Dynactin has two antagonistic regulatory domains and exerts opposing effects on dynein motility

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

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

Positioning of endoplasmic reticulum exit sites around the Golgi depends on BicaudalD2 and Rab6 activity

The endoplasmic reticulum (ER) is involved in biogenesis, modification and transport of secreted and membrane proteins. The ER membranes are spread throughout the cell cytoplasm as well as the export domains known as ER exit sites (ERES). A subpopulation of ERES is centrally localized proximal to the Golgi apparatus. The significance of this subpopulation on ER-to-Golgi transport remains unclear. Transport carriers (TCs) form at the ERES via a COPII-dependent mechanism and move to Golgi on microtubule (MT) tracks. It was shown previously that ERES are distributed along MTs and undergo chaotic short-range movements and sporadic rapid long-range movements. The long-range movements of ERES are impaired by either depolymerization of MTs or inhibition of dynein, suggesting that ERES central concentration is mediated by dynein activity. We demonstrate that the processive movements of ERES are frequently coupled with the TC departure. Using the Sar1a[H79G]-induced ERES clustering at the perinuclear region, we identified BicaudalD2 (BicD2) and Rab6 as components of the dynein adaptor complex which drives perinuclear ERES concentration at the cell center. BicD2 partially colocalized with ERES and with TC. Peri-Golgi ERES localization was significantly affected by inhibition of BicD2 function with its N-terminal fragment or inhibition of Rab6 function with its dominant-negative mutant. Golgi accumulation of secretory protein was delayed by inhibition of Rab6 and BicD2. Thus, we conclude that a BicD2/Rab6 dynein adaptor is required for maintenance of Golgi-associated ERES. We propose that Golgi-associated ERES may enhance the efficiency of the ER-to-Golgi transport.

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.

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.

Conformational Flexibility of p150Glued(1-191) Subunit of Dynactin Assembled with Microtubules

Microtubule (MT)-associated proteins perform diverse functions in cells. These functions are dependent on their interactions with MTs. Dynactin, a cofactor of dynein motor, assists the binding of dynein to various organelles and is crucial to the long-distance processivity of dynein-based complexes. The largest subunit of dynactin, the p150^Glued, contains an N-terminus segment that is responsible for the MT-binding interactions and long-range processivity of dynactin. We employed solution and magic angle spinning NMR spectroscopy to characterize the structure and dynamics of the p150^Glued N-terminal region, both free and in complex with polymerized MTs. This 191-residue region encompasses the cytoskeleton-associated protein glycine-rich domain, the basic domain, and serine/proline-rich (SP-rich) domain. We demonstrate that the basic and SP-rich domains are intrinsically disordered in solution and significantly enhance the binding affinity to MTs as these regions contain the second MT-binding site on the p150^Glued subunit. The majority of the basic and SP-rich domains are predicted to be random coil, whereas the segments S111-I116, A124-R132, and K144-T146 in the basic domain contain short α-helical or β-sheet structures. These three segments possibly encompass the MT-binding site. Surprisingly, the protein retains a high degree of flexibility upon binding to MTs except for the regions that are directly involved in the binding interactions with MTs. This conformational flexibility may be essential for the biological functions of the p150^Glued subunit.

Host proteins involved in microtubule-dependent HIV-1 intracellular transport and uncoating

Microtubules and microtubule-associated proteins are critical for cargo transport throughout the cell. Many viruses are able to usurp these transport systems for their own replication and spread. HIV-1 utilizes these proteins for many of its early events postentry, including transport, uncoating and reverse transcription. The molecular motor proteins dynein and kinesin-1 are the primary drivers of cargo transport, and HIV-1 utilizes these proteins for infection. In this Review, we highlight recent developments in the understanding of how HIV-1 hijacks motor transport, the key cellular and viral proteins involved, and the ways that transport influences other steps in the HIV-1 lifecycle.

CLIP-170 is essential for MTOC repositioning during T cell activation by regulating dynein localisation on the cell surface

The microtubule-organizing centre (MTOC) is repositioned to the centre of the contacted cell surface, the immunological synapse, during T cell activation. However, our understanding of its molecular mechanism remains limited. Here, we found that the microtubule plus-end tracking cytoplasmic linker protein 170 (CLIP-170) plays a novel role in MTOC repositioning using fluorescence imaging. Inhibition of CLIP-170 phosphorylation impaired both MTOC repositioning and interleukin-2 (IL-2) expression. T cell stimulation induced some fraction of dynein to colocalise with CLIP-170 and undergo plus-end tracking. Concurrently, it increased dynein in minus-end-directed movement. It also increased dynein relocation to the centre of the contact surface. Dynein not colocalised with CLIP-170 showed both an immobile state and minus-end-directed movement at a velocity in good agreement with the velocity of MTOC repositioning, which suggests that dynein at the immunological synapse may pull the microtubules and the MTOC. Although CLIP-170 is phosphorylated by AMP-activated protein kinase (AMPK) irrespective of stimulation, phosphorylated CLIP-170 is essential for dynein recruitment to plus-end tracking and for dynein relocation. This indicates that dynein relocation results from coexistence of plus-end- and minus-end-directed translocation. In conclusion, CLIP-170 plays an indispensable role in MTOC repositioning and full activation of T cells by regulating dynein localisation.

HIV-1 Engages a Dynein-Dynactin-BICD2 Complex for Infection and Transport to the Nucleus

Human immunodeficiency virus type 1 (HIV-1) infection depends on efficient intracytoplasmic transport of the incoming viral core to the target cell nucleus. Evidence suggests that this movement is facilitated by the microtubule motor dynein, a large multiprotein complex that interacts with dynactin and cargo-specific adaptor proteins for retrograde movement via microtubules. Dynein adaptor proteins are necessary for activating dynein movement and for linking specific cargoes to dynein. We hypothesized that HIV-1 engages the dynein motor complex via an adaptor for intracellular transport. Here, we show that small interfering RNA depletion of the dynein heavy chain, components of the dynactin complex, and the dynein adaptor BICD2 reduced cell permissiveness to HIV-1 infection. Cell depletion of dynein heavy chain and BICD2 resulted in impaired HIV-1 DNA accumulation in the nucleus and decreased retrograde movement of the virus. Biochemical studies revealed that dynein components and BICD2 associate with capsid-like assemblies of the HIV-1 CA protein in cell extracts and that purified recombinant BICD2 binds to CA assemblies in vitro Association of dynein with CA assemblies was reduced upon immunodepletion of BICD2 from cell extracts. We conclude that BICD2 is a capsid-associated dynein adaptor utilized by HIV-1 for transport to the nucleus.IMPORTANCE During HIV-1 infection, the virus must travel across the cytoplasm to enter the nucleus. The host cell motor protein complex dynein has been implicated in HIV-1 intracellular transport. We show that expression of the dynein heavy chain, components of the dynein-associated dynactin complex, and the dynein adaptor BICD2 in target cells are important for HIV-1 infection and nuclear entry. BICD2 interacts with the HIV-1 capsid in vitro, suggesting that it functions as a capsid-specific adaptor for HIV-1 intracellular transport. Our work identifies specific host proteins involved in microtubule-dependent HIV-1 intracellular transport and highlights the BICD2-capsid interaction as a potential target for antiviral therapy.

The cytoplasmic dynein transport machinery and its many cargoes

Cytoplasmic dynein 1 is an important microtubule-based motor in many eukaryotic cells. Dynein has critical roles both in interphase and during cell division. Here, we focus on interphase cargoes of dynein, which include membrane-bound organelles, RNAs, protein complexes and viruses. A central challenge in the field is to understand how a single motor can transport such a diverse array of cargoes and how this process is regulated. The molecular basis by which each cargo is linked to dynein and its cofactor dynactin has started to emerge. Of particular importance for this process is a set of coiled-coil proteins - activating adaptors - that both recruit dynein-dynactin to their cargoes and activate dynein motility.

Coin Tossing Explains the Activity of Opposing Microtubule Motors on Phagosomes

How the opposing activity of kinesin and dynein motors generates polarized distribution of organelles inside cells is poorly understood and hotly debated [1, 2]. Possible explanations include stochastic mechanical competition [3, 4], coordinated regulation by motor-associated proteins [5-7], mechanical activation of motors [8], and lipid-induced organization [9]. Here, we address this question by using phagocytosed latex beads to generate early phagosomes (EPs) that move bidirectionally along microtubules (MTs) in an in vitro assay [9]. Dynein/kinesin activity on individual EPs is recorded as real-time force generation of the motors against an optical trap. Activity of one class of motors frequently coincides with, or is rapidly followed by opposite motors. This leads to frequent and rapid reversals of EPs in the trap. Remarkably, the choice between dynein and kinesin can be explained by the tossing of a coin. Opposing motors therefore appear to function stochastically and independently of each other, as also confirmed by observing no effect on kinesin function when dynein is inhibited on the EPs. A simple binomial probability calculation based on the geometry of EP-microtubule contact explains the observed activity of dynein and kinesin on phagosomes. This understanding of intracellular transport in terms of a hypothetical coin, if it holds true for other cargoes, provides a conceptual framework to explain the polarized localization of organelles inside cells.