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

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.

A KIF1C-CNBP motor-adaptor complex for trafficking mRNAs to cell protrusions

mRNA localization to subcellular compartments is a widely used mechanism that functionally contributes to numerous processes. mRNA targeting can be achieved upon recognition of RNA cargo by molecular motors. However, our molecular understanding of how this is accomplished is limited, especially in higher organisms. We focus on a pathway that targets mRNAs to peripheral protrusions of mammalian cells and is important for cell migration. Trafficking occurs through active transport on microtubules, mediated by the KIF1C kinesin. Here, we identify the RNA-binding protein CNBP, as a factor required for mRNA localization to protrusions. CNBP binds directly to GA-rich sequences in the 3'UTR of protrusion targeted mRNAs. CNBP also interacts with KIF1C and is required for KIF1C recruitment to mRNAs and for their trafficking on microtubules to the periphery. This work provides a molecular mechanism for KIF1C recruitment to mRNA cargo and reveals a motor-adaptor complex for mRNA transport to cell protrusions.

Presynaptic perspective: Axonal transport defects in neurodevelopmental disorders

Disruption of synapse assembly and maturation leads to a broad spectrum of neurodevelopmental disorders. Presynaptic proteins are largely synthesized in the soma, where they are packaged into precursor vesicles and transported into distal axons to ensure precise assembly and maintenance of presynapses. Due to their morphological features, neurons face challenges in the delivery of presynaptic cargos to nascent boutons. Thus, targeted axonal transport is vital to build functional synapses. A growing number of mutations in genes encoding the transport machinery have been linked to neurodevelopmental disorders. Emerging lines of evidence have started to uncover presynaptic mechanisms underlying axonal transport defects, thus broadening the view of neurodevelopmental disorders beyond postsynaptic mechanisms. In this review, we discuss presynaptic perspectives of neurodevelopmental disorders by focusing on impaired axonal transport and disturbed assembly and maintenance of presynapses. We also discuss potential strategies for restoring axonal transport as an early therapeutic intervention.

To be or not to be: orb, the fusome and oocyte specification in Drosophila

In the fruit fly Drosophila melanogaster, two cells in a cyst of 16 interconnected cells have the potential to become the oocyte, but only one of these will assume an oocyte fate as the cysts transition through regions 2a and 2b of the germarium. The mechanism of specification depends on a polarized microtubule network, a dynein dependent Egl:BicD mRNA cargo complex, a special membranous structure called the fusome and its associated proteins, and the translational regulator orb. In this work, we have investigated the role of orb and the fusome in oocyte specification. We show here that specification is a stepwise process. Initially, orb mRNAs accumulate in the two pro-oocytes in close association with the fusome. This association is accompanied by the activation of the orb autoregulatory loop, generating high levels of Orb. Subsequently, orb mRNAs become enriched in only one of the pro-oocytes, the presumptive oocyte, and this is followed, with a delay, by Orb localization to the oocyte. We find that fusome association of orb mRNAs is essential for oocyte specification in the germarium, is mediated by the orb 3' UTR, and requires Orb protein. We also show that the microtubule minus end binding protein Patronin functions downstream of orb in oocyte specification. Finally, in contrast to a previously proposed model for oocyte selection, we find that the choice of which pro-oocyte becomes the oocyte does not seem to be predetermined by the amount of fusome material in these two cells, but instead depends upon a competition for orb gene products.

Tropomyosin 1-I/C coordinates kinesin-1 and dynein motors during oskar mRNA transport

Dynein and kinesin motors mediate long-range intracellular transport, translocating towards microtubule minus and plus ends, respectively. Cargoes often undergo bidirectional transport by binding to both motors simultaneously. However, it is not known how motor activities are coordinated in such circumstances. In the Drosophila female germline, sequential activities of the dynein-dynactin-BicD-Egalitarian (DDBE) complex and of kinesin-1 deliver oskar messenger RNA from nurse cells to the oocyte, and within the oocyte to the posterior pole. We show through in vitro reconstitution that Tm1-I/C, a tropomyosin-1 isoform, links kinesin-1 in a strongly inhibited state to DDBE-associated oskar mRNA. Nuclear magnetic resonance spectroscopy, small-angle X-ray scattering and structural modeling indicate that Tm1-I/C suppresses kinesin-1 activity by stabilizing its autoinhibited conformation, thus preventing competition with dynein until kinesin-1 is activated in the oocyte. Our work reveals a new strategy for ensuring sequential activity of microtubule motors.

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.

dTtc1, a conserved tetratricopeptide repeat protein, is required for maturation of Drosophila egg chambers via its role in stabilizing electron transport chain components

We recently identified the Drosophila ortholog of TTC1 (dTtc1) as an interacting partner of Egalitarian, an RNA adaptor of the Dynein motor. In order to better understand the function of this relatively uncharacterized protein, we depleted dTtc1 in the Drosophila female germline. Depletion of dTtc1 resulted in defective oogenesis and no mature eggs were produced. A closer examination revealed that mRNA cargoes normally transported by Dynein were relatively unaffected. However, mitochondria in dTtc1 depleted egg chambers displayed an extremely swollen phenotype. Ultrastructural analysis revealed a lack of cristae. These phenotypes were not observed upon disruption of Dynein. Thus, this function of dTtc1 is likely to be Dynein independent. Consistent with a role for dTtc1 in mitochondrial biology, a published proteomics screen revealed that dTtc1 interacts with numerous components of electron transport chain (ETC) complexes. Our results indicate that the expression level of several of these ETC components was significantly reduced upon depletion of dTtc1. Importantly, this phenotype was completely rescued upon expression of wild-type GFP-dTtc1 in the depleted background. Lastly, we demonstrate that the mitochondrial phenotype caused by a lack of dTtc1 is not restricted to the germline but is also observed in somatic tissues. Our model suggests that dTtc1, likely in combination with cytoplasmic chaperones, is required for stabilizing ETC components.

Ndel1 disfavors dynein-dynactin-adaptor complex formation in two distinct ways

Dynein is the primary minus-end-directed microtubule motor protein. To achieve activation, dynein binds to the dynactin complex and an adaptor to form the "activated dynein complex." The protein Lis1 aids activation by binding to dynein and promoting its association with dynactin and the adaptor. Ndel1 and its paralog Nde1 are dynein- and Lis1-binding proteins that help control dynein localization within the cell. Cell-based assays suggest that Ndel1-Nde1 also work with Lis1 to promote dynein activation, although the underlying mechanism is unclear. Using purified proteins and quantitative binding assays, here we found that the C-terminal region of Ndel1 contributes to dynein binding and negatively regulates binding to Lis1. Using single-molecule imaging and protein biochemistry, we observed that Ndel1 inhibits dynein activation in two distinct ways. First, Ndel1 disfavors the formation of the activated dynein complex. We found that phosphomimetic mutations in the C-terminal domain of Ndel1 increase its ability to inhibit dynein-dynactin-adaptor complex formation. Second, we observed that Ndel1 interacts with dynein and Lis1 simultaneously and sequesters Lis1 away from its dynein-binding site. In doing this, Ndel1 prevents Lis1-mediated dynein activation. Together, our work suggests that in vitro, Ndel1 is a negative regulator of dynein activation, which contrasts with cellular studies where Ndel1 promotes dynein activity. To reconcile our findings with previous work, we posit that Ndel1 functions to scaffold dynein and Lis1 together while keeping dynein in an inhibited state. We speculate that Ndel1 release can be triggered in cellular settings to allow for timed dynein activation.

The meiotic LINC complex component KASH5 is an activating adaptor for cytoplasmic dynein

Cytoplasmic dynein-driven movement of chromosomes during prophase I of mammalian meiosis is essential for synapsis and genetic exchange. Dynein connects to chromosome telomeres via KASH5 and SUN1 or SUN2, which together span the nuclear envelope. Here, we show that KASH5 promotes dynein motility in vitro, and cytosolic KASH5 inhibits dynein's interphase functions. KASH5 interacts with a dynein light intermediate chain (DYNC1LI1 or DYNC1LI2) via a conserved helix in the LIC C-terminal, and this region is also needed for dynein's recruitment to other cellular membranes. KASH5's N-terminal EF-hands are essential as the interaction with dynein is disrupted by mutation of key calcium-binding residues, although it is not regulated by cellular calcium levels. Dynein can be recruited to KASH5 at the nuclear envelope independently of dynactin, while LIS1 is essential for dynactin incorporation into the KASH5-dynein complex. Altogether, we show that the transmembrane protein KASH5 is an activating adaptor for dynein and shed light on the hierarchy of assembly of KASH5-dynein-dynactin complexes.

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.

Ndel1 modulates dynein activation in two distinct ways

Dynein is the primary minus-end-directed microtubule motor [1]. To achieve activation, dynein binds to the dynactin complex and an adaptor to form the "activated dynein complex" [2, 3]. The protein Lis1 aids activation by binding to dynein and promoting its association with dynactin and adaptor [4, 5]. Ndel1 and its orthologue Nde1 are dynein and Lis1 binding proteins that help control where dynein localizes within the cell [6]. Cell-based assays suggest that Ndel1/Nde1 also work with Lis1 to promote dynein activation, although the underlying mechanism is unclear [6]. Using purified proteins and quantitative binding assays, we found that Ndel1's C-terminal region contributes to binding to dynein and negatively regulates binding to Lis1. Using single-molecule imaging and protein biochemistry, we observed that Ndel1 inhibits dynein activation in two distinct ways. First, Ndel1 disfavors the formation of the activated dynein complex. We found that phosphomimetic mutations in Ndel1's C-terminal domain increase its ability to inhibit dynein-dynactin-adaptor complex formation. Second, we observed that Ndel1 interacts with dynein and Lis1 simultaneously and sequesters Lis1 away from its dynein binding site. In doing this, Ndel1 prevents Lis1-mediated dynein activation. Our work suggests that in vitro, Ndel1 is a negative regulator of dynein activation, which contrasts with cellular studies where Ndel1 promotes dynein activity. To reconcile our findings with previous work, we posit that Ndel1 functions to scaffold dynein and Lis1 together while keeping dynein in an inhibited state. We speculate that Ndel1 release can be triggered in cellular settings to allow for timed dynein activation.

Subcellular spatial transcriptomics identifies three mechanistically different classes of localizing RNAs

Intracellular RNA localization is a widespread and dynamic phenomenon that compartmentalizes gene expression and contributes to the functional polarization of cells. Thus far, mechanisms of RNA localization identified in Drosophila have been based on a few RNAs in different tissues, and a comprehensive mechanistic analysis of RNA localization in a single tissue is lacking. Here, by subcellular spatial transcriptomics we identify RNAs localized in the apical and basal domains of the columnar follicular epithelium (FE) and we analyze the mechanisms mediating their localization. Whereas the dynein/BicD/Egl machinery controls apical RNA localization, basally-targeted RNAs require kinesin-1 to overcome a default dynein-mediated transport. Moreover, a non-canonical, translation- and dynein-dependent mechanism mediates apical localization of a subgroup of dynein-activating adaptor-encoding RNAs (BicD, Bsg25D, hook). Altogether, our study identifies at least three mechanisms underlying RNA localization in the FE, and suggests a possible link between RNA localization and dynein/dynactin/adaptor complex formation in vivo.

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.

Doublecortin and JIP3 are neural-specific counteracting regulators of dynein-mediated retrograde trafficking

Mutations in the microtubule (MT)-binding protein doublecortin (DCX) or in the MT-based molecular motor dynein result in lissencephaly. However, a functional link between DCX and dynein has not been defined. Here, we demonstrate that DCX negatively regulates dynein-mediated retrograde transport in neurons from Dcx^-/y or Dcx^-/y;Dclk1^-/- mice by reducing dynein's association with MTs and disrupting the composition of the dynein motor complex. Previous work showed an increased binding of the adaptor protein C-Jun-amino-terminal kinase-interacting protein 3 (JIP3) to dynein in the absence of DCX. Using purified components, we demonstrate that JIP3 forms an active motor complex with dynein and its cofactor dynactin with two dyneins per complex. DCX competes with the binding of the second dynein, resulting in a velocity reduction of the complex. We conclude that DCX negatively regulates dynein-mediated retrograde transport through two critical interactions by regulating dynein binding to MTs and regulating the composition of the dynein motor complex.

In vivo proximity biotin ligation identifies the interactome of Egalitarian, a Dynein cargo adaptor

Numerous motors of the Kinesin family contribute to plus-end-directed microtubule transport. However, almost all transport towards the minus-end of microtubules involves Dynein. Understanding the mechanism by which Dynein transports this vast diversity of cargo is the focus of intense research. In selected cases, adaptors that link a particular cargo with Dynein have been identified. However, the sheer diversity of cargo suggests that additional adaptors must exist. We used the Drosophila egg chamber as a model to address this issue. Within egg chambers, Egalitarian is required for linking mRNA with Dynein. However, in the absence of Egalitarian, Dynein transport into the oocyte is severely compromised. This suggests that additional cargoes might be linked to Dynein in an Egalitarian-dependent manner. We therefore used proximity biotin ligation to define the interactome of Egalitarian. This approach yielded several novel interacting partners, including P body components and proteins that associate with Dynein in mammalian cells. We also devised and validated a nanobody-based proximity biotinylation strategy that can be used to define the interactome of any GFP-tagged protein.

Linking transport and translation of mRNAs with endosomes and mitochondria

In eukaryotic cells, proteins are targeted to their final subcellular locations with precise timing. A key underlying mechanism is the active transport of cognate mRNAs, which in many systems can be linked intimately to membrane trafficking. A prominent example is the long-distance endosomal transport of mRNAs and their local translation. Here, we describe current highlights of fundamental mechanisms of the underlying transport process as well as of biological functions ranging from endosperm development in plants to fungal pathogenicity and neuronal processes. Translation of endosome-associated mRNAs often occurs at the cytoplasmic surface of endosomes, a process that is needed for membrane-assisted formation of heteromeric protein complexes and for accurate subcellular targeting of proteins. Importantly, endosome-coupled translation of mRNAs encoding mitochondrial proteins, for example, seems to be particularly important for efficient organelle import and for regulating subcellular mitochondrial activity. In essence, these findings reveal a new mechanism of loading newly synthesised proteins onto endocytic membranes enabling intimate crosstalk between organelles. The novel link between endosomes and mitochondria adds an inspiring new level of complexity to trafficking and organelle biology.

Dynein light chain-dependent dimerization of Egalitarian is essential for maintaining oocyte fate in Drosophila

Egalitarian (Egl) is an RNA adaptor for the Dynein motor and is thought to link numerous, perhaps hundreds, of mRNAs with Dynein. Dynein, in turn, is responsible for the transport and localization of these mRNAs. Studies have shown that efficient mRNA binding by Egl requires the protein to dimerize. We recently demonstrated that Dynein light chain (Dlc) is responsible for facilitating the dimerization of Egl. Mutations in Egl that fail to interact with Dlc do not dimerize, and as such, are defective for mRNA binding. Consequently, this mutant does not efficiently associate with BicaudalD (BicD), the factor responsible for linking the Egl/mRNA complex with Dynein. In this report, we tested whether artificially dimerizing this Dlc-binding mutant using a leucine zipper would restore mRNA binding and rescue mutant phenotypes in vivo. Interestingly, we found that although artificial dimerization of Egl restored BicD binding, it only partially restored mRNA binding. As a result, Egl-dependent phenotypes, such as oocyte specification and mRNA localization, were only partially rescued. We hypothesize that Dlc-mediated dimerization of Egl results in a three-dimensional conformation of the Egl dimer that is best suited for mRNA binding. Although the leucine zipper restores Egl dimerization, it likely does not enable Egl to assemble into the conformation required for maximal mRNA binding activity.

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.

The Trypanosoma brucei subpellicular microtubule array is organized into functionally discrete subdomains defined by microtubule associated proteins

Microtubules are inherently dynamic cytoskeletal polymers whose length and organization can be altered to perform essential functions in eukaryotic cells, such as providing tracks for intracellular trafficking and forming the mitotic spindle. Microtubules can be bundled to create more stable structures that collectively propagate force, such as in the flagellar axoneme, which provides motility. The subpellicular microtubule array of the protist parasite Trypanosoma brucei, the causative agent of African sleeping sickness, is a remarkable example of a highly specialized microtubule bundle. It is comprised of a single layer of microtubules that are crosslinked to each other and to the overlying plasma membrane. The array microtubules appear to be highly stable and remain intact throughout the cell cycle, but very little is known about the pathways that tune microtubule properties in trypanosomatids. Here, we show that the subpellicular microtubule array is organized into subdomains that consist of differentially localized array-associated proteins at the array posterior, middle, and anterior. The array-associated protein PAVE1 stabilizes array microtubules at the cell posterior and is essential for maintaining its tapered shape. PAVE1 and the newly identified protein PAVE2 form a complex that binds directly to the microtubule lattice, demonstrating that they are a true kinetoplastid-specific MAP. TbAIR9, which localizes to the entirety of the subpellicular array, is necessary for maintaining the localization of array-associated proteins within their respective subdomains of the array. The arrangement of proteins within the array likely tunes the local properties of array microtubules and creates the asymmetric shape of the cell, which is essential for parasite viability.

Optimal RNA binding by Egalitarian, a Dynein cargo adaptor, is critical for maintaining oocyte fate in Drosophila

The Dynein motor is responsible for the localization of numerous mRNAs within Drosophila oocytes and embryos. The RNA binding protein, Egalitarian (Egl), is thought to link these various RNA cargoes with Dynein. Although numerous studies have shown that Egl is able to specifically associate with these RNAs, the nature of these interactions has remained elusive. Egl contains a central RNA binding domain that shares limited homology with an exonuclease, yet Egl binds to RNA without degrading it. Mutations have been identified within Egl that disrupt its association with its protein interaction partners, BicaudalD (BicD) and Dynein light chain (Dlc), but no mutants have been described that are specifically defective for RNA binding. In this report, we identified a series of positively charged residues within Egl that are required for RNA binding. Using corresponding RNA binding mutants, we demonstrate that specific RNA cargoes are more reliant on maximal Egl RNA biding activity for their correct localization in comparison to others. We also demonstrate that specification and maintenance of oocyte fate requires maximal Egl RNA binding activity. Even a subtle reduction in Egl's RNA binding activity completely disrupts this process. Our results show that efficient RNA localization at the earliest stages of oogenesis is required for specification of the oocyte and restriction of meiosis to a single cell.

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.

Walking the line: mechanisms underlying directional mRNA transport and localisation in neurons and beyond

Messenger RNA (mRNA) localisation enables a high degree of spatiotemporal control on protein synthesis, which contributes to establishing the asymmetric protein distribution required to set up and maintain cellular polarity. As such, a tight control of mRNA localisation is essential for many biological processes during development and in adulthood, such as body axes determination in Drosophila melanogaster and synaptic plasticity in neurons. The mechanisms controlling how mRNAs are localised, including diffusion and entrapment, local degradation and directed active transport, are largely conserved across evolution and have been under investigation for decades in different biological models. In this review, we will discuss the standing of the field regarding directional mRNA transport in light of the recent discovery that RNA can hitchhike on cytoplasmic organelles, such as endolysosomes, and the impact of these transport modalities on our understanding of neuronal function during development, adulthood and in neurodegeneration.

Dimerisation of the PICTS complex via LC8/Cut-up drives co-transcriptional transposon silencing in Drosophila

In animal gonads, the PIWI-interacting RNA (piRNA) pathway guards genome integrity in part through the co-transcriptional gene silencing of transposon insertions. In Drosophila ovaries, piRNA-loaded Piwi detects nascent transposon transcripts and instructs heterochromatin formation through the Panoramix-induced co-transcriptional silencing (PICTS) complex, containing Panoramix, Nxf2 and Nxt1. Here, we report that the highly conserved dynein light chain LC8/Cut-up (Ctp) is an essential component of the PICTS complex. Loss of Ctp results in transposon de-repression and a reduction in repressive chromatin marks specifically at transposon loci. In turn, Ctp can enforce transcriptional silencing when artificially recruited to RNA and DNA reporters. We show that Ctp drives dimerisation of the PICTS complex through its interaction with conserved motifs within Panoramix. Artificial dimerisation of Panoramix bypasses the necessity for its interaction with Ctp, demonstrating that conscription of a protein from a ubiquitous cellular machinery has fulfilled a fundamental requirement for a transposon silencing complex.

Dynamic multimerization of Dab2-Myosin VI complexes regulates cargo processivity while minimizing cortical actin reorganization

Myosin VI ensembles on endocytic cargo facilitate directed transport through a dense cortical actin network. Myosin VI is recruited to clathrin-coated endosomes via the cargo adaptor Dab2. Canonically, it has been assumed that the interactions between a motor and its cargo adaptor are stable. However, it has been demonstrated that the force generated by multiple stably attached motors disrupts local cytoskeletal architecture, potentially compromising transport. In this study, we demonstrate that dynamic multimerization of myosin VI-Dab2 complexes facilitates cargo processivity without significant reorganization of cortical actin networks. Specifically, we find that Dab2 myosin interacting region (MIR) binds myosin VI with a moderate affinity (184 nM) and single-molecule kinetic measurements demonstrate a high rate of turnover (1 s⁻¹) of the Dab2 MIR–myosin VI interaction. Single-molecule motility shows that saturating Dab2-MIR concentration (2 μM) promotes myosin VI homodimerization and processivity with run lengths comparable with constitutive myosin VI dimers. Cargo-mimetic DNA origami scaffolds patterned with Dab2 MIR-myosin VI complexes are weakly processive, displaying sparse motility on single actin filaments and “stop-and-go” motion on a cellular actin network. On a minimal actin cortex assembled on lipid bilayers, unregulated processive movement by either constitutive myosin V or VI dimers results in actin remodeling and foci formation. In contrast, Dab2 MIR–myosin VI interactions preserve the integrity of a minimal cortical actin network. Taken together, our study demonstrates the importance of dynamic motor–cargo association in enabling cargo transportation without disrupting cytoskeletal organization.

Cytoskeletal organization through multivalent interactions

The cytoskeleton consists of polymeric protein filaments with periodic lattices displaying identical binding sites, which establish a multivalent platform for the binding of a plethora of filament-associated ligand proteins. Multivalent ligand proteins can tether themselves to the filaments through one of their binding sites, resulting in an enhanced reaction kinetics for the remaining binding sites. In this Opinion, we discuss a number of cytoskeletal phenomena underpinned by such multivalent interactions, namely (1) generation of entropic forces by filament crosslinkers, (2) processivity of molecular motors, (3) spatial sorting of proteins, and (4) concentration-dependent unbinding of filament-associated proteins. These examples highlight that cytoskeletal filaments constitute the basis for the formation of microenvironments, which cytoskeletal ligand proteins can associate with and, once engaged, can act within at altered reaction kinetics. We thus argue that multivalency is one of the properties crucial for the functionality of the cytoskeleton.

Patterning the Drosophila embryo: A paradigm for RNA-based developmental genetic regulation

Embryonic anterior–posterior patterning is established in Drosophila melanogaster by maternally expressed genes. The mRNAs of several of these genes accumulate at either the anterior or posterior pole of the oocyte via a number of mechanisms. Many of these mRNAs are also under elaborate translational regulation. Asymmetric RNA localization coupled with spatially restricted translation ensures that their proteins are restricted to the position necessary for the developmental process that they drive. Bicoid (Bcd), the anterior determinant, and Oskar (Osk), the determinant for primordial germ cells and posterior patterning, have been studied particularly closely. In early embryos an anterior–posterior gradient of Bcd is established, activating transcription of different sets of zygotic genes depending on local Bcd concentration. At the posterior pole, Osk seeds formation of polar granules, ribonucleoprotein complexes that accumulate further mRNAs and proteins involved in posterior patterning and germ cell specification. After fertilization, polar granules associate with posterior nuclei and mature into nuclear germ granules. Osk accumulates in these granules, and either by itself or as part of the granules, stimulates germ cell division.

This article is categorized under:

RNA Export and Localization > RNA Localization

Translation > Translation Regulation

RNA in Disease and Development > RNA in Development

Activation and Regulation of Cytoplasmic Dynein

Cytoplasmic dynein is an AAA+ motor that drives the transport of many intracellular cargoes towards the minus end of microtubules (MTs). Previous in vitro studies characterized isolated dynein as an exceptionally weak motor that moves slowly and diffuses on an MT. Recent studies altered this view by demonstrating that dynein remains in an autoinhibited conformation on its own, and processive motility is activated when it forms a ternary complex with dynactin and a cargo adaptor. This complex assembles more efficiently in the presence of Lis1, providing an explanation for why Lis1 is a required cofactor for most cytoplasmic dynein-driven processes in cells. This review describes how dynein motility is activated and regulated by cargo adaptors and accessory proteins.

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.

Conformational diversity of dynactin sidearm and domain organization of its subunit p150

Dynactin is a principal regulator of the minus-end directed microtubule motor dynein. The sidearm of dynactin is essential for binding to microtubules and regulation of dynein activity. Although our understanding of the structure of the dynactin backbone (Arp1 rod) has greatly improved recently, structural details of the sidearm subcomplex remain elusive. Here, we report the flexible nature and diverse conformations of dynactin sidearm observed by electron microscopy. Using nanogold labeling and deletion mutant analysis, we determined the domain organization of the largest subunit p150 and discovered that its coiled-coil (CC1), dynein-binding domain, adopted either a folded or an extended form. Furthermore, the entire sidearm exhibited several characteristic forms, and the equilibrium among them depended on salt concentrations. These conformational diversities of the dynactin complex provide clues to understanding how it binds to microtubules and regulates dynein.

Dynactin p150 promotes processive motility of DDB complexes by minimizing diffusional behavior of dynein

Cytoplasmic dynein is activated by forming a complex with dynactin and the adaptor protein BicD2. We used interferometric scattering (iSCAT) microscopy to track dynein-dynactin-BicD2 (DDB) complexes in vitro and developed a regression-based algorithm to classify switching between processive, diffusive, and stuck motility states. We find that DDB spends 65% of its time undergoing processive stepping, 4% undergoing 1D diffusion, and the remaining time transiently stuck to the microtubule. Although the p150 subunit was previously shown to enable dynactin diffusion along microtubules, blocking p150 enhanced the proportion of time DDB diffused and reduced the time DDB processively walked. Thus, DDB diffusive behavior most likely results from dynein switching into an inactive (diffusive) state, rather than p150 tethering the complex to the microtubule. DDB-kinesin-1 complexes, formed using a DNA adapter, moved slowly and persistently, and blocking p150 led to a 70 nm/s plus-end shift in the average velocity of the complexes, in quantitative agreement with the shift of isolated DDB into the diffusive state. The data suggest a DDB activation model in which dynactin p150 enhances dynein processivity not solely by acting as a diffusive tether that maintains microtubule association, but rather by acting as an allosteric activator that promotes a conformation of dynein optimal for processive stepping. In bidirectional cargo transport driven by the opposing activities of kinesin and dynein-dynactin-BicD2, the dynactin p150 subunit promotes retrograde transport and could serve as a target for regulators of transport.

A reconstituted mammalian APC-kinesin complex selectively transports defined packages of axonal mRNAs

Through the asymmetric distribution of messenger RNAs (mRNAs), cells spatially regulate gene expression to create cytoplasmic domains with specialized functions. In neurons, mRNA localization is required for essential processes such as cell polarization, migration, and synaptic plasticity underlying long-term memory formation. The essential components driving cytoplasmic mRNA transport in neurons and mammalian cells are not known. We report the first reconstitution of a mammalian mRNA transport system revealing that the tumor suppressor adenomatous polyposis coli (APC) forms stable complexes with the axonally localized β-actin and β2B-tubulin mRNAs, which are linked to a kinesin-2 via the cargo adaptor KAP3. APC activates kinesin-2, and both proteins are sufficient to drive specific transport of defined mRNA packages. Guanine-rich sequences located in 3'UTRs of axonal mRNAs increase transport efficiency and balance the access of different mRNAs to the transport system. Our findings reveal a minimal set of proteins sufficient to transport mammalian mRNAs.

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

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

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.

The Egalitarian binding partners Dynein light chain and Bicaudal-D act sequentially to link mRNA to the Dynein motor

A conserved mechanism of polarity establishment is the localization of mRNA to specific cellular regions. Although it is clear that many mRNAs are transported along microtubules, much less is known about the mechanism by which these mRNAs are linked to microtubule motors. The RNA binding protein Egalitarian (Egl) is necessary for localization of several mRNAs in Drosophila oocytes and embryos. Egl also interacts with Dynein light chain (Dlc) and Bicaudal-D (BicD). The role of Dlc and BicD in mRNA localization has remained elusive. Both proteins are required for oocyte specification, as is Egl. Null alleles in these genes result in an oogenesis block. In this report, we used an shRNA-depletion strategy to overcome the oogenesis block. Our findings reveal that the primary function of Dlc is to promote Egl dimerization. Loss of dimerization compromises the ability of Egl to bind RNA. Consequently, Egl is not bound to cargo, and is not able to efficiently associate with BicD and the Dynein motor. Our results therefore identify the key molecular steps required for assembling a localization-competent mRNP.

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.

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 localization and transport

RNA localization serves numerous purposes from controlling development and differentiation to supporting the physiological activities of cells and organisms. After a brief introduction into the history of the study of mRNA localization I will focus on animal systems, describing in which cellular compartments and in which cell types mRNA localization was observed and studied. In recent years numerous novel localization patterns have been described, and countless mRNAs have been documented to accumulate in specific subcellular compartments. These fascinating revelations prompted speculations about the purpose of localizing all these mRNAs. In recent years experimental evidence for an unexpected variety of different functions has started to emerge. Aside from focusing on the functional aspects, I will discuss various ways of localizing mRNAs with a focus on the mechanism of active and directed transport on cytoskeletal tracks. Structural studies combined with imaging of transport and biochemical studies have contributed to the enormous recent progress, particularly in understanding how dynein/dynactin/BicD (DDB) dependent transport on microtubules works. This transport process actively localizes diverse cargo in similar ways to the minus end of microtubules and, at least in flies, also individual mRNA molecules. A sophisticated mechanism ensures that cargo loading licenses processive transport.

Optogenetic dissection of mitotic spindle positioning in vivo

The position of the mitotic spindle determines the plane of cell cleavage, and thereby daughter cell location, size, and content. Spindle positioning is driven by dynein-mediated pulling forces exerted on astral microtubules, which requires an evolutionarily conserved complex of Gα∙GDP, GPR-1/2^Pins/LGN, and LIN-5^Mud/NuMA proteins. To examine individual functions of the complex components, we developed a genetic strategy for light-controlled localization of endogenous proteins in C. elegans embryos. By replacing Gα and GPR-1/2 with a light-inducible membrane anchor, we demonstrate that Gα∙GDP, Gα∙GTP, and GPR-1/2 are not required for pulling-force generation. In the absence of Gα and GPR-1/2, cortical recruitment of LIN-5, but not dynein itself, induced high pulling forces. The light-controlled localization of LIN-5 overruled normal cell-cycle and polarity regulation and provided experimental control over the spindle and cell-cleavage plane. Our results define Gα∙GDP–GPR-1/2^Pins/LGN as a regulatable membrane anchor, and LIN-5^Mud/NuMA as a potent activator of dynein-dependent spindle-positioning forces.

A cell about to divide must decide where exactly to cut itself in two. Split right down the middle, and the two daughter cells will be identical; offset the cleavage plane to one side, and the resulting siblings will have different sizes, places and fates.

In animals, the splitting of cells is dictated by the location of the spindle, a structure that forms when cable-like microtubules stretch from the cell membrane to attach to the chromosomes. At the membrane, a group of proteins tugs on the microtubules to bring the spindle into the correct position. One of these proteins, dynein, is a motor that uses microtubules as its track to pull the spindle into place. What the other parts of the complex do is still unclear, but a general assumption is that they may be serving as an anchor for dynein.

To test this model, Fielmich, Schmidt et al. removed one or more proteins from the complex in the developing embryos of the nematode worm Caenorhabditis elegans. A light-activated system then linked the remaining proteins to the membrane by tying them to an artificial anchor. Two of the proteins in the complex could be replaced with the artificial anchor, but pulling forces were absent when dynein was artificially tied to the membrane. This indicates that the motor being anchored at the edge of the cell is not enough for it to pull on microtubules. Instead, the experiments showed that dynein needs to be activated by another component of the complex, a protein called LIN-5. This suggests that individual proteins in the complex have specialized roles that go beyond simply tethering dynein. In fact, steering where LIN-5 was attached on the membrane helped to control the location of the spindle, and therefore of the cleavage plane.

As mammals have a protein similar to LIN-5, dissecting the roles of the components involved in positioning the spindle in C. elegans could help to understand normal and abnormal human development. In addition, these results demonstrate that creating artificial interactions between proteins using light is a powerful technique to study biological processes.

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.