Structural basis for cargo binding and autoinhibition of Bicaudal-D1 by a parallel coiled-coil with homotypic registry

Molecular mechanism for recognition of the cargo adapter Rab6GTP by the dynein adapter BicD2

Rab6 is a key modulator of protein secretion. The dynein adapter Bicaudal D2 (BicD2) recruits the motors cytoplasmic dynein and kinesin-1 to Rab6GTP-positive vesicles for transport; however, it is unknown how BicD2 recognizes Rab6. Here, we establish a structural model for recognition of Rab6GTP by BicD2, using structure prediction and mutagenesis. The binding site of BicD2 spans two regions of Rab6 that undergo structural changes upon the transition from the GDP- to GTP-bound state, and several hydrophobic interface residues are rearranged, explaining the increased affinity of the active GTP-bound state. Mutations of Rab6GTP that abolish binding to BicD2 also result in reduced co-migration of Rab6GTP/BicD2 in cells, validating our model. These mutations also severely diminished the motility of Rab6-positive vesicles in cells, highlighting the importance of the Rab6GTP/BicD2 interaction for overall motility of the multi-motor complex that contains both kinesin-1 and dynein. Our results provide insights into trafficking of secretory and Golgi-derived vesicles and will help devise therapies for diseases caused by BicD2 mutations, which selectively affect the affinity to Rab6 and other cargoes.

Molecular mechanism of dynein-dynactin complex assembly by LIS1

Cytoplasmic dynein is a microtubule motor vital for cellular organization and division. It functions as a ~4-megadalton complex containing its cofactor dynactin and a cargo-specific coiled-coil adaptor. However, how dynein and dynactin recognize diverse adaptors, how they interact with each other during complex formation, and the role of critical regulators such as lissencephaly-1 (LIS1) protein (LIS1) remain unclear. In this study, we determined the cryo-electron microscopy structure of dynein-dynactin on microtubules with LIS1 and the lysosomal adaptor JIP3. This structure reveals the molecular basis of interactions occurring during dynein activation. We show how JIP3 activates dynein despite its atypical architecture. Unexpectedly, LIS1 binds dynactin's p150 subunit, tethering it along the length of dynein. Our data suggest that LIS1 and p150 constrain dynein-dynactin to ensure efficient complex formation.

Novel Insights into the Role of Chromatin Remodeler MORC2 in Cancer

A newly discovered chromatin remodeler, MORC2, is a Microrchidia (MORC) family member. MORC2 acts as a chromatin remodeler by binding to the DNA and changing chromatin conformation using its ATPase domain. MORC2 is highly expressed in a variety of human cancers. It controls diverse signaling pathways essential for cancer development through its target genes and interacting partners. MORC2 promotes cancer cells' growth, invasion, and migration by regulating the expression of genes involved in these processes. MORC2 is localized primarily in the nucleus and is also found in the cytoplasm. In the cytoplasm, MORC2 interacts with adenosine triphosphate (ATP)-citrate lyase (ACLY) to promote lipogenesis and cholesterogenesis in cancer. In the nucleus, MORC2 interacts with the transcription factor c-Myc to control the transcription of genes involved in glucose metabolism to drive cancer cell migration and invasion. Furthermore, MORC2 recruits on to the promoters of tumor suppressor genes to repress their transcription and expression to promote oncogenesis. In addition to its crucial function in oncogenesis, it plays a vital role in DNA repair. Overall, this review concisely summarizes the current knowledge about MORC2-regulated molecular pathways involved in cancer.

A Structural Model for the Core Nup358-BicD2 Interface

Dynein motors facilitate the majority of minus-end-directed transport events on microtubules. The dynein adaptor Bicaudal D2 (BicD2) recruits the dynein machinery to several cellular cargo for transport, including Nup358, which facilitates a nuclear positioning pathway that is essential for the differentiation of distinct brain progenitor cells. Previously, we showed that Nup358 forms a "cargo recognition α-helix" upon binding to BicD2; however, the specifics of the BicD2-Nup358 interface are still not well understood. Here, we used AlphaFold2, complemented by two additional docking programs (HADDOCK and ClusPro) as well as mutagenesis, to show that the Nup358 cargo-recognition α-helix binds to BicD2 between residues 747 and 774 in an anti-parallel manner, forming a helical bundle. We identified two intermolecular salt bridges that are important to stabilize the interface. In addition, we uncovered a secondary interface mediated by an intrinsically disordered region of Nup358 that is directly N-terminal to the cargo-recognition α-helix and binds to BicD2 between residues 774 and 800. This is the same BicD2 domain that binds to the competing cargo adapter Rab6, which is important for the transport of Golgi-derived and secretory vesicles. Our results establish a structural basis for cargo recognition and selection by the dynein adapter BicD2, which facilitates transport pathways that are important for brain development.

BICD2 phosphorylation regulates dynein function and centrosome separation in G2 and M

The activity of dynein is regulated by a number of adaptors that mediate its interaction with dynactin, effectively activating the motor complex while also connecting it to different cargos. The regulation of adaptors is consequently central to dynein physiology but remains largely unexplored. We now describe that one of the best-known dynein adaptors, BICD2, is effectively activated through phosphorylation. In G2, phosphorylation of BICD2 by CDK1 promotes its interaction with PLK1. In turn, PLK1 phosphorylation of a single residue in the N-terminus of BICD2 results in a structural change that facilitates the interaction with dynein and dynactin, allowing the formation of active motor complexes. Moreover, modified BICD2 preferentially interacts with the nucleoporin RanBP2 once RanBP2 has been phosphorylated by CDK1. BICD2 phosphorylation is central for dynein recruitment to the nuclear envelope, centrosome tethering to the nucleus and centrosome separation in the G2 and M phases of the cell cycle. This work reveals adaptor activation through phosphorylation as crucial for the spatiotemporal regulation of dynein activity.

Role of Nesprin-2 and RanBP2 in BICD2-associated brain developmental disorders

Bicaudal D2 (BICD2) is responsible for recruiting cytoplasmic dynein to diverse forms of subcellular cargo for their intracellular transport. Mutations in the human BICD2 gene have been found to cause an autosomal dominant form of spinal muscular atrophy (SMA-LED2), and brain developmental defects. Whether and how the latter mutations are related to roles we and others have identified for BICD2 in brain development remains little understood. BICD2 interacts with the nucleoporin RanBP2 to recruit dynein to the nuclear envelope (NE) of Radial Glial Progenitor cells (RGPs) to mediate their well-known but mysterious cell-cycle-regulated interkinetic nuclear migration (INM) behavior, and their subsequent differentiation to form cortical neurons. We more recently found that BICD2 also mediates NE dynein recruitment in migrating post-mitotic neurons, though via a different interactor, Nesprin-2. Here, we report that Nesprin-2 and RanBP2 compete for BICD2-binding in vitro. To test the physiological implications of this behavior, we examined the effects of known BICD2 mutations using in vitro biochemical and in vivo electroporation-mediated brain developmental assays. We find a clear relationship between the ability of BICD2 to bind RanBP2 vs. Nesprin-2 in controlling of nuclear migration and neuronal migration behavior. We propose that mutually exclusive RanBP2-BICD2 vs. Nesprin-2-BICD2 interactions at the NE play successive, critical roles in INM behavior in RGPs and in post-mitotic neuronal migration and errors in these processes contribute to specific human brain malformations.

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.

Multivalency, autoinhibition, and protein disorder in the regulation of interactions of dynein intermediate chain with dynactin and the nuclear distribution protein

As the only major retrograde transporter along microtubules, cytoplasmic dynein plays crucial roles in the intracellular transport of organelles and other cargoes. Central to the function of this motor protein complex is dynein intermediate chain (IC), which binds the three dimeric dynein light chains at multivalent sites, and dynactin p150Glued and nuclear distribution protein (NudE) at overlapping sites of its intrinsically disordered N-terminal domain. The disorder in IC has hindered cryo-electron microscopy and X-ray crystallography studies of its structure and interactions. Here we use a suite of biophysical methods to reveal how multivalent binding of the three light chains regulates IC interactions with p150Glued and NudE. Using IC from Chaetomium thermophilum, a tractable species to interrogate IC interactions, we identify a significant reduction in binding affinity of IC to p150Glued and a loss of binding to NudE for constructs containing the entire N-terminal domain as well as for full-length constructs when compared to the tight binding observed with short IC constructs. We attribute this difference to autoinhibition caused by long-range intramolecular interactions between the N-terminal single α-helix of IC, the common site for p150Glued, and NudE binding, and residues closer to the end of the N-terminal domain. Reconstitution of IC subcomplexes demonstrates that autoinhibition is differentially regulated by light chains binding, underscoring their importance both in assembly and organization of IC, and in selection between multiple binding partners at the same site.

Conformational transitions of the Spindly adaptor underlie its interaction with Dynein and Dynactin

Cytoplasmic Dynein 1, or Dynein, is a microtubule minus end-directed motor. Dynein motility requires Dynactin and a family of activating adaptors that stabilize the Dynein-Dynactin complex and promote regulated interactions with cargo in space and time. How activating adaptors limit Dynein activation to specialized subcellular locales is unclear. Here, we reveal that Spindly, a mitotic Dynein adaptor at the kinetochore corona, exists natively in a closed conformation that occludes binding of Dynein-Dynactin to its CC1 box and Spindly motif. A structure-based analysis identified various mutations promoting an open conformation of Spindly that binds Dynein-Dynactin. A region of Spindly downstream from the Spindly motif and not required for cargo binding faces the CC1 box and stabilizes the intramolecular closed conformation. This region is also required for robust kinetochore localization of Spindly, suggesting that kinetochores promote Spindly activation to recruit Dynein. Thus, our work illustrates how specific Dynein activation at a defined cellular locale may require multiple factors.

Antiviral function and viral antagonism of the rapidly evolving dynein activating adaptor NINL

Viruses interact with the intracellular transport machinery to promote viral replication. Such host-virus interactions can drive host gene adaptation, leaving signatures of pathogen-driven evolution in host genomes. Here, we leverage these genetic signatures to identify the dynein activating adaptor, ninein-like (NINL), as a critical component in the antiviral innate immune response and as a target of viral antagonism. Unique among genes encoding components of active dynein complexes, NINL has evolved under recurrent positive (diversifying) selection, particularly in its carboxy-terminal cargo-binding region. Consistent with a role for NINL in host immunity, we demonstrate that NINL knockout cells exhibit an impaired response to interferon, resulting in increased permissiveness to viral replication. Moreover, we show that proteases encoded by diverse picornaviruses and coronaviruses cleave and disrupt NINL function in a host- and virus-specific manner. Our work reveals the importance of NINL in the antiviral response and the utility of using signatures of host-virus genetic conflicts to uncover new components of antiviral immunity and targets of viral antagonism.

In vitro characterization of the full-length human dynein-1 cargo adaptor BicD2

Cargo adaptors are crucial in coupling motor proteins with their respective cargos and regulatory proteins. BicD2 is a prominent example within the cargo adaptor family. BicD2 is able to recruit the microtubule motor dynein to RNA, viral particles, and nuclei. The BicD2-mediated interaction between the nucleus and dynein is implicated in mitosis, interkinetic nuclear migration (INM) in radial glial progenitor cells, and neuron precursor migration during embryonic neocortex development. In vitro studies involving full-length cargo adaptors are difficult to perform due to the hydrophobic character, low-expression levels, and intrinsic flexibility of cargo adaptors. Here, we report the recombinant production of full-length human BicD2 and confirm its biochemical activity by interaction studies with RanBP2. We also describe pH-dependent conformational changes of BicD2 using cryoelectron microscopy (cryo-EM), template-free structure predictions, and biophysical tools. Our results will help define the biochemical parameters for the in vitro reconstitution of higher-order BicD2 protein complexes.

Coil-to-α-helix transition at the Nup358-BicD2 interface activates BicD2 for dynein recruitment

Nup358, a protein of the nuclear pore complex, facilitates a nuclear positioning pathway that is essential for many biological processes, including neuromuscular and brain development. Nup358 interacts with the dynein adaptor Bicaudal D2 (BicD2), which in turn recruits the dynein machinery to position the nucleus. However, the molecular mechanisms of the Nup358/BicD2 interaction and the activation of transport remain poorly understood. Here for the first time, we show that a minimal Nup358 domain activates dynein/dynactin/BicD2 for processive motility on microtubules. Using nuclear magnetic resonance titration and chemical exchange saturation transfer, mutagenesis, and circular dichroism spectroscopy, a Nup358 α-helix encompassing residues 2162–2184 was identified, which transitioned from a random coil to an α-helical conformation upon BicD2 binding and formed the core of the Nup358-BicD2 interface. Mutations in this region of Nup358 decreased the Nup358/BicD2 interaction, resulting in decreased dynein recruitment and impaired motility. BicD2 thus recognizes Nup358 through a ‘cargo recognition α-helix,’ a structural feature that may stabilize BicD2 in its activated state and promote processive dynein motility.

MORC protein family-related signature within human disease and cancer

The microrchidia (MORC) family of proteins is a highly conserved nuclear protein superfamily, whose members contain common domain structures (GHKL-ATPase, CW-type zinc finger and coiled-coil domain) yet exhibit diverse biological functions. Despite the advancing research in previous decades, much of which focuses on their role as epigenetic regulators and in chromatin remodeling, relatively little is known about the role of MORCs in tumorigenesis and pathogenesis. MORCs were first identified as epigenetic regulators and chromatin remodelers in germ cell development. Currently, MORCs are regarded as disease genes that are involved in various human disorders and oncogenes in cancer progression and are expected to be the important biomarkers for diagnosis and treatment. A new paradigm of expanded MORC family function has raised questions regarding the regulation of MORCs and their biological role at the subcellular level. Here, we systematically review the progress of researching MORC members with respect to their domain architectures, diverse biological functions, and distribution characteristics and discuss the emerging roles of the aberrant expression or mutation of MORC family members in human disorders and cancer development. Furthermore, the illustration of related mechanisms of the MORC family has made MORCs promising targets for developing diagnostic tools and therapeutic treatments for human diseases, including cancers.

Cryo-EM reveals the complex architecture of dynactin's shoulder region and pointed end

Dynactin is a 1.1 MDa complex that activates the molecular motor dynein for ultra-processive transport along microtubules. In order to do this, it forms a tripartite complex with dynein and a coiled-coil adaptor. Dynactin consists of an actin-related filament whose length is defined by its flexible shoulder domain. Despite previous cryo-EM structures, the molecular architecture of the shoulder and pointed end of the filament is still poorly understood due to the lack of high-resolution information in these regions. Here we combine multiple cryo-EM datasets and define precise masking strategies for particle signal subtraction and 3D classification. This overcomes domain flexibility and results in high-resolution maps into which we can build the shoulder and pointed end. The unique architecture of the shoulder securely houses the p150 subunit and positions the four identical p50 subunits in different conformations to bind dynactin's filament. The pointed end map allows us to build the first structure of p62 and reveals the molecular basis for cargo adaptor binding to different sites at the pointed end.

Coiled-coil registry shifts in the F684I mutant of Bicaudal D result in cargo-independent activation of dynein motility

The dynein adaptor Drosophila Bicaudal D (BicD) is auto-inhibited and activates dynein motility only after cargo is bound, but the underlying mechanism is elusive. In contrast, we show that the full-length BicD/F684I mutant activates dynein processivity even in the absence of cargo. Our X-ray structure of the C-terminal domain of the BicD/F684I mutant reveals a coiled-coil registry shift; in the N-terminal region, the two helices of the homodimer are aligned, whereas they are vertically shifted in the wild-type. One chain is partially disordered and this structural flexibility is confirmed by computations, which reveal that the mutant transitions back and forth between the two registries. We propose that a coiled-coil registry shift upon cargo-binding activates BicD for dynein recruitment. Moreover, the human homolog BicD2/F743I exhibits diminished binding of cargo adaptor Nup358, implying that a coiled-coil registry shift may be a mechanism to modulate cargo selection for BicD2-dependent transport pathways.

The FTS-Hook-FHIP (FHF) complex interacts with AP-4 to mediate perinuclear distribution of AP-4 and its cargo ATG9A

The heterotetrameric adaptor protein complex 4 (AP-4) is a component of a protein coat associated with the trans-Golgi network (TGN). Mutations in AP-4 subunits cause a complicated form of autosomal-recessive hereditary spastic paraplegia termed AP-4-deficiency syndrome. Recent studies showed that AP-4 mediates export of the transmembrane autophagy protein ATG9A from the TGN to preautophagosomal structures. To identify additional proteins that cooperate with AP-4 in ATG9A trafficking, we performed affinity purification-mass spectrometry followed by validation of the hits by biochemical and functional analyses. This approach resulted in the identification of the fused toes homolog-Hook-FHIP (FHF) complex as a novel AP-4 accessory factor. We found that the AP-4-FHF interaction is mediated by direct binding of the AP-4 μ4 subunit to coiled-coil domains in the Hook1 and Hook2 subunits of FHF. Knockdown of FHF subunits resulted in dispersal of AP-4 and ATG9A from the perinuclear region of the cell, consistent with the previously demonstrated role of the FHF complex in coupling organelles to the microtubule (MT) retrograde motor dynein-dynactin. These findings thus uncover an additional mechanism for the distribution of ATG9A within cells and provide further evidence for a role of protein coats in coupling transport vesicles to MT motors.

PTPN23 binds the dynein adaptor BICD1 and is required for endocytic sorting of neurotrophin receptors

Signalling by target-derived neurotrophins is essential for the correct development of the nervous system and its maintenance throughout life. Several aspects concerning the lifecycle of neurotrophins and their receptors have been characterised over the years, including the formation, endocytosis and trafficking of signalling-competent ligand-receptor complexes. However, the molecular mechanisms directing the sorting of activated neurotrophin receptors are still elusive. Previously, our laboratory identified Bicaudal-D1 (BICD1), a dynein motor adaptor, as a key factor for lysosomal degradation of brain-derived neurotrophic factor (BDNF)-activated TrkB (also known as NTRK2) and p75NTR (also known as NGFR) in motor neurons. Here, using a proteomics approach, we identified protein tyrosine phosphatase, non-receptor type 23 (PTPN23), a member of the endosomal sorting complexes required for transport (ESCRT) machinery, in the BICD1 interactome. Molecular mapping revealed that PTPN23 is not a canonical BICD1 cargo; instead, PTPN23 binds the N-terminus of BICD1, which is also essential for the recruitment of cytoplasmic dynein. In line with the BICD1-knockdown phenotype, loss of PTPN23 leads to increased accumulation of BDNF-activated p75NTR and TrkB in swollen vacuole-like compartments, suggesting that neuronal PTPN23 is a novel regulator of the endocytic sorting of neurotrophin receptors.

Role of Microtubule-Associated Factors in HIF1α Nuclear Translocation

Adaptation to hypoxia is essential for regulating the survival and functions of hypoxic cells; it is mainly mediated by the hypoxia-inducible factor 1 (HIF1). The alpha subunit of HIF1 (HIF1α) is a well-known regulatory component of HIF1, which is tightly controlled by various types of HIF1α-regulating processes. Previous research has shown that microtubule-regulated HIF1α nuclear translocation is a key factor for HIF1 activation under hypoxia. In this review, we summarize experimental reports on the role of microtubule-associated factors, such as microtubule, dynein, and dynein adaptor protein, in nuclear translocation of HIF1α. Based upon scientific evidence, we propose a bicaudal D homolog (BICD) as a novel HIF1α translocation regulating factor. A deeper understanding of the mechanism of the action of regulatory factors in controlling HIF1α nuclear translocation will provide novel insights into cell biology under hypoxia.

Adapter Proteins for Opposing Motors Interact Simultaneously with Nuclear Pore Protein Nup358

Nup358 is a protein subunit of the nuclear pore complex that recruits the opposing microtubule motors kinesin-1 and dynein [via the dynein adaptor Bicaudal D2 (BicD2)] to the nuclear envelope. This pathway is important for positioning of the nucleus during the early steps of mitotic spindle assembly and also essential for an important process in brain development. It is unknown whether dynein and kinesin-1 interact with Nup358 simultaneously or whether they compete. Here, we have reconstituted and characterized a minimal complex of kinesin-1 light chain 2 (KLC2) and Nup358. The proteins interact through a W-acidic motif in Nup358, which is highly conserved among vertebrates but absent in insects. While Nup358 and KLC2 form predominantly monomers, their interaction results in the formation of 2:2 complexes, and the W-acidic motif is required for the oligomerization. In active motor complexes, BicD2 and KLC2 each form dimers. Notably, we show that the dynein adaptor BicD2 and KLC2 interact simultaneously with Nup358, resulting in the formation of 2:2:2 complexes. Mutation of the W-acidic motif results in the formation of 1:1:1 complexes. On the basis of our data, we propose that Nup358 recruits simultaneously one kinesin-1 motor and one dynein motor via BicD2 to the nucleus. We hypothesize that the binding sites are close enough to promote direct interactions between these motor recognition domains, which may be important for the regulation of the motility of these opposing motors. Our data provide important insights into a nuclear positioning pathway that is crucial for brain development and faithful chromosome segregation.

Role of Coiled-Coil Registry Shifts in the Activation of Human Bicaudal D2 for Dynein Recruitment upon Cargo Binding

Dynein adaptors such as Bicaudal D2 (BicD2) recognize cargo for dynein-dependent transport, and cargo-bound adaptors are required to activate dynein for processive transport, but the mechanism of action is unknown. Here we report the X-ray structure of the cargo-binding domain of human BicD2 and investigate the structural dynamics of the coiled-coil. Our molecular dynamics simulations support the fact that BicD2 can switch from a homotypic coiled-coil registry, in which both helices of the homodimer are aligned, to an asymmetric registry, where a portion of one helix is vertically shifted, as both states are similarly stable and defined by distinct conformations of F743. The F743I variant increases dynein recruitment in the Drosophila homologue, whereas the human R747C variant causes spinal muscular atrophy. We report spontaneous registry shifts for both variants, which may be the cause for BicD2 hyperactivation and disease. We propose that a registry shift upon cargo binding may activate autoinhibited BicD2 for dynein recruitment.

Dynein activators and adaptors at a glance

Cytoplasmic dynein-1 (hereafter dynein) is an essential cellular motor that drives the movement of diverse cargos along the microtubule cytoskeleton, including organelles, vesicles and RNAs. A long-standing question is how a single form of dynein can be adapted to a wide range of cellular functions in both interphase and mitosis. Recent progress has provided new insights - dynein interacts with a group of activating adaptors that provide cargo-specific and/or function-specific regulation of the motor complex. Activating adaptors such as BICD2 and Hook1 enhance the stability of the complex that dynein forms with its required activator dynactin, leading to highly processive motility toward the microtubule minus end. Furthermore, activating adaptors mediate specific interactions of the motor complex with cargos such as Rab6-positive vesicles or ribonucleoprotein particles for BICD2, and signaling endosomes for Hook1. In this Cell Science at a Glance article and accompanying poster, we highlight the conserved structural features found in dynein activators, the effects of these activators on biophysical parameters, such as motor velocity and stall force, and the specific intracellular functions they mediate.

Dynein activator Hook1 is required for trafficking of BDNF-signaling endosomes in neurons

Axonal transport is required for neuronal development and survival. Transport from the axon to the soma is driven by the molecular motor cytoplasmic dynein, yet it remains unclear how dynein is spatially and temporally regulated. We find that the dynein effector Hook1 mediates transport of TrkB-BDNF-signaling endosomes in primary hippocampal neurons. Hook1 comigrates with a subpopulation of Rab5 endosomes positive for TrkB and BDNF, which exhibit processive retrograde motility with faster velocities than the overall Rab5 population. Knockdown of Hook1 significantly reduced the motility of BDNF-signaling endosomes without affecting the motility of other organelles. In microfluidic chambers, Hook1 depletion resulted in a significant decrease in the flux and processivity of BDNF-Qdots along the mid-axon, an effect specific for Hook1 but not Hook3. Hook1 depletion inhibited BDNF trafficking to the soma and blocked downstream BDNF- and TrkB-dependent signaling to the nucleus. Together, these studies support a model in which differential association with cargo-specific effectors efficiently regulates dynein in neurons.

BICD1 mediates HIF1α nuclear translocation in mesenchymal stem cells during hypoxia adaptation

Hypoxia inducible factor 1α (HIF1α) is a master regulator leading to metabolic adaptation, an essential physiological process to maintain the survival of stem cells under hypoxia. However, it is poorly understood how HIF1α translocates into the nucleus in stem cells under hypoxia. Here, we investigated the role of a motor adaptor protein Bicaudal D homolog 1 (BICD1) in dynein-mediated HIF1α nuclear translocation and the effect of BICD1 regulation on hypoxia adaptation and its therapeutic potential on human umbilical cord blood-derived mesenchymal stem cells (UCB-MSCs). In our results, silencing of BICD1 but not BICD2 abolished HIF1α nuclear translocation and its activity. BICD1 overexpression further enhanced hypoxia-induced HIF1α nuclear translocation. Hypoxia stimulated direct bindings of HIF1α to BICD1 and the intermediate chain of dynein (Dynein IC), which was abolished by BICD1 silencing. Akt inhibition reduced the binding of BICD1 to HIF1α and nuclear translocation of HIF1α. Conversely, Akt activation or GSK3β silencing further enhanced the hypoxia-induced HIF1α nuclear translocation. Furthermore, BICD1 silencing abolished hypoxia-induced glycolytic reprogramming and increased mitochondrial ROS accumulation and apoptosis in UCB-MSCs under hypoxia. In the mouse skin wound healing model, the transplanted cell survival and skin wound healing capacities of hypoxia-pretreated UCB-MSCs were reduced by BICD1 silencing and further increased by GSK3β silencing. In conclusion, we demonstrated that BICD1-induced HIF1α nuclear translocation is critical for hypoxia adaptation, which determines the regenerative potential of UCB-MSCs.

A Quantitative Model for BicD2/Cargo Interactions

Dynein adaptor proteins such as Bicaudal D2 (BicD2) are integral components of the dynein transport machinery, as they recognize cargoes for cell cycle-specific transport and link them to the motor complex. Human BicD2 switches from selecting secretory and Golgi-derived vesicles for transport in G1 and S phase (by recognizing Rab6^GTP), to selecting the nucleus for transport in G2 phase (by recognizing nuclear pore protein Nup358), but the molecular mechanisms governing this switch are elusive. Here, we have developed a quantitative model for BicD2/cargo interactions that integrates affinities, oligomeric states, and cellular concentrations of the reactants. BicD2 and cargo form predominantly 2:2 complexes. Furthermore, the affinity of BicD2 toward its cargo Nup358 is higher than that toward Rab6^GTP. Based on our calculations, an estimated 1000 BicD2 molecules per cell would be recruited to the nucleus through Nup358 in the absence of regulation. Notably, RanGTP is a negative regulator of the Nup358/BicD2 interaction that weakens the affinity by a factor of 10 and may play a role in averting dynein recruitment to the nucleus outside of the G2 phase. However, our quantitative model predicts that an additional negative regulator remains to be identified. In the absence of negative regulation, the affinity of Nup358 would likely be sufficient to recruit BicD2 to the nucleus in G2 phase. Our quantitative model makes testable predictions of how cellular transport events are orchestrated. These transport processes are important for brain development, cell cycle control, signaling, and neurotransmission at synapses.

RNA-directed activation of cytoplasmic dynein-1 in reconstituted transport RNPs

Polarised mRNA transport is a prevalent mechanism for spatial control of protein synthesis. However, the composition of transported ribonucleoprotein particles (RNPs) and the regulation of their movement are poorly understood. We have reconstituted microtubule minus end-directed transport of mRNAs using purified components. A Bicaudal-D (BicD) adaptor protein and the RNA-binding protein Egalitarian (Egl) are sufficient for long-distance mRNA transport by the dynein motor and its accessory complex dynactin, thus defining a minimal transport-competent RNP. Unexpectedly, the RNA is required for robust activation of dynein motility. We show that a cis-acting RNA localisation signal promotes the interaction of Egl with BicD, which licenses the latter protein to recruit dynein and dynactin. Our data support a model for BicD activation based on RNA-induced occupancy of two Egl-binding sites on the BicD dimer. Scaffolding of adaptor protein assemblies by cargoes is an attractive mechanism for regulating intracellular transport.

In our cells, tiny molecular motors transport the components necessary for life’s biological processes from one location to another. They do so by loading their cargo, and burning up chemical fuel to carry it along pathways made of filaments. For example, one such motor, called dynein, can move molecules of messenger RNA (mRNA) to specific locations within the cell. There, the mRNA will be used as a template to create proteins, which will operate at exactly the right place.

Transporting mRNA in this way is critical in processes such as embryonic development and the formation of memories; yet, this mechanism is still poorly understood. Previous work suggested that the mRNA is simply a passenger of the dynein motor, but McClintock et al. asked if this is really the case. Instead, could mRNA regulate its own sorting by controlling the activity of dynein?

Studying mRNA trafficking within the complex molecular environment of a cell is challenging, so mRNA transporting machinery was recreated in the laboratory. Only the proteins necessary to build a working system were included in the experiments. In addition to the filaments, the components included dynein and a complex of proteins known as dynactin, which allows the motor to move together with a protein called BICD2. A protein named Egalitarian was used to link the mRNA to BICD2.

By filming fluorescently labelled proteins and mRNAs, McClintock et al. discovered that mRNA strongly promotes the movement of the dynein motor. A structured section in the mRNA acts as a docking area for two copies of Egalitarian. This activates BICD2, which then binds to dynein and dynactin, thereby completing the transport machinery. According to these results, the mRNA directs the assembly of the system that will carry it within the cell.

Viruses such as HIV and herpesvirus hijack dynein motors to have their genetic information moved around a cell in order to propagate infection. Understanding precisely how mRNA is transported may help to develop new strategies to fight these viruses.

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

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

The human cytoplasmic dynein interactome reveals novel activators of motility

In human cells, cytoplasmic dynein-1 is essential for long-distance transport of many cargos, including organelles, RNAs, proteins, and viruses, towards microtubule minus ends. To understand how a single motor achieves cargo specificity, we identified the human dynein interactome by attaching a promiscuous biotin ligase (‘BioID’) to seven components of the dynein machinery, including a subunit of the essential cofactor dynactin. This method reported spatial information about the large cytosolic dynein/dynactin complex in living cells. To achieve maximal motile activity and to bind its cargos, human dynein/dynactin requires ‘activators’, of which only five have been described. We developed methods to identify new activators in our BioID data, and discovered that ninein and ninein-like are a new family of dynein activators. Analysis of the protein interactomes for six activators, including ninein and ninein-like, suggests that each dynein activator has multiple cargos.

DOI:http://dx.doi.org/10.7554/eLife.28257.001

Probing the Energetics of Dynactin Filament Assembly and the Binding of Cargo Adaptor Proteins Using Molecular Dynamics Simulation and Electrostatics-Based Structural Modeling

Dynactin, a large multiprotein complex, binds with the cytoplasmic dynein-1 motor and various adaptor proteins to allow recruitment and transportation of cellular cargoes toward the minus end of microtubules. The structure of the dynactin complex is built around an actin-like minifilament with a defined length, which has been visualized in a high-resolution structure of the dynactin filament determined by cryo-electron microscopy (cryo-EM). To understand the energetic basis of dynactin filament assembly, we used molecular dynamics simulation to probe the intersubunit interactions among the actin-like proteins, various capping proteins, and four extended regions of the dynactin shoulder. Our simulations revealed stronger intersubunit interactions at the barbed and pointed ends of the filament and involving the extended regions (compared with the interactions within the filament), which may energetically drive filament termination by the capping proteins and recruitment of the actin-like proteins by the extended regions, two key features of the dynactin filament assembly process. Next, we modeled the unknown binding configuration among dynactin, dynein tails, and a number of coiled-coil adaptor proteins (including several Bicaudal-D and related proteins and three HOOK proteins), and predicted a key set of charged residues involved in their electrostatic interactions. Our modeling is consistent with previous findings of conserved regions, functional sites, and disease mutations in the adaptor proteins and will provide a structural framework for future functional and mutational studies of these adaptor proteins. In sum, this study yielded rich structural and energetic information about dynactin and associated adaptor proteins that cannot be directly obtained from the cryo-EM structures with limited resolutions.

Hook Adaptors Induce Unidirectional Processive Motility by Enhancing the Dynein-Dynactin Interaction

Cytoplasmic dynein drives the majority of minus end-directed vesicular and organelle motility in the cell. However, it remains unclear how dynein is spatially and temporally regulated given the variety of cargo that must be properly localized to maintain cellular function. Recent work has suggested that adaptor proteins provide a mechanism for cargo-specific regulation of motors. Of particular interest, studies in fungal systems have implicated Hook proteins in the regulation of microtubule motors. Here we investigate the role of mammalian Hook proteins, Hook1 and Hook3, as potential motor adaptors. We used optogenetic approaches to specifically recruit Hook proteins to organelles and observed rapid transport of peroxisomes to the perinuclear region of the cell. This rapid and efficient translocation of peroxisomes to microtubule minus ends indicates that mammalian Hook proteins activate dynein rather than kinesin motors. Biochemical studies indicate that Hook proteins interact with both dynein and dynactin, stabilizing the formation of a supramolecular complex. Complex formation requires the N-terminal domain of Hook proteins, which resembles the calponin-homology domain of end-binding (EB) proteins but cannot bind directly to microtubules. Single-molecule motility assays using total internal reflection fluorescence microscopy indicate that both Hook1 and Hook3 effectively activate cytoplasmic dynein, inducing longer run lengths and higher velocities than the previously characterized dynein activator bicaudal D2 (BICD2). Together, these results suggest that dynein adaptors can differentially regulate dynein to allow for organelle-specific tuning of the motor for precise intracellular trafficking.

Bicaudal D Family of Motor Adaptors: Linking Dynein Motility to Cargo Binding

Transport of different intracellular cargoes along cytoskeleton filaments is essential for the morphogenesis and function of a broad variety of eukaryotic cells. Intracellular transport is mediated by cytoskeletal motors including myosin, kinesin, and dynein, which are typically linked to various cargoes by adaptor proteins. Recent studies suggest that adaptor proteins can also act as essential transport cofactors, which control motor activity and coordination. Characterization of the evolutionary conserved Bicaudal D (BICD) family of dynein adaptor proteins has provided important insights into the fundamental mechanisms governing cargo trafficking. This review highlights the advances in the current understanding of how BICD adaptors regulate microtubule-based transport and how they contribute to developmental processes and human disease.