosome reaction and in the regulation of the blood–testis barrier.

Epidermal growth factor stimulates extracellular-signal regulated kinase phosphorylation of a novel site on cytoplasmic Dynein intermediate chain 2

Extracellular-signal regulated kinase (ERK) signaling is required for a multitude of physiological and patho-physiological processes. However, the identities of the proteins that ERK phosphorylates to elicit these responses are incompletely known. Using an affinity purification methodology of general utility, here we identify cytoplasmic dynein intermediate chain 2 (DYNC1I-2, IC-2) as a novel substrate for ERK following epidermal growth factor receptor stimulation of fibroblasts. IC-2 is a subunit of cytoplasmic dynein, a minus-end directed motor protein necessary for transport of diverse cargos along microtubules. Emerging data support the hypothesis that post-translational modification regulates dynein but the signaling mechanisms used are currently unknown. We find that ERK phosphorylates IC-2 on a novel, highly conserved Serine residue proximal to the binding site for the p150Glued subunit of the cargo adapter dynactin. Surprisingly, neither constitutive phosphorylation nor a phosphomimetic substitution of this Serine influences binding of p150Glued to IC-2. These data suggest that ERK phosphorylation of IC-2 regulates dynein function through mechanisms other than its interaction with dynactin.

Trk activation of the ERK1/2 kinase pathway stimulates intermediate chain phosphorylation and recruits cytoplasmic dynein to signaling endosomes for retrograde axonal transport

The retrograde transport of Trk-containing endosomes from the axon to the cell body by cytoplasmic dynein is necessary for axonal and neuronal survival. We investigated the recruitment of dynein to signaling endosomes in rat embryonic neurons and PC12 cells. We identified a novel phosphoserine on the dynein intermediate chains (ICs), and we observed a time-dependent neurotrophin-stimulated increase in intermediate chain phosphorylation on this site in both cell types. Pharmacological studies, overexpression of constitutively active MAP kinase kinase, and an in vitro assay with recombinant proteins demonstrated that the intermediate chains are phosphorylated by the MAP kinase ERK1/2, extracellular signal-regulated kinase, a major downstream effector of Trk. Live cell imaging with fluorescently tagged IC mutants demonstrated that the dephosphomimic mutants had significantly reduced colocalization with Trk and Rab7, but not a mitochondrial marker. The phosphorylated intermediate chains were enriched on immunoaffinity-purified Trk-containing organelles. Inhibition of ERK reduced the amount of phospho-IC and the total amount of dynein that copurified with the signaling endosomes. In addition, inhibition of ERK1/2 reduced the motility of Rab7- and TrkB-containing endosomes and the extent of their colocalization with dynein in axons. NGF-dependent survival of sympathetic neurons was significantly reduced by the overexpression of the dephosphomimic mutant IC-1B-S80A, but not WT IC-1B, further demonstrating the functional significance of phosphorylation on this site. These results demonstrate that neurotrophin binding to Trk initiates the recruitment of cytoplasmic dynein to signaling endosomes through ERK1/2 phosphorylation of intermediate chains for their subsequent retrograde transport in axons.

New insights into the mechanism for chromosome alignment in metaphase

During mitosis, duplicated sister chromatids are properly aligned at the metaphase plate of the mitotic spindle before being segregated into two daughter cells. This requires a complex process to ensure proper interactions between chromosomes and spindle microtubules. The kinetochore, the proteinaceous complex assembled at the centromere region on each chromosome, serves as the microtubule attachment site and powers chromosome movement in mitosis. Numerous proteins/protein complexes have been implicated in the connection between kinetochores and dynamic microtubules. Recent studies have advanced our understanding on the nature of the interface between kinetochores and microtubule plus ends in promoting and maintaining their stable attachment. These efforts have demonstrated the importance of this process to ensure accurate chromosome segregation, an issue which has great significance for understanding and controlling abnormal chromosome segregation (aneuploidy) in human genetic diseases and in cancer progression.

She1-mediated inhibition of dynein motility along astral microtubules promotes polarized spindle movements

BACKGROUND

Cytoplasmic dynein motility along microtubules is critical for diverse cellular processes ranging from vesicular transport to nuclear envelope breakdown to mitotic spindle alignment. In yeast, we have proposed a regulated-offloading model to explain how dynein motility drives microtubule sliding along the cortex, powering transport of the nucleus into the mother-bud neck [1, 2]: the dynein regulator She1 limits dynein offloading by gating the recruitment of dynactin to the astral microtubule plus end, a prerequisite for offloading to the cortex. However, whether She1 subsequently affects cortically anchored dynein activity during microtubule sliding is unclear.

RESULTS

Using single-molecule motility assays, we show that She1 strongly inhibits dynein movement along microtubules, acting directly on the motor domain in a manner independent of dynactin. She1 has no effect on the motility of either Kip2, a kinesin that utilizes the same microtubule track as dynein, or human kinesin-1, demonstrating the specificity of She1 for the dynein motor. At single-molecule resolution, She1 binds tightly to and exhibits diffusional behavior along microtubules. Diffusive She1 collides with and pauses motile dynein motors, prolonging their attachment to the microtubule. Furthermore, Aurora B/Ipl1 directly phosphorylates She1, and this modification appears to enhance the diffusive behavior of She1 along microtubules and its potency against dynein. In cells, She1 dampens productive microtubule-cortex interactions specifically in the mother compartment, polarizing spindle movements toward the bud cell.

CONCLUSIONS

Our data reveal how inhibitory microtubule-associated proteins selectively regulate motor activity to achieve unidirectional nuclear transport and demonstrate a direct link between cell-cycle machinery and dynein pathway activity.

Golgin160 recruits the dynein motor to position the Golgi apparatus

Membrane motility is a fundamental characteristic of all eukaryotic cells. One of the best-known examples is that of the mammalian Golgi apparatus, where constant inward movement of Golgi membranes results in its characteristic position near the centrosome. While it is clear that the minus-end-directed motor dynein is required for this process, the mechanism and regulation of dynein recruitment to Golgi membranes remains unknown. Here, we show that the Golgi protein golgin160 recruits dynein to Golgi membranes. This recruitment confers centripetal motility to membranes and is regulated by the GTPase Arf1. Further, during cell division, motor association with membranes is regulated by the dissociation of the receptor-motor complex from membranes. These results identify a cell-cycle-regulated membrane receptor for a molecular motor and suggest a mechanistic basis for achieving the dramatic changes in organelle positioning seen during cell division.