Accumulation of cytoplasmic dynein and dynactin at microtubule plus ends in Aspergillus nidulans is kinesin dependent

The mechanism(s) by which microtubule plus-end tracking proteins are targeted is unknown. In the filamentous fungus Aspergillus nidulans, both cytoplasmic dynein and NUDF, the homolog of the LIS1 protein, localize to microtubule plus ends as comet-like structures. Herein, we show that NUDM, the p150 subunit of dynactin, also forms dynamic comet-like structures at microtubule plus ends. By examining proteins tagged with green fluorescent protein in different loss-of-function mutants, we demonstrate that dynactin and cytoplasmic dynein require each other for microtubule plus-end accumulation, and the presence of cytoplasmic dynein is also important for NUDF's plus-end accumulation. Interestingly, deletion of NUDF increases the overall accumulation of dynein and dynactin at plus ends, suggesting that NUDF may facilitate minus-end-directed dynein movement. Finally, we demonstrate that a conventional kinesin, KINA, is required for the microtubule plus-end accumulation of cytoplasmic dynein and dynactin, but not of NUDF.     

The deacetylase HDAC6 regulates aggresome formation and cell viability in response to misfolded protein stress

The efficient clearance of cytotoxic misfolded protein aggregates is critical for cell survival. Misfolded protein aggregates are transported and removed from the cytoplasm by dynein motors via the microtubule network to a novel organelle termed the aggresome where they are processed. However, the means by which dynein motors recognize misfolded protein cargo, and the cellular factors that regulate aggresome formation, remain unknown. We have discovered that HDAC6, a microtubule-associated deacetylase, is a component of the aggresome. We demonstrate that HDAC6 has the capacity to bind both polyubiquitinated misfolded proteins and dynein motors, thereby acting to recruit misfolded protein cargo to dynein motors for transport to aggresomes. Indeed, cells deficient in HDAC6 fail to clear misfolded protein aggregates from the cytoplasm, cannot form aggresomes properly, and are hypersensitive to the accumulation of misfolded proteins. These findings identify HDAC6 as a crucial player in the cellular management of misfolded protein-induced stress.     

LIS1, CLIP-170's key to the dynein/dynactin pathway

CLIP-170 is a plus-end tracking protein which may act as an anticatastrophe factor. It has been proposed to mediate the association of dynein/dynactin to microtubule (MT) plus ends, and it also binds to kinetochores in a dynein/dynactin-dependent fashion, both via its C-terminal domain. This domain contains two zinc finger motifs (proximal and distal), which are hypothesized to mediate protein-protein interactions. LIS1, a protein implicated in brain development, acts in several processes mediated by the dynein/dynactin pathway by interacting with dynein and other proteins. Here we demonstrate colocalization and direct interaction between CLIP-170 and LIS1. In mammalian cells, LIS1 recruitment to kinetochores is dynein/dynactin dependent, and recruitment there of CLIP-170 is dependent on its site of binding to LIS1, located in the distal zinc finger motif. Overexpression of CLIP-170 results in a zinc finger-dependent localization of a phospho-LIS1 isoform and dynactin to MT bundles, raising the possibility that CLIP-170 and LIS1 regulate dynein/dynactin binding to MTs. This work suggests that LIS1 is a regulated adapter between CLIP-170 and cytoplasmic dynein at sites involved in cargo-MT loading, and/or in the control of MT dynamics.     

Aggresomes: A Cellular Response to Misfolded Proteins

Intracellular deposition of misfolded protein aggregates into ubiquitin-rich cytoplasmic inclusions is linked to the pathogenesis of many diseases. Why these aggregates form despite the existence of cellular machinery to recognize and degrade misfolded protein and how they are delivered to cytoplasmic inclusions are not known. We have investigated the intracellular fate of cystic fibrosis transmembrane conductance regulator (CFTR), an inefficiently folded integral membrane protein which is degraded by the cytoplasmic ubiquitin-proteasome pathway. Overexpression or inhibition of proteasome activity in transfected human embryonic kidney or Chinese hamster ovary cells led to the accumulation of stable, high molecular weight, detergent-insoluble, multiubiquitinated forms of CFTR. Using immunofluorescence and transmission electron microscopy with immunogold labeling, we demonstrate that undegraded CFTR molecules accumulate at a distinct pericentriolar structure which we have termed the aggresome. Aggresome formation is accompanied by redistribution of the intermediate filament protein vimentin to form a cage surrounding a pericentriolar core of aggregated, ubiquitinated protein. Disruption of microtubules blocks the formation of aggresomes. Similarly, inhibition of proteasome function also prevented the degradation of unassembled presenilin-1 molecules leading to their aggregation and deposition in aggresomes. These data lead us to propose that aggresome formation is a general response of cells which occurs when the capacity of the proteasome is exceeded by the production of aggregation-prone misfolded proteins.     

Cystic fibrosis transmembrane regulator missing the first four transmembrane segments increases wild type and DeltaF508 processing

We previously generated an adenoassociated viral gene therapy vector, rAAV-Delta264 cystic fibrosis transmembrane conductance regulator (CFTR), missing the first four transmembrane domains of CFTR. When infected into monkey lungs, Delta264 CFTR increased the levels of endogenous wild type CFTR protein. To understand this process, we transfected Delta264 CFTR plasmid cDNA into COS7 cells, and we noted that protein expression from the truncation mutant is barely detectable when compared with wild type or DeltaF508 CFTR. Delta264 CFTR protein expression increases dramatically when cells are treated with proteasome inhibitors. Cycloheximide experiments show that Delta264 CFTR is degraded faster than DeltaF508 CFTR. VCP and HDAC6, two proteins involved in retrograde translocation from endoplasmic reticulum to cytosol for proteasomal and aggresomal degradation, coimmunoprecipitate with Delta264 CFTR. In cotransfection studies in COS7 cells and in transfection of Delta264 CFTR into cells stably expressing wild type and DeltaF508 CFTR, Delta264 CFTR increases wild type CFTR protein and increases levels of maturation of immature band B to mature band C of DeltaF508 CFTR. Thus the adenoassociated viral vector, rAAV-Delta264 CFTR, is a highly promising cystic fibrosis gene therapy vector because it increases the amount of mature band C protein both from wild type and DeltaF508 CFTR and associates with key elements in quality control mechanism of CFTR.     

Characterization and dynamics of aggresome formation by a cytosolic GFP-chimera.

Formation of a novel structure, the aggresome, has been proposed to represent a general cellular response to the presence of misfolded proteins (Johnston, J.A., C.L. Ward, and R.R. Kopito. 1998. J. Cell Biol. 143:1883-1898; Wigley, W.C., R.P. Fabunmi, M.G. Lee, C.R. Marino, S. Muallem, G.N. DeMartino, and P.J. Thomas. 1999. J. Cell Biol. 145:481-490). To test the generality of this finding and characterize aspects of aggresome composition and its formation, we investigated the effects of overexpressing a cytosolic protein chimera (GFP-250) in cells. Overexpression of GFP-250 caused formation of aggresomes and was paralleled by the redistribution of the intermediate filament protein vimentin as well as by the recruitment of the proteasome, and the Hsp70 and the chaperonin systems of chaperones. Interestingly, GFP-250 within the aggresome appeared not to be ubiquitinated. In vivo time-lapse analysis of aggresome dynamics showed that small aggregates form within the periphery of the cell and travel on microtubules to the MTOC region where they remain as distinct but closely apposed particulate structures. Overexpression of p50/dynamitin, which causes the dissociation of the dynactin complex, significantly inhibited the formation of aggresomes, suggesting that the minus-end-directed motor activities of cytoplasmic dynein are required for aggresome formation. Perinuclear aggresomes interfered with correct Golgi localization and disrupted the normal astral distribution of microtubules. However, ER-to-Golgi protein transport occurred normally in aggresome containing cells. Our results suggest that aggresomes can be formed by soluble, nonubiquitinated proteins as well as by integral transmembrane ubiquitinated ones, supporting the hypothesis that aggresome formation might be a general cellular response to the presence of misfolded proteins.     

A Neurospora crassa Arp1 mutation affecting cytoplasmic dynein and dynactin localization

Of the actin-related proteins, Arp1 is the most similar to conventional actin, and functions solely as a component of the multisubunit complex dynactin. Dynactin has been identified as an activator of the microtubule-associated motor cytoplasmic dynein. The role of Arp1 within dynactin is two-fold: (1) it serves as a structural scaffold protein for other dynactin subunits; and (2) it has been proposed to link dynactin, and thereby dynein, with membranous cargo via interaction with spectrin. Using the filamentous fungus Neurospora crassa, we have identified genes encoding subunits of cytoplasmic dynein and dynactin. In this study, we describe a genetic screen for N. crassa Arp1 (ro-4) mutants that are defective for dynactin function. We report that the ro-4(E8) mutant is unusual in that it shows alterations in the localization of cytoplasmic dynein and dynactin and in microtubule organization. In the mutant, dynein/dynactin complexes co-localize with bundled microtubules at hyphal tips. Given that dynein transports membranous cargo from hyphal tips to distal regions, the cytoplasmic dynein and dynactin complexes that accumulate along microtubule tracts at hyphal tips in the ro-4(E8) mutant may have either reduced motor activity or be delayed for activation of motor activity following cargo binding.     

Derlin-1 promotes the efficient degradation of the cystic fibrosis transmembrane conductance regulator (CFTR) and CFTR folding mutants

A complex involving Derlin-1 and p97 mediates the retrotranslocation and endoplasmic reticulum (ER)-associated degradation of misfolded proteins in yeast and is used by certain viruses to promote host cell protein degradation (Romisch, K. (2005) Annu. Rev. Cell Dev. Biol. 21, 435-456; Lilley, B. N., and Ploegh, H. L. (2004) Nature 429, 834-840; Ye, Y., Shibata, Y., Yun, C., Ron, D., and Rapoport, T. A. (2004) Nature 429, 841-847). We asked whether the components of this pathway are involved in the endoplasmic reticulum-associated degradation of the mammalian integral membrane protein, the cystic fibrosis transmembrane conductance regulator (CFTR), a substrate for the ubiquitin-proteasome system. We report that Derlin-1 and p97 formed complexes with CFTR in human airway epithelial cells. Derlin-1 interacted with nonubiquitylated CFTR, whereas p97 associated with ubiquitylated CFTR. Exogenous expression of Derlin-1 led to its co-localization with CFTR in the ER where it reduced wild type (WT) CFTR expression and efficiently degraded the disease-associated CFTR folding mutants, DeltaF508 and G85E (>90%). Consistent with this, Derlin-1 also reduced the amount of WT or DeltaF508 CFTR appearing in detergent-in-soluble aggregates. An approximately 70% knockdown of endogenous Derlin-1 by RNA interference increased the steady-state levels of WT and DeltaF508 CFTR by 10-15-fold, reflecting its significant role in CFTR degradation. Derlin-1 mediated the degradation of N-terminal CFTR fragments corresponding to the first transmembrane domain of CFTR, but CFTR fragments that incorporated additional domains were degraded less efficiently. These findings suggest that Derlin-1 recognizes misfolded, nonubiquitylated CFTR to initiate its dislocation and degradation early in the course of CFTR biogenesis, perhaps by detecting structural instability within the first transmembrane domain.     

Small heat-shock proteins select deltaF508-CFTR for endoplasmic reticulum-associated degradation

Secreted proteins that fail to achieve their native conformations, such as cystic fibrosis transmembrane conductance regulator (CFTR) and particularly the DeltaF508-CFTR variant can be selected for endoplasmic reticulum (ER)-associated degradation (ERAD) by molecular chaperones. Because the message corresponding to HSP26, which encodes a small heat-shock protein (sHsp) in yeast was up-regulated in response to CFTR expression, we examined the impact of sHsps on ERAD. First, we observed that CFTR was completely stabilized in cells lacking two partially redundant sHsps, Hsp26p and Hsp42p. Interestingly, the ERAD of a soluble and a related integral membrane protein were unaffected in yeast deleted for the genes encoding these sHsps, and CFTR polyubiquitination was also unaltered, suggesting that Hsp26p/Hsp42p are not essential for polyubiquitination. Next, we discovered that DeltaF508-CFTR degradation was enhanced when a mammalian sHsp, alphaA-crystallin, was overexpressed in human embryonic kidney 293 cells, but wild-type CFTR biogenesis was unchanged. Because alphaA-crystallin interacted preferentially with DeltaF508-CFTR and because purified alphaA-crystallin suppressed the aggregation of the first nucleotide-binding domain of CFTR, we suggest that sHsps maintain the solubility of DeltaF508-CFTR during the ERAD of this polypeptide.     

Neurospora crassa ro-10 and ro-11 genes encode novel proteins required for nuclear distribution

Movement and distribution of nuclei in fungi have been shown to be dependent on cytoplasmic microtubules and the microtubule-associated motor cytoplasmic dynein. We have isolated hundreds of Neurospora crassa mutants, known as ropy, that are defective in nuclear distribution. Three of the ro genes, ro-1, ro-3 and ro-4, have been shown to encode subunits of either cytoplasmic dynein or the dynein activator complex, dynactin. In this report, we describe the isolation and initial characterization of two additional ro genes, ro-10 and ro-11. ro-10 and ro-11 are non-essential genes that encode novel 24 kDa and 75 kDa proteins respectively. Both ro-10 and ro-11 mutants retain the ability to generate long cytoplasmic microtubule tracks, suggesting that the nuclear distribution defect is not caused by a gross defect in the microtubule cytoskeleton. RO10, as well as RO4 (actin-related protein ARP1, the most abundant subunit of dynactin), appears to be required for the stability of RO3 (p150Glued), the largest subunit of dynactin. We propose that ro-10 mutants lack proper nuclear distribution, because RO10 is either a subunit of dynactin and required for dynactin activity or essential for assembly of the dynactin complex. ro-11 mutations have no effect on RO1 or RO3 levels and have only a very slight effect on the localization pattern of cytoplasmic dynein and dynactin. The role of RO11 in the movement and distribution of nuclei in N. crassa hyphae remains unknown.     

Nudel/NudE and Lis1 promote dynein and dynactin interaction in the context of spindle morphogenesis

Lis1, Nudel/NudE, and dynactin are regulators of cytoplasmic dynein, a minus end–directed, microtubule (MT)-based motor required for proper spindle assembly and orientation. In vitro studies have shown that dynactin promotes processive movement of dynein on MTs, whereas Lis1 causes dynein to enter a persistent force-generating state (referred to here as dynein stall). Yet how the activities of Lis1, Nudel/NudE, and dynactin are coordinated to regulate dynein remains poorly understood in vivo. Working in Xenopus egg extracts, we show that Nudel/NudE facilitates the binding of Lis1 to dynein, which enhances the recruitment of dynactin to dynein. We further report a novel Lis1-dependent dynein–dynactin interaction that is essential for the organization of mitotic spindle poles. Finally, using assays for MT gliding and spindle assembly, we demonstrate an antagonistic relationship between Lis1 and dynactin that allows dynactin to relieve Lis1-induced dynein stall on MTs. Our findings suggest the interesting possibility that Lis1 and dynactin could alternately engage with dynein to allow the motor to promote spindle assembly.     

CLIP-170 interacts with dynactin complex and the APC-binding protein EB1 by different mechanisms

CLIP-170 is a "cytoplasmic linker protein" implicated in endosome-microtubule interactions and in control of microtubule dynamics. CLIP-170 localizes dynamically to growing microtubule plus ends, colocalizing with the dynein activator dynactin and the APC-binding protein EB1. This shared "plus-end tracking" behavior suggests that CLIP-170 might interact with dynactin and/or EB1. We have used site-specific mutagenesis of CLIP-170 and a transfection/colocalization assay to address this question in mammalian tissue culture cells. Our results indicate that CLIP-170 interacts, directly or indirectly, with both dynactin and EB1. We find that the CLIP-170/dynactin interaction is mediated by the second metal binding motif of the CLIP-170 tail. In contrast, the CLIP-170/EB1 interaction requires neither metal binding motif. In addition, our experiments suggest that the CLIP-170/dynactin interaction occurs via the shoulder/sidearm subcomplex of dynactin and can occur in the cytosol (i.e., it does not require microtubule binding). These results have implications for the targeting of both dynactin and EB1 to microtubule plus ends. Our data suggest that the CLIP-170/dynactin interaction can target dynactin complex to microtubule plus ends, although dynactin likely also targets MT plus ends directly via the microtubule binding motif of the p150(Glued) subunit. We find that CLIP-170 mutants alter p150(Glued) localization without affecting EB1, indicating that EB1 can target microtubule plus ends independently of dynactin.     

A requirement for cytoplasmic dynein and dynactin in intermediate filament network assembly and organization

We present evidence that vimentin intermediate filament (IF) motility in vivo is associated with cytoplasmic dynein. Immunofluorescence reveals that subunits of dynein and dynactin are associated with all structural forms of vimentin in baby hamster kidney-21 cells. This relationship is also supported by the presence of numerous components of dynein and dynactin in IF-enriched cytoskeletal preparations. Overexpression of dynamitin biases IF motility toward the cell surface, leading to a perinuclear clearance of IFs and their redistribution to the cell surface. IF-enriched cytoskeletal preparations from dynamitin-overexpressing cells contain decreased amounts of dynein, actin-related protein-1, and p150Glued relative to controls. In contrast, the amount of dynamitin is unaltered in these preparations, indicating that it is involved in linking vimentin cargo to dynactin. The results demonstrate that dynein and dynactin are required for the normal organization of vimentin IF networks in vivo. These results together with those of previous studies also suggest that a balance among the microtubule (MT) minus and plus end-directed motors, cytoplasmic dynein, and kinesin are required for the assembly and maintenance of type III IF networks in interphase cells. Furthermore, these motors are to a large extent responsible for the long recognized relationships between vimentin IFs and MTs.     

Cytoplasmic dynein/dynactin drives kinetochore protein transport to the spindle poles and has a role in mitotic spindle checkpoint inactivation.

We discovered that many proteins located in the kinetochore outer domain, but not the inner core, are depleted from kinetochores and accumulate at spindle poles when ATP production is suppressed in PtK1 cells, and that microtubule depolymerization inhibits this process. These proteins include the microtubule motors CENP-E and cytoplasmic dynein, and proteins involved with the mitotic spindle checkpoint, Mad2, Bub1R, and the 3F3/2 phosphoantigen. Depletion of these components did not disrupt kinetochore outer domain structure or alter metaphase kinetochore microtubule number. Inhibition of dynein/dynactin activity by microinjection in prometaphase with purified p50 "dynamitin" protein or concentrated 70.1 anti-dynein antibody blocked outer domain protein transport to the spindle poles, prevented Mad2 depletion from kinetochores despite normal kinetochore microtubule numbers, reduced metaphase kinetochore tension by 40%, and induced a mitotic block at metaphase. Dynein/dynactin inhibition did not block chromosome congression to the spindle equator in prometaphase, or segregation to the poles in anaphase when the spindle checkpoint was inactivated by microinjection with Mad2 antibodies. Thus, a major function of dynein/dynactin in mitosis is in a kinetochore disassembly pathway that contributes to inactivation of the spindle checkpoint.     

Apoptotic cleavage of cytoplasmic dynein intermediate chain and p150(Glued) stops dynein-dependent membrane motility.

Cytoplasmic dynein is the major minus end-directed microtubule motor in animal cells, and associates with many of its cargoes in conjunction with the dynactin complex. Interaction between cytoplasmic dynein and dynactin is mediated by the binding of cytoplasmic dynein intermediate chains (CD-IC) to the dynactin subunit, p150(Glued). We have found that both CD-IC and p150(Glued) are cleaved by caspases during apoptosis in cultured mammalian cells and in Xenopus egg extracts. Xenopus CD-IC is rapidly cleaved at a conserved aspartic acid residue adjacent to its NH(2)-terminal p150(Glued) binding domain, resulting in loss of the otherwise intact cytoplasmic dynein complex from membranes. Cleavage of CD-IC and p150(Glued) in apoptotic Xenopus egg extracts causes the cessation of cytoplasmic dynein--driven endoplasmic reticulum movement. Motility of apoptotic membranes is restored by recruitment of intact cytoplasmic dynein and dynactin from control cytosol, or from apoptotic cytosol supplemented with purified cytoplasmic dynein--dynactin, demonstrating the dynamic nature of the association of cytoplasmic dynein and dynactin with their membrane cargo.     

Post-translational disruption of the delta F508 cystic fibrosis transmembrane conductance regulator (CFTR)-molecular chaperone complex with geldanamycin stabilizes delta F508 CFTR in the rabbit reticulocyte lysate

The DeltaF508 mutation of cystic fibrosis transmembrane conductance regulator (CFTR) is a trafficking mutant, which is retained and degraded in the endoplasmic reticulum by the ubiquitin-proteasome pathway. The mutant protein fails to reach a completely folded conformation that is no longer a substrate for ubiquitination ("stable B"). Wild type protein reaches this state with 25% efficiency. In this study the rabbit reticulocyte lysate with added microsomal membranes has been used to reproduce the post-translational events in the folding of wild type and DeltaF508 CFTR. In this system wild type CFTR does not reach the stable B form if the post-translational temperature is 37 degrees C, whereas at 30 degrees C the behavior of both wild type and mutant proteins mimics that observed in the cell. Geldanamycin stabilizes DeltaF508 CFTR with respect to ubiquitination only when added post-translationally. The interaction of wild type and mutant CFTR with the molecular chaperones heat shock cognate 70 (hsc70) and heat shock protein 90 (hsp90) has been assessed. Release of wild type protein from hsc70 coincides with the cessation of ubiquitination and formation of stable B. Geldanamycin immediately prevents the binding of hsp90 to DeltaF508 CFTR, and after a delay releases it from hsc70. Release of mutant protein from hsc70 also coincides with the formation of stable B DeltaF508 CFTR.     

In vivo roles of the basic domain of dynactin p150 in microtubule plus-end tracking and dynein function

Microtubule (MT) plus-end-tracking proteins accumulate at MT plus ends for various cellular functions, but their targeting mechanisms are not fully understood (Akhmanova A and Steinmetz MO. Tracking the ends: a dynamic protein network controls the fate of microtubule tips. Nat Rev Mol Cell Biol 2008;9:309-322.). Here, we tested in the filamentous fungus Aspergillus nidulans the requirement for plus-end localization of dynactin p150, a protein essential for dynein function. Deletion of the N-terminal MT-binding region of p150 significantly diminishes the MT plus-end accumulation of both dynein heavy chain and p150, and causes a partial defect in nuclear distribution. Surprisingly, within the MT-binding region, the basic domain is more critical than the CAP-Gly (cytoskeleton-associated protein glycine-rich) domain for maintaining plus-end tracking of p150, as well as for the functions of dynein in nuclear distribution and early endosome movement. Our results show that the basic domain of A. nidulans p150 is important for p150-MT interaction both in vivo and in vitro, and the basic amino acids within this domain are crucial for the plus-end accumulation of p150 in the wild-type background and for the p150-MT interaction in the ΔkinA (kinesin-1) background. We suggest that the basic amino acids are required for the electrostatic interaction between p150 and MTs, which is important for kinesin-1-mediated plus-end targeting of dynactin and dynein in A. nidulans.     

Parkin-mediated K63-linked polyubiquitination targets misfolded DJ-1 to aggresomes via binding to HDAC6

Sequestration of misfolded proteins into pericentriolar inclusions called aggresomes is a means that cells use to minimize misfolded protein-induced cytotoxicity. However, the molecular mechanism by which misfolded proteins are recruited to aggresomes remains unclear. Mutations in the E3 ligase parkin cause autosomal recessive Parkinson's disease that is devoid of Lewy bodies, which are similar to aggresomes. Here, we report that parkin cooperates with heterodimeric E2 enzyme UbcH13/Uev1a to mediate K63-linked polyubiquitination of misfolded DJ-1. K63-linked polyubiquitination of misfolded DJ-1 serves as a signal for interaction with histone deacetylase 6, an adaptor protein that binds the dynein-dynactin complex. Through this interaction, misfolded DJ-1 is linked to the dynein motor and transported to aggresomes. Furthermore, fibroblasts lacking parkin display deficits in targeting misfolded DJ-1 to aggresomes. Our findings reveal a signaling role for K63-linked polyubiquitination in dynein-mediated transport, identify parkin as a key regulator in the recruitment of misfolded DJ-1 to aggresomes, and have important implications regarding the biogenesis of Lewy bodies.     

Interaction of the p62 subunit of dynactin with Arp1 and the cortical actin cytoskeleton

Targeting of the minus-end directed microtubule motor cytoplasmic dynein to a wide array of intracellular substrates appears to be mediated by an accessory factor known as dynactin [1-4]. Dynactin is a multi-subunit complex that contains a short actin-related protein 1 (Arp 1) filament with capZ at the barbed end and p62 at the pointed end [5]. The location of the p62 subunit and the proposed role for dynactin as a multifunctional targeting complex raise the possibility of a dual role for p62 in dynein targeting and in Arp1 pointed-end capping. In order to gain further insight into the role of p62 in dynactin function, we have cloned cDNAs that encode two full-length isoforms of the protein from rat brain. We found that p62 is homologous to the nuclear migration protein Ropy-2 from Neurospora [6]; both proteins contain a zinc-binding motif that resembles the LIM domain of several other cytoskeletal proteins [7]. Overexpression of p62 in cultured mammalian cells revealed colocalization with cortical actin, stress fibers, and focal adhesion sites, sites of potential interaction between microtubules and the cell cortex [8,9]. The p62 protein also colocalized with polymers of overexpressed wild-type or barbed-end-mutant Arp1, but not with a pointed-end mutant. Deletion of the LIM domain abolished targeting of p62 to focal-adhesion sites but did not interfere with binding of p62 to actin or Arp1. These data implicate p62 in Arp1 pointed-end binding and suggest additional roles in linking dynein and dynactin to the cortical cytoskeleton.     

Cytoplasmic dynein-mediated assembly of pericentrin and gamma tubulin onto centrosomes

Centrosome assembly is important for mitotic spindle formation and if defective may contribute to genomic instability in cancer. Here we show that in somatic cells centrosome assembly of two proteins involved in microtubule nucleation, pericentrin and gamma tubulin, is inhibited in the absence of microtubules. A more potent inhibitory effect on centrosome assembly of these proteins is observed after specific disruption of the microtubule motor cytoplasmic dynein by microinjection of dynein antibodies or by overexpression of the dynamitin subunit of the dynein binding complex dynactin. Consistent with these observations is the ability of pericentrin to cosediment with taxol-stabilized microtubules in a dynein- and dynactin-dependent manner. Centrosomes in cells with reduced levels of pericentrin and gamma tubulin have a diminished capacity to nucleate microtubules. In living cells expressing a green fluorescent protein-pericentrin fusion protein, green fluorescent protein particles containing endogenous pericentrin and gamma tubulin move along microtubules at speeds of dynein and dock at centrosomes. In Xenopus extracts where gamma tubulin assembly onto centrioles can occur without microtubules, we find that assembly is enhanced in the presence of microtubules and inhibited by dynein antibodies. From these studies we conclude that pericentrin and gamma tubulin are novel dynein cargoes that can be transported to centrosomes on microtubules and whose assembly contributes to microtubule nucleation.