Biology subjects

Bullock, S. L.

Publications and source records attributed to Bullock, S. L..

12 recordsLinked to original sources

bioRxiv2026

FXR proteins and BICD2 cooperate to promote dynein-mediated RNP transport and stress granule assembly

Intracellular trafficking of ribonucleoprotein particles (RNPs) is a widespread strategy for regulating gene expression, yet how this process is orchestrated in mammalian systems remains unclear. Here, we show that Bicaudal D2 (BICD2), a partner of the microtubule motor dynein, is complexed with RNA-binding proteins (RBPs) in human cells. We also reveal that two of these RBPs - the Fragile X-related proteins FXR1 and FXR2 - bind directly to BICD2 and recruit mRNAs to dynein. Surprisingly, given its conventional designation as a cargo adaptor, BICD2 is not needed for dynein to interact with FXR1 and FXR2. Using in vitro reconstitution, we demonstrate that BICD2 instead activates dynein motility in concert with the FXR proteins. These data support a model in which BICD2 is recruited to pre-existing RNP-motor complexes, where it is licensed by the FXR proteins to activate processive movement. We further provide evidence that FXR proteins, BICD2 and dynein cooperate to promote rapid formation of stress granules, large RNPs that form in the cytoplasm in response to cellular insults. Collectively, our work reveals molecular mechanisms underlying RNP transport and stress granule assembly in human cells and indicates that cargo recruitment and motor activation are separable steps in dynein-mediated transport.

cell biology↗

bioRxiv2026

Dynein-dependent positioning of multiple organelles regulates adaptive gene expression during oxidative stress

Cells respond to environmental insults through well-characterized changes to their transcriptome and proteome. The contribution that reorganization of intracellular architecture makes to the adaptive stress response is much less understood. Here, we reveal a conserved spatial response to oxidative stress in which the microtubule motor dynein enhances perinuclear clustering of multiple membrane-bound organelles. These events are triggered by reactive oxygen species and protein kinase C and occur without increased association of dynein with its activator dynactin, pointing to a non-canonical mechanism for stimulating retrograde transport. Using a chemically-inducible system for dispersing individual organelles, we show that perinuclear localization of multiple compartments potentiates oxidative stress-induced gene expression, with distinct genes depending on inputs from either combinations of organelles or individual compartments. Together, these findings implicate positioning of multiple membrane-bound compartments in gene expression regulation and identify coordinated organelle trafficking by dynein as an additional regulatory layer in the cellular stress response.

cell biology↗

bioRxiv2025

Imp, a key regulator of transposable elements, cell growth, and differentiation genes during embryogenesis

Imps are a highly conserved family of RNA-binding proteins involved in embryonic development, cancer progression, and neurogenesis. However, the molecular pathways and RNAs regulated by Imp to control these processes remain poorly understood. Embryos derived from Imp mutant germline clones arrest development, and transcriptome analysis revealed significant dysregulation of genes involved in cell growth, differentiation, tube morphogenesis, neuronal projection development, and RNA metabolism, along with de-repression of transposable element (TE) RNAs. Consistent with these findings, Imp mutant embryos display TE-overexpression phenotypes, are smaller in size, and exhibit defective organ development, including impaired tracheal branching and gastrulation. Reduced levels of Imp at the larval neuromuscular junction (NMJ) impair synaptic bouton formation and decrease adult longevity. RIP-seq experiments showed that Imp-associated RNAs are enriched for TE RNAs. Proteomic analyses confirmed that several TE-encoded proteins are upregulated in Imp mutant embryos. Specifically, the Ty1 family retrotransposon Copia was derepressed. Consistent with recent findings that Copia is a potent inhibitor of synaptogenesis, its upregulation likely contributes to the impaired NMJ formation and broader embryonic defects observed in Imp mutants. Moreover, Imp associates with piRNA pathway proteins, ensures Piwi nuclear localization, and--like piwi mutants--its loss disrupts TE silencing and causes position-effect variegation (PEV) defects. The analysis of Imp complexes further points to potential mechanisms by which Imp may regulate TE expression. Overall, these results indicate that Imp maintains genome stability and ensures proper developmental progression and neuronal activity by regulating post-transcriptional processes and suppressing transposons.

developmental biology↗

bioRxiv2025

Cryo-EM structures of Egl-BicD-RNA complexes reveal how diverse mRNAs are selected for subcellular localization

Localization of mRNAs is a widespread mechanism for controlling where proteins operate in cells and underpins many fundamental processes, from embryonic patterning to synaptic plasticity. This spatial control is mediated by RNA-binding proteins (RBPs) that interact with localization signals within target mRNAs. However, these signals lack overt sequence or structural patterns, even when targeted by the same protein, raising the question of how specificity is achieved. Here, we investigate this question using the Drosophila RBP Egalitarian (Egl), which couples developmentally important mRNAs to microtubule-based transport via Bicaudal D (BicD) and the dynein motor protein. We present cryo-EM structures of Egl- BicD in complex with six different RNAs. Egl uses multiple non-canonical double-stranded RNA-binding domains to cooperatively form a recognition pocket around a localization signal. Despite substantial variation in length and sequence, each signal adopts a bent stem-loop conformation that, together with base-pair identities at two defined sites, drives Egl binding. We further demonstrate that Egl dimers couple RNA binding to transport initiation through coincident detection of two RNA elements within the same transcript. Collectively, we show that diverse localizing mRNAs are recognized through a combination of shape, positional sequence features, and number of structured RNA elements. This work provides a framework for understanding how other RBPs engage highly variable mRNAs.

cell biology↗

bioRxiv2024

Centrocortin potentiates co-translational localization of its mRNA to the centrosome via dynein

Centrosomes rely upon proteins within the pericentriolar material to nucleate and organize microtubules. Several mRNAs also reside at centrosomes, although less is known about how and why they accumulate there. We previously showed that local Centrocortin (Cen) mRNA supports centrosome separation, microtubule organization, and viability in Drosophila embryos. Here, using Cen mRNA as a model, we examine mechanisms of centrosomal mRNA localization. We find that while the Cen N-terminus is sufficient for protein enrichment at centrosomes, multiple domains cooperate to concentrate Cen mRNA at this location. We further identify an N-terminal motif within Cen that is conserved among dynein cargo adaptor proteins and test its contribution to RNA localization. Our results support a model whereby Cen protein enables the accumulation of its own mRNA to centrosomes through a mechanism requiring active translation, microtubules, and the dynein motor complex. Taken together, our data uncover the basis of translation-dependent localization of a centrosomal RNA required for mitotic integrity. SummaryEnrichment of Centrocortin (Cen) mRNA at centrosomes is required for mitotic fidelity. This study describes a mechanism underlying co-translational Cen mRNA targeting involving microtubules, the dynein motor, and a highly conserved dynein binding motif within the Cen coding sequence.

cell biology↗

bioRxiv2023

A force-sensitive mutation reveals a spindle assembly checkpoint-independent role for dynein in anaphase progression

The cytoplasmic dynein-1 (dynein) motor organizes cells by shaping microtubule networks and moving a large variety of cargoes along them. However, dyneins diverse roles complicate in vivo studies of its functions significantly. To address this issue, we have used gene editing to generate a series of missense mutations in Drosophila Dynein heavy chain (Dhc). We find that mutations associated with human neurological disease cause a range of defects in larval and adult flies, including impaired cargo trafficking in neurons. We also describe a novel mutation in the microtubule-binding domain (MTBD) of Dhc that, remarkably, causes metaphase arrest of mitotic spindles in the embryo but does not impair other dynein-dependent processes. We demonstrate that the mitotic arrest is independent of dyneins well-established roles in silencing the spindle assembly checkpoint. In vitro reconstitution and optical trapping assays reveal that the mutation only impairs the performance of dynein under load. In silico all-atom molecular dynamics simulations show that this effect correlates with increased flexibility of the MTBD, as well as an altered orientation of the stalk domain, with respect to the microtubule. Collectively, our data point to a novel role of dynein in anaphase progression that depends on the motor operating in a specific load regime. More broadly, our work illustrates how cytoskeletal transport processes can be dissected in vivo by manipulating mechanical properties of motors.

cell biology↗

bioRxiv2023

HEATR5B associates with dynein-dynactin and selectively promotes motility of AP1-bound endosomal membranes

The dynein motor complex mediates polarised trafficking of a wide variety of organelles, intracellular vesicles and macromolecules. These functions are dependent on the dynactin complex, which helps recruit cargoes to dyneins tail region and activates motor movement. How dynein and dynactin orchestrate trafficking of diverse cargoes is unclear. Here, we identify HEATR5B, an interactor of the AP1 clathrin adaptor complex, as a novel player in dynein-dynactin function. HEATR5B is one of several proteins recovered in a biochemical screen for proteins whose association with the human dynein tail complex is augmented by dynactin. We show that HEATR5B binds directly to the dynein tail and dynactin and stimulates motility of AP1-associated endosomal membranes in human cells. We also demonstrate that the HEATR5B homologue in Drosophila is an essential gene that promotes dynein-based transport of AP1-bound membranes to the Golgi apparatus. As HEATR5B lacks the coiled-coil architecture typical of dynein adaptors, our data point to a non-canonical process orchestrating motor function on a specific cargo. We additionally show that HEATR5B promotes association of AP1 with endosomal membranes in a dynein-independent manner. Thus, HEATR5B co-ordinates multiple events in AP1-based trafficking.

cell biology↗

bioRxiv2023

Genome-scale requirements for dynein-based trafficking revealed by a high-content arrayed CRISPR screen

The cytoplasmic dynein-1 (dynein) motor plays a key role in cellular organisation by transporting a wide variety of cellular constituents towards the minus ends of microtubules. However, relatively little is known about how the biosynthesis, assembly and functional diversity of the motor is orchestrated. To address this issue, we have conducted an arrayed CRISPR loss-of-function screen in human cells using the distribution of dynein-tethered peroxisomes and early endosomes as readouts. From a guide RNA library targeting 18,253 genes, 195 validated hits were recovered and parsed into those impacting multiple dynein cargoes and those whose effects are restricted to a subset of cargoes. Clustering of high-dimensional phenotypic fingerprints generated from multiplexed images revealed co-functional genes involved in many cellular processes, including several candidate novel regulators of core dynein functions. Mechanistic analysis of one of these proteins, the RNA-binding protein SUGP1, provides evidence that it promotes cargo trafficking by sustaining functional expression of the dynein activator LIS1. Our dataset represents a rich source of new hypotheses for investigating microtubule-based transport, as well as several other aspects of cellular organisation that were captured by our high-content imaging.

cell biology↗

bioRxiv2022

Tropomyosin 1-I/C co-ordinates 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 Drosophila, sequential activities of the dynein-dynactin-BicD-Egalitarian (DDBE) complex and of kinesin-1 deliver oskar mRNA from nurse cells to the oocyte, and within the oocyte to the posterior pole. Here, through in vitro reconstitution, we show that Tm1-I/C, a Tropomyosin-1 isoform, links kinesin-1 in an inactive state to DDBE-associated oskar mRNA. NMR 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 a tug-of-war between the opposite polarity motors until kinesin-1 is activated in the oocyte. Our work reveals a novel strategy ensuring sequential activity of microtubule motors.

biochemistry↗

bioRxiv2019

Lis1 activates dynein motility by pairing it with dynactin

Lissencephaly-1 (Lis1) is a key cofactor for dynein-mediated intracellular transport towards the minus-ends of microtubules (MTs). It remains unclear whether Lis1 serves as an inhibitor or an activator of mammalian dynein motility. Here we use single-molecule imaging and optical trapping to show that Lis1 does not directly alter the stepping and force production of individual dynein motors assembled with dynactin and a cargo adaptor. Instead, Lis1 binding releases dynein from its auto-inhibited state and thereby promotes the formation of an active complex with dynactin. Lis1 also favors recruitment of two dyneins to dynactin, resulting in increased velocity, higher force production and more effective competition against kinesin in a tug-of-war. Lis1 dissociates from motile complexes, indicating that its primary role is to orchestrate the assembly of the transport machinery. These results provide a mechanistic explanation for why Lis1 is required for efficient transport of many dynein-associated cargoes in cells.

biophysics↗

bioRxiv2018

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 stabilises 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.

cell biology↗

bioRxiv2016

DYNC1H1 mutations associated with neurological diseases compromise processivity of dynein-dynactin-cargo adaptor complexes

Mutations in the human DYNC1H1 gene are associated with neurological diseases. DYNC1H1 encodes the heavy chain of cytoplasmic dynein-1, a 1.4 MDa motor complex that traffics organelles, vesicles and macromolecules towards microtubule minus ends. The effects of the DYNC1H1 mutations on dynein motility, and consequently their links to neuropathology, are not understood. Here, we address this issue using a recombinant expression system for human dynein coupled to single-molecule resolution in vitro motility assays. We functionally characterise 14 DYNC1H1 mutations identified in humans diagnosed with malformations in cortical development (MCD) or spinal muscular atrophy with lower extremity predominance (SMALED), as well as three mutations that cause motor and sensory defects in mice. Two of the human mutations, R1962C and H3822P, strongly interfere with dyneins core mechanochemical properties. The remaining mutations selectively compromise the processive mode of dynein movement that is activated by binding to the accessory complex dynactin and the cargo adaptor BICD2. Mutations with the strongest effects on dynein motility in vitro are associated with MCD. The vast majority of mutations do not affect binding of dynein to dynactin and BICD2, and are therefore expected to result in linkage of cargoes to dynein-dynactin complexes that have defective long-range motility. This observation offers an explanation for the dominant effects of DYNC1H1 mutations in vivo. Collectively, our results suggest that compromised processivity of cargo-motor assemblies contributes to human neurological disease and provide insight into the influence of different regions of the heavy chain on dynein motility.

cell biology↗