Functional assessment of the V390F mutation in the CCTδ subunit of chaperonin containing tailless complex polypeptide 1

The chaperonin containing tailless complex polypeptide 1 (CCT) is a multi-subunit molecular chaperone. It is found in the cytoplasm of all eukaryotic cells, where the oligomeric form plays an essential role in the folding of predominantly the cytoskeletal proteins actin and tubulin. Both the CCT oligomer and monomeric subunits also display functions that extend beyond folding, which are often associated with microtubules and actin filaments. Here, we assess the functional significance of the CCTδ V390F mutation, reported in several cancer cell lines. Upon transfection into B16F1 mouse melanoma cells, GFP-CCTδV390F incorporates into the CCT oligomer more readily than GFP-CCTδ. Furthermore, unlike GFP-CCTδ, GFP-CCTδV390F does not interact with the dynactin complex component, p150Glued. As CCTδ has previously been implicated in altered migration in wound healing assays, we assessed the behaviour of GFP-CCTδV390F and other mutants of CCTδ, previously used to assess functional interactions with p150Glued, in chemotaxis assays. We developed the assay system to incorporate a layer of the inert hydrogel GrowDex® to provide a 3D matrix for chemotaxis assessment and found subtle differences in the migration of B16F1 cells, depending on the presence of the hydrogel.

Kazrin C entraps early endosomes at the pericentriolar region and facilitates endocytic recycling

We identified kazrin C as a human protein that inhibits clathrin-mediated endocytosis when overexpressed. We now generated kazrin knock out and GFP-kazrin C expressing MEF lines to investigate in detail its function in endocytic traffic. We find that kazrin depletion delays recycling of internalized material and causes accumulation and dispersal of early endosomes (EE), indicating a role in transport from the early to the perinuclear recycling endosomes (RE). Consistently, we found that the C-terminal domain of kazrin C, predicted to be an intrinsically disordered region (IDR), specifically interacts with several endosomal components, including Epsin Homology Domain (EHD) proteins, γ-adaptin, and phosphatidyl-inositol-3 phosphate. Further, kazrin C shares homology with dynein/dynactin adaptors, it directly interacts with the dynactin complex and the dynein light intermediate chain LIC1, and overexpressed GFP-kazrin C forms condensates that entrap EE in the vicinity of the centrosome, in a microtubule-dependent manner. Altogether, the data indicates that kazrin C facilitates cargo recycling by trapping EE or EE-derived transport intermediates at the perinuclear region, where transfer of cargo to the RE might occur.

RUFY3 links Arl8b and JIP4-Dynein complex to regulate lysosome size and positioning

The whole-cell scale spatial organization of lysosomes is regulated by their bidirectional motility on microtubule tracks. Small GTP-binding (G) protein, Arl8b, stimulates the anterograde transport of lysosomes by recruiting adaptor protein SKIP (also known as PLEKHM2), which in turn couples the microtubule motor kinesin-1. Here, we have identified an Arl8b effector, RUN and FYVE domain-containing protein family member 3, RUFY3, which drives the retrograde transport of lysosomes. Artificial targeting of RUFY3 to the surface of mitochondria was sufficient to drive their perinuclear positioning. We find that RUFY3 interacts with the JIP4-Dynein-Dynactin complex and mediates Arl8b association with the retrograde motor complex. The mobile fraction of the total lysosomes per cell was significantly enhanced upon RUFY3 depletion, suggesting that RUFY3 maintains the lysosomes clustering within the perinuclear cloud. Expectedly, RUFY3 knockdown disrupted the perinuclear positioning of lysosomes upon nutrient starvation and/or serum depletion, although lysosome continued to undergo fusion with autophagosomes. Interestingly, lysosome fission events were more frequent in RUFY3-depleted cells and accordingly, there was a striking reduction in lysosome size, an effect that was also observed in dynein and JIP4 depleted cells. These findings indicate that the dynein-dependent “perinuclear cloud” arrangement of lysosomes also regulates the size of these proteolytic compartments and, likely, their cellular roles.

DCTN1 Binds to TDP-43 and Regulates TDP-43 Aggregation

A common pathological hallmark of several neurodegenerative diseases, including amyotrophic lateral sclerosis, is cytoplasmic mislocalization and aggregation of nuclear RNA-binding protein TDP-43. Perry disease, which displays inherited atypical parkinsonism, is a type of TDP-43 proteinopathy. The causative gene DCTN1 encodes the largest subunit of the dynactin complex. Dynactin associates with the microtubule-based motor cytoplasmic dynein and is required for dynein-mediated long-distance retrograde transport. Perry disease-linked missense mutations (e.g., p.G71A) reside within the CAP-Gly domain and impair the microtubule-binding abilities of DCTN1. However, molecular mechanisms by which such DCTN1 mutations cause TDP-43 proteinopathy remain unclear. We found that DCTN1 bound to TDP-43. Biochemical analysis using a panel of truncated mutants revealed that the DCTN1 CAP-Gly-basic supradomain, dynactin domain, and C-terminal region interacted with TDP-43, preferentially through its C-terminal region. Remarkably, the p.G71A mutation affected the TDP-43-interacting ability of DCTN1. Overexpression of DCTN1G71A, the dynactin-domain fragment, or C-terminal fragment, but not the CAP-Gly-basic fragment, induced cytoplasmic mislocalization and aggregation of TDP-43, suggesting functional modularity among TDP-43-interacting domains of DCTN1. We thus identified DCTN1 as a new player in TDP-43 cytoplasmic-nuclear transport, and showed that dysregulation of DCTN1-TDP-43 interactions triggers mislocalization and aggregation of TDP-43, thus providing insights into the pathological mechanisms of Perry disease and other TDP-43 proteinopathies.

Recycling of autophagosomal components from autolysosomes by recycler complex

Autolysosomes contain components both from autophagosomes and lysosomes. The contents inside the autophagosomal lumen are degraded during autophagy, while the fate of autophagosomal components on autolysosomal membrane remains unknown. Here, we found the autophagosomal membrane and transmembrane proteins are not degraded, but recycled from autolysosomes. We named this process autophagosomal components recycling (ACR). We further identified a multiprotein complex composed of SNX4, SNX5 and SNX17 essential for ACR which we termed “recycler”. In this, SNX4 and SNX5 form a heterodimer that recognizes an autophagosomal cargo STX17 and is required for generating membrane curvature on autolysosomes both via their BAR domains, to mediate the cargo sorting process. SNX17 interacts with both the dynein-dynactin complex and the SNX4-SNX5 dimer to facilitate retrograde transport of STX17. Depletion of any subunit of recycler completely blocks ACR, and also inhibits autophagy. Our discovery of ACR and identification of recycler reveal an important retrieval and recycling pathway on autolysosomes.

Sequential dynein effectors regulate axonal autophagosome motility in a maturation-dependent pathway

Autophagy is a degradative pathway required to maintain neuronal homeostasis. Neuronal autophagosomes form constitutively at the axon terminal and mature via lysosomal fusion during dynein-mediated transport to the soma. How the dynein-autophagosome interaction is regulated during maturation is unknown. Here, we identify a series of handoffs between dynein effectors as autophagosomes transit along the axons of primary neurons. In the distal axon, JIP1 initiates autophagosomal transport, while autophagosomes in the mid-axon require HAP1 and Huntingtin for motility. We demonstrate that HAP1 is a bonafide dynein activator, binding the dynein-dynactin complex via canonical and noncanonical interactions. JIP3 is found on most axonal autophagosomes but specifically regulates the transport of acidified autolysosomes. Inhibiting autophagosomal transport disrupts maturation, while inhibiting autophagosomal maturation perturbs the association and function of dynein effectors. Thus maturation and transport are tightly linked. These results describe a novel maturation-based dynein effector handoff on neuronal autophagosomes that is key to autophagosomal motility, cargo degradation, and the maintenance of axonal health.SummaryNeuronal autophagosomes form in the distal axon and mature via fusion with lysosomes during their dynein-driven transport to the soma. Dynein is regulated on autophagosomes by distinct effector proteins—JIP1, HAP1, and JIP3—depending on location and autophagosomal maturity. In this sequential pathway, transport and maturation state are tightly linked to maintain neuronal health.

Lis1 promotes the formation of maximally activated cytoplasmic dynein-1 complexes

Cytoplasmic dynein-1 is a molecular motor that drives nearly all minus-end-directed microtubule-based transport in human cells, performing functions ranging from retrograde axonal transport to mitotic spindle assembly1,2. Activated dynein complexes consist of one or two dynein dimers, the dynactin complex, and an “activating adaptor”, with maximal velocity seen with two dimers present (Fig. 1a)3-6. Little is known about how this massive ∼4MDa complex is assembled. Using purified recombinant human proteins, we uncovered a novel role for the dynein-binding protein, Lis1, in the formation of fully activated dynein complexes containing two dynein dimers. Lis1 is required for maximal velocity of complexes activated by proteins representing three different families of activating adaptors: BicD2, Hook3, and Ninl. Once activated dynein complexes have formed, they do not require the presence of Lis1 for sustained maximal velocity. Using cryo-electron microscopy we show that human Lis1 binds to dynein at two sites on dynein’s motor domain, similar to yeast dynein7. We propose that the ability of Lis1 to bind at these sites may function in multiple stages of assembling the motile human dynein/ dynactin/ activating adaptor complex.

DnaJB6 is a RanGTP-regulated protein involved in dynein-dependent microtubule organization during mitosis

SummaryBipolar spindle organization is essential for the faithful segregation of chromosomes during cell division. This organization relies on the collective activities of motor proteins. The minus-end directed dynein motor complex generates spindle inward forces and plays a major role in spindle pole focusing. The dynactin complex regulates many dynein functions increasing its processivity and force production.Here we show that DnaJB6 is a novel RanGTP regulated protein. It interacts with dynactin p150Glued in a RanGTP-dependent manner specifically in M-phase and promotes spindle pole focusing and dynein force generation. Our data suggest a novel mechanism by which RanGTP regulates dynein activity during M-phase.Summary statementWe describe DnaJB6 as a novel RanGTP-regulated protein important for spindle assembly. Our data suggest that RanGTP regulates dynein-dependent inward spindle force generation and pole focusing through DnaJB6

Crosslinking Proteomics Indicates Effects of Simvastatin on the TLR2 interactome and Reveals ACTR1A as a Novel Regulator of the TLR2 Signal Cascade

Toll-like receptor 2 (TLR2) is a pattern recognition receptor that, upon ligation by microbial molecules, interacts with other proteins to initiate pro-inflammatory responses by the cell. Statins (hydroxymethylglutaryl coenzyme A reductase inhibitors), drugs widely prescribed to reduce hypercholesterolemia, are reported to have both pro- and anti-inflammatory effects upon cells. Some of these responses are presumed to be driven by effects on signaling proteins at the plasma membrane, but the underlying mechanisms remain obscure. We reasoned that profiling the effect of statins on the repertoire of TLR2-interacting proteins might provide novel insights into the mechanisms by which statins impact inflammation. In order to study the TLR2 interactome, we designed a co-immunoprecipitation (IP)-based cross-linking proteomics study. A hemagglutinin (HA)-tagged-TLR2 transfected HEK293 cell line was utilized to precipitate the TLR2 interactome upon cell exposure to the TLR2 agonist Pam3CSK4 and simvastatin, singly and in combination. To stabilize protein interactors, we utilized two different chemical cross-linkers with different spacer chain lengths. Proteomic analysis revealed important combinatorial effects of simvastatin and Pam3CSK4 on the TLR2 interactome. After stringent data filtering, we identified alpha-centractin (ACTR1A), an actin-related protein and subunit of the dynactin complex, as a potential interactor of TLR2. The interaction was validated using biochemical methods. RNA interference studies revealed an important role for ACTR1A in induction of pro-inflammatory cytokines. Taken together, we report that statins remodel the TLR2 interactome, and we identify ACTR1A, a part of the dynactin complex, as a novel regulator of TLR2-mediated immune signaling pathways.