Asymmetric Molecular Architecture of the Human γ-Tubulin Ring Complex.

The γ-tubulin ring complex (γ-TuRC) is an essential regulator of centrosomal and acentrosomal microtubule formation, yet its structure is not known. Here, we present a cryo-EM reconstruction of the native human γ-TuRC at ∼3.8 Å resolution, revealing an asymmetric, cone-shaped structure. Pseudo-atomic models indicate that GCP4, GCP5, and GCP6 form distinct Y-shaped assemblies that structurally mimic GCP2/GCP3 subcomplexes distal to the γ-TuRC "seam." We also identify an unanticipated structural bridge that includes an actin-like protein and spans the γ-TuRC lumen. Despite its asymmetric architecture, the γ-TuRC arranges γ-tubulins into a helical geometry poised to nucleate microtubules. Diversity in the γ-TuRC subunits introduces large (>100,000 Å2) surfaces in the complex that allow for interactions with different regulatory factors. The observed compositional complexity of the γ-TuRC could self-regulate its assembly into a cone-shaped structure to control microtubule formation across diverse contexts, e.g., within biological condensates or alongside existing filaments.

Allosteric activation of VCP, an AAA unfoldase, by small molecule mimicry.

The loss of function of AAA (ATPases associated with diverse cellular activities) mechanoenzymes has been linked to diseases, and small molecules that activate these proteins can be powerful tools to probe mechanisms and test therapeutic hypotheses. Unlike chemical inhibitors that can bind a single conformational state to block enzyme function, activator binding must be permissive to different conformational states needed for mechanochemistry. However, we do not know how AAA proteins can be activated by small molecules. Here, we focus on valosin-containing protein (VCP)/p97, an AAA unfoldase whose loss of function has been linked to protein aggregation-based disorders, to identify druggable sites for chemical activators. We identified VCP ATPase Activator 1 (VAA1), a compound that dose-dependently stimulates VCP ATPase activity up to ~threefold. Our cryo-EM studies resulted in structures (ranging from ~2.9 to 3.7 Å-resolution) of VCP in apo and ADP-bound states and revealed that VAA1 binds an allosteric pocket near the C-terminus in both states. Engineered mutations in the VAA1-binding site confer resistance to VAA1, and furthermore, modulate VCP activity. Mutation of a phenylalanine residue in the VCP C-terminal tail that can occupy the VAA1 binding site also stimulates ATPase activity, suggesting that VAA1 acts by mimicking this interaction. Together, our findings uncover a druggable allosteric site and a mechanism of enzyme regulation that can be tuned through small molecule mimicry.

Cryo-EM shows how dynactin recruits two dyneins for faster movement.

Dynein and its cofactor dynactin form a highly processive microtubule motor in the presence of an activating adaptor, such as BICD2. Different adaptors link dynein and dynactin to distinct cargoes. Here we use electron microscopy and single-molecule studies to show that adaptors can recruit a second dynein to dynactin. Whereas BICD2 is biased towards recruiting a single dynein, the adaptors BICDR1 and HOOK3 predominantly recruit two dyneins. We find that the shift towards a double dynein complex increases both the force and speed of the microtubule motor. Our 3.5 Å resolution cryo-electron microscopy reconstruction of a dynein tail-dynactin-BICDR1 complex reveals how dynactin can act as a scaffold to coordinate two dyneins side-by-side. Our work provides a structural basis for understanding how diverse adaptors recruit different numbers of dyneins and regulate the motile properties of the dynein-dynactin transport machine.

Structure of human cytoplasmic dynein-2 primed for its power stroke.

Members of the dynein family, consisting of cytoplasmic and axonemal isoforms, are motors that move towards the minus ends of microtubules. Cytoplasmic dynein-1 (dynein-1) plays roles in mitosis and cellular cargo transport, and is implicated in viral infections and neurodegenerative diseases. Cytoplasmic dynein-2 (dynein-2) performs intraflagellar transport and is associated with human skeletal ciliopathies. Dyneins share a conserved motor domain that couples cycles of ATP hydrolysis with conformational changes to produce movement. Here we present the crystal structure of the human cytoplasmic dynein-2 motor bound to the ATP-hydrolysis transition state analogue ADP.vanadate. The structure reveals a closure of the motor's ring of six AAA+ domains (ATPases associated with various cellular activites: AAA1-AAA6). This induces a steric clash with the linker, the key element for the generation of movement, driving it into a conformation that is primed to produce force. Ring closure also changes the interface between the stalk and buttress coiled-coil extensions of the motor domain. This drives helix sliding in the stalk which causes the microtubule binding domain at its tip to release from the microtubule. Our structure answers the key questions of how ATP hydrolysis leads to linker remodelling and microtubule affinity regulation.

In vitro reconstitution of a highly processive recombinant human dynein complex.

Cytoplasmic dynein is an approximately 1.4 MDa multi-protein complex that transports many cellular cargoes towards the minus ends of microtubules. Several in vitro studies of mammalian dynein have suggested that individual motors are not robustly processive, raising questions about how dynein-associated cargoes can move over long distances in cells. Here, we report the production of a fully recombinant human dynein complex from a single baculovirus in insect cells. Individual complexes very rarely show directional movement in vitro. However, addition of dynactin together with the N-terminal region of the cargo adaptor BICD2 (BICD2N) gives rise to unidirectional dynein movement over remarkably long distances. Single-molecule fluorescence microscopy provides evidence that BICD2N and dynactin stimulate processivity by regulating individual dynein complexes, rather than by promoting oligomerisation of the motor complex. Negative stain electron microscopy reveals the dynein-dynactin-BICD2N complex to be well ordered, with dynactin positioned approximately along the length of the dynein tail. Collectively, our results provide insight into a novel mechanism for coordinating cargo binding with long-distance motor movement.

Structure of the γ-tubulin ring complex-capped microtubule.

Microtubules are composed of α-tubulin and ß-tubulin dimers positioned head-to-tail to form protofilaments that associate laterally in varying numbers. It is not known how cellular microtubules assemble with the canonical 13-protofilament architecture, resulting in micrometer-scale α/ß-tubulin tracks for intracellular transport that align with, rather than spiral along, the long axis of the filament. We report that the human ~2.3 MDa γ-tubulin ring complex (γ-TuRC), an essential regulator of microtubule formation that contains 14 γ-tubulins, selectively nucleates 13-protofilament microtubules. Cryogenic electron microscopy reconstructions of γ-TuRC-capped microtubule minus ends reveal the extensive intra-domain and inter-domain motions of γ-TuRC subunits that accommodate luminal bridge components and establish lateral and longitudinal interactions between γ-tubulins and α-tubulins. Our structures suggest that γ-TuRC, an inefficient nucleation template owing to its splayed conformation, can transform into a compacted cap at the microtubule minus end and set the lattice architecture of cellular microtubules.

Structure of the γ-tubulin ring complex-capped microtubule.

Microtubules are composed of α/ß-tubulin dimers positioned head-to-tail to form protofilaments that associate laterally in varying numbers. It is not known how cellular microtubules assemble with the canonical 13-protofilament architecture, resulting in micrometer-scale α/ß-tubulin tracks for intracellular transport that align with, rather than spiral along, the filament's long-axis. We report that the human ∼2.3MDa γ-tubulin ring complex (γ-TuRC), an essential regulator of microtubule formation that contains 14 γ-tubulins, selectively nucleates 13-protofilament microtubules. Cryo-EM reconstructions of γ-TuRC-capped microtubule minus-ends reveal the extensive intra- and inter-domain motions of γ-TuRC subunits that accommodate its actin-containing luminal bridge and establish lateral and longitudinal interactions between γ- and α-tubulins. Our structures reveal how free γ-TuRC, an inefficient nucleation template due to its splayed conformation, transforms into a stable cap that blocks addition or loss of α/ß-tubulins from minus-ends and sets the lattice architecture of cellular microtubules. One Sentence Summary: Structural insights into how the γ-tubulin ring complex nucleates and caps a 13-protofilament microtubule.

The structure of sulfoindocarbocyanine 3 terminally attached to dsDNA via a long, flexible tether.

Fluorescence resonance energy transfer (FRET) is an important source of long-range distance information in macromolecules. However, extracting maximum information requires knowledge of fluorophore, donor and acceptor, positions on the macromolecule. We previously determined the structure of the indocarbocyanine fluorophores Cy3 and Cy5 attached to DNA via three-carbon atom tethers, showing that they stacked onto the end of the helix in a manner similar to an additional basepair. Our recent FRET study has suggested that when they are attached via a longer 13-atom tether, these fluorophores are repositioned relative to the terminal basepair by a rotation of ∼30°, while remaining stacked. In this study, we have used NMR to extend our structural understanding to the commonly used fluorophore sulfoindocarbocyanine-3 (sCy3) attached to the 5'-terminus of the double-helical DNA via a 13-atom flexible tether (L13). We find that L13-sCy3 remains predominantly stacked onto the end of the duplex, but adopts a significantly different conformation, from that of either Cy3 or Cy5 attached by 3-atom tethers, with the long axes of the fluorophore and the terminal basepair approximately parallel. This result is in close agreement with our FRET data, supporting the contention that FRET data can be used to provide orientational information.

Biochemical reconstitutions reveal principles of human γ-TuRC assembly and function.

The formation of cellular microtubule networks is regulated by the γ-tubulin ring complex (γ-TuRC). This ∼2.3 MD assembly of >31 proteins includes γ-tubulin and GCP2-6, as well as MZT1 and an actin-like protein in a "lumenal bridge" (LB). The challenge of reconstituting the γ-TuRC has limited dissections of its assembly and function. Here, we report a biochemical reconstitution of the human γ-TuRC (γ-TuRC-GFP) as a ∼35 S complex that nucleates microtubules in vitro. In addition, we generate a subcomplex, γ-TuRCΔLB-GFP, which lacks MZT1 and actin. We show that γ-TuRCΔLB-GFP nucleates microtubules in a guanine nucleotide-dependent manner and with similar efficiency as the holocomplex. Electron microscopy reveals that γ-TuRC-GFP resembles the native γ-TuRC architecture, while γ-TuRCΔLB-GFP adopts a partial cone shape presenting only 8-10 γ-tubulin subunits and lacks a well-ordered lumenal bridge. Our results show that the γ-TuRC can be reconstituted using a limited set of proteins and suggest that the LB facilitates the self-assembly of regulatory interfaces around a microtubule-nucleating "core" in the holocomplex.

Targeting allostery in the Dynein motor domain with small molecule inhibitors.

Cytoplasmic dyneins are AAA (ATPase associated with diverse cellular activities) motor proteins responsible for microtubule minus-end-directed intracellular transport. Dynein's unusually large size, four distinct nucleotide-binding sites, and conformational dynamics pose challenges for the design of potent and selective chemical inhibitors. Here we use structural approaches to develop a model for the inhibition of a well-characterized S. cerevisiae dynein construct by pyrazolo-pyrimidinone-based compounds. These data, along with functional assays of dynein motility and mutagenesis studies, suggest that the compounds inhibit dynein by engaging the regulatory ATPase sites in the AAA3 and AAA4 domains, and not by interacting with dynein's main catalytic site in the AAA1 domain. A double Walker B mutation of the AAA3 and AAA4 sites substantially reduces enzyme activity, suggesting that targeting these regulatory domains is sufficient to inhibit dynein. Our findings reveal how chemical inhibitors can be designed to disrupt allosteric communication across dynein's AAA domains.

MZT Proteins Form Multi-Faceted Structural Modules in the γ-Tubulin Ring Complex.

Microtubule organization depends on the γ-tubulin ring complex (γ-TuRC), a ∼2.3-MDa nucleation factor comprising an asymmetric assembly of γ-tubulin and GCP2-GCP6. However, it is currently unclear how the γ-TuRC-associated microproteins MZT1 and MZT2 contribute to the structure and regulation of the holocomplex. Here, we report cryo-EM structures of MZT1 and MZT2 in the context of the native human γ-TuRC. MZT1 forms two subcomplexes with the N-terminal α-helical domains of GCP3 or GCP6 (GCP-NHDs) within the γ-TuRC "lumenal bridge." We determine the X-ray structure of recombinant MZT1/GCP6-NHD and find it is similar to that within the native γ-TuRC. We identify two additional MZT/GCP-NHD-like subcomplexes, one of which is located on the outer face of the γ-TuRC and comprises MZT2 and GCP2-NHD in complex with a centrosomin motif 1 (CM1)-containing peptide. Our data reveal how MZT1 and MZT2 establish multi-faceted, structurally mimetic "modules" that can expand structural and regulatory interfaces in the γ-TuRC.

The CryoEM structure of the Saccharomyces cerevisiae ribosome maturation factor Rea1.

The biogenesis of 60S ribosomal subunits is initiated in the nucleus where rRNAs and proteins form pre-60S particles. These pre-60S particles mature by transiently interacting with various assembly factors. The ~5000 amino-acid AAA+ ATPase Rea1 (or Midasin) generates force to mechanically remove assembly factors from pre-60S particles, which promotes their export to the cytosol. Here we present three Rea1 cryoEM structures. We visualise the Rea1 engine, a hexameric ring of AAA+ domains, and identify an α-helical bundle of AAA2 as a major ATPase activity regulator. The α-helical bundle interferes with nucleotide-induced conformational changes that create a docking site for the substrate binding MIDAS domain on the AAA +ring. Furthermore, we reveal the architecture of the Rea1 linker, which is involved in force generation and extends from the AAA+ ring. The data presented here provide insights into the mechanism of one of the most complex ribosome maturation factors.

How dynein and dynactin transport cargos: a structural perspective.

Recent structures of the dynein motor in three different conformations reveal how it uses ATP hydrolysis to move along microtubules. Attention is now turning to how cytoplasmic dynein-1 and dynactin act together to carry cargos. Cryo-electron microscopy (cryo-EM) has revealed the structure of dynactin and how it binds dynein in the presence of a cargo adaptor protein Bicaudal-D2 (BICD2). Future questions will include how dynein-1 transports so many different cargos and how the 2.4MDa dynein/dynactin transport machine is regulated.

The structure of the dynactin complex and its interaction with dynein.

Dynactin is an essential cofactor for the microtubule motor cytoplasmic dynein-1. We report the structure of the 23-subunit dynactin complex by cryo-electron microscopy to 4.0 angstroms. Our reconstruction reveals how dynactin is built around a filament containing eight copies of the actin-related protein Arp1 and one of ß-actin. The filament is capped at each end by distinct protein complexes, and its length is defined by elongated peptides that emerge from the α-helical shoulder domain. A further 8.2 angstrom structure of the complex between dynein, dynactin, and the motility-inducing cargo adaptor Bicaudal-D2 shows how the translational symmetry of the dynein tail matches that of the dynactin filament. The Bicaudal-D2 coiled coil runs between dynein and dynactin to stabilize the mutually dependent interactions between all three components.