Mutations in the heavy chain of cytoplasmic dynein suppress the nudF nuclear migration mutation of Aspergillus nidulans

To identify proteins that interact directly or indirectly with the NUDF protein, which is required for nuclear migration in Aspergillus nidulans, we initiated a screen for extragenic suppressors of the heat-sensitive nudF6 mutation. Suppressor mutations in at least five genes, designated snfA–snfE, caused improved growth and nuclear migration at high temperatures compared to the nudF6 parent. Two snfC mutations mapped near the nudA gene, which encodes the cytoplasmic dynein heavy chain, and could be repaired by transformation with wild-type nudA DNA, demonstrating that they are mutations in nudA. The snfC mutations are bypass suppressors of nudF and genetic evidence indicated that NUDA and NUDF act in the same nuclear migration pathway. Taken together, our data suggests that NUDF affects nuclear migration by acting on the dynein motor system. Received: 4 January 1997 / Accepted: 26 February 1997     

Extragenic Suppressors of Nudc3, a Mutation That Blocks Nuclear Migration in Aspergillus Nidulans

Nuclear migration plays an important role in the growth and development of many organisms including the filamentous fungus Aspergillus nidulans. We have cloned three genes from A. nidulans, nudA, nudC, and nudF, in which mutations affect nuclear migration. The nudA gene encodes the heavy chain of cytoplasmic dynein. The nudC gene encodes a 22-kD protein. The nudF gene was identified as an extracopy suppressor of the temperature sensitive (ts(-)) nudC3 mutation. The nudC3 mutation substantially decreases the intracellular concentration of the nudF protein at restrictive temperature. This is restored toward the normal level by an extra copy of nudF. To identify other genes whose products interact directly or indirectly with the NUDC protein, we have isolated a set of extragenic suppressors of the nudC3 temperature-sensitive mutation. Genetic analysis of 16 such extragenic suppressors showed them to represent nine different genes, designated sncA-sncI (for suppressor of nudC). sncA-sncH were either dominant or semidominant in diploids homozygous for nudC3 and heterozygous for the snc mutations. All of the suppressors reversed the ts(-) phenotype of nudC3 by restoring the intracellular concentration of the NUDF protein.     

Mutations in the heavy chain of cytoplasmic dynein suppress the nudF nuclear migration mutation of Aspergillus nidulans

To identify proteins that interact directly or indirectly with the NUDF protein, which is required for nuclear migration in Aspergillus nidulans, we initiated a screen for extragenic suppressors of the heat-sensitive nudF6 mutation. Suppressor mutations in at least five genes, designated snfA–snfE, caused improved growth and nuclear migration at high temperatures compared to the nudF6 parent. Two snfC mutations mapped near the nudA gene, which encodes the cytoplasmic dynein heavy chain, and could be repaired by transformation with wild-type nudA DNA, demonstrating that they are mutations in nudA. The snfC mutations are bypass suppressors of nudF and genetic evidence indicated that NUDA and NUDF act in the same nuclear migration pathway. Taken together, our data suggests that NUDF affects nuclear migration by acting on the dynein motor system.     

Roles of NUDE and NUDF proteins of Aspergillus nidulans: insights from intracellular localization and overexpression effects

The NUDF protein of the filamentous fungus Aspergillus nidulans functions in the cytoplasmic dynein pathway. It binds several proteins, including the NUDE protein. Green fluorescent protein-tagged NUDF and NUDA (dynein heavy chain) localize to linearly moving dashes ("comets") that coincide with microtubule ends. Herein, deletion of the nudE gene did not eliminate the comets of NUDF and NUDA, but affected the behavior of NUDA. Comets were also observed with the green fluorescent protein-tagged NUDE and its nonfunctional C-terminal domain. In addition, overexpressed NUDA and NUDE accumulated in specks that were either immobile or bounced randomly. Neither comets nor specks were observed with the functional N-terminal domain of NUDE, indicating that these structures are not essential for NUDE function. Furthermore, NUDF overproduction totally suppressed deletion of the nudE gene. This implies that the function of NUDE is secondary to that of NUDF. Unexpectedly, NUDF overproduction inhibited one conditional nudA mutant and all tested apsA mutants. An allele-specific interaction between the nudF and nudA genes is consistent with a direct interaction between NUDF and dynein heavy chain. Because APSA and its yeast homolog Num1p are cortical proteins, an interaction between the nudF and apsA genes suggests a role for NUDF at the cell cortex.     

An Alpha Tubulin Mutation Suppresses Nuclear Migration Mutations in Aspergillus Nidulans

Microtubules and cytoplasmic dynein, a microtubule-dependent motor, are required for nuclei to move along the hyphae of filamentous fungi. Nuclear migration in Aspergillus nidulans is blocked by heat-sensitive (hs(-)) mutations in the nudA gene, which encodes dynein heavy chain, and the nudF gene, which encodes a G protein β-subunit-like protein. Hs(-) mutations in the nudC and nudG genes also prevent nuclear migration. We have isolated extragenic suppressor mutations that reverse the hs(-) phenotypes caused by these mutations. Here we show that one nudF suppressor also suppresses hs(-) mutations in nudA, nudC, and nudG and deletions in nudA and nudF. This suppressor mutation is in the tubA alpha tubulin gene, and its characteristics suggest that it destabilizes microtubules. The mutation alters microtubule staining and confers sensitivity to cold and benomyl, two treatments that destabilize microtubules. Treatment with low concentrations of benomyl also suppresses the hs(-) nudA, nudC, nudF, and nudG mutations and the nudA and nudF deletions. Suppression of the hs(-) nudA mutation and the nudA deletion is especially interesting because these strains lack active dynein heavy chain. Together, these results suggest that microtubule destabilization allows nuclei to migrate even in the absence of cytoplasmic dynein motor function.     

Labeling of Chlamydomonas 18 S dynein polypeptides by 8-azidoadenosine 5'-triphosphate, a photoaffinity analog of ATP

The 18 S dynein from the outer arm of Chlamydomonas flagella is composed of an alpha subunit containing an alpha heavy chain (Mr = approximately 340,000) and an Mr = 16,000 light chain, and a beta subunit containing a beta heavy chain (Mr = approximately 340,000), two intermediate chains (Mr = 78,000 and 69,000), and seven light chains (Mr = 8,000-20,000). Both subunits contain ATPase activity. We have used 8-azidoadenosine 5'-triphosphate (8-N3 ATP), a photoaffinity analog of ATP, to investigate the ATP-binding sites of intact 18 S dynein. 8-N3ATP is a competitive inhibitor of 18 S dynein's ATPase activity and is itself hydrolyzed by 18 S dynein; moreover, 18 S dynein's hydrolysis of ATP and 8-N3ATP is inhibited by vanadate to the same extent. 8-N3ATP therefore appears to interact with at least one of 18 S dynein's ATP hydrolytic sites in the same way as does ATP. When [alpha- or gamma-32P]8-N3ATP is incubated with 18 S dynein in the presence of UV irradiation, label is incorporated primarily into the alpha, beta, and Mr = 78,000 chains; a much smaller amount is incorporated into the Mr = 69,000 chain. The light chains are not labeled. The incorporation is UV-dependent, ATP-sensitive, and blocked by preincubation of the enzyme with vanadate plus low concentrations of ATP or ADP. These results suggest that the alpha heavy chain contains the site of ATP binding and hydrolysis in the alpha subunit. In the beta subunit, the beta heavy chain and one or both intermediate chains may contain ATP-binding sites.     

Cytoplasmic dynein's mitotic spindle pole localization requires a functional anaphase-promoting complex, gamma-tubulin, and NUDF/LIS1 in Aspergillus nidulans

In Aspergillus nidulans, cytoplasmic dynein and NUDF/LIS1 are found at the spindle poles during mitosis, but they seem to be targeted to this location via different mechanisms. The spindle pole localization of cytoplasmic dynein requires the function of the anaphase-promoting complex (APC), whereas that of NUDF does not. Moreover, although NUDF's localization to the spindle poles does not require a fully functional dynein motor, the function of NUDF is important for cytoplasmic dynein's targeting to the spindle poles. Interestingly, a gamma-tubulin mutation, mipAR63, nearly eliminates the localization of cytoplasmic dynein to the spindle poles, but it has no apparent effect on NUDF's spindle pole localization. Live cell analysis of the mipAR63 mutant revealed a defect in chromosome separation accompanied by unscheduled spindle elongation before the completion of anaphase A, suggesting that gamma-tubulin may recruit regulatory proteins to the spindle poles for mitotic progression. In A. nidulans, dynein is not apparently required for mitotic progression. In the presence of a low amount of benomyl, a microtubule-depolymerizing agent, however, a dynein mutant diploid strain exhibits a more pronounced chromosome loss phenotype than the control, indicating that cytoplasmic dynein plays a role in chromosome segregation.     

NudF, a nuclear migration gene in Aspergillus nidulans, is similar to the human LIS-1 gene required for neuronal migration.

During a study of the genetics of nuclear migration in the filamentous fungus Aspergillus nidulans, we cloned a gene, nudF, which is required for nuclear migration during vegetative growth as well as development. The NUDF protein level is controlled by another protein NUDC, and extra copies of the nudF gene can suppress the nudC3 mutation. nudF encodes a protein with 42% sequence identity to the human LIS-1 (Miller-Dieker lissencephaly-1) gene, which is required for proper neuronal migration during brain development. This strong similarity suggests that the LIS-1 gene product may have a function similar to that of NUDF and supports previous findings to suggest that nuclear migration may play a role in neuronal migration.     

The LIS1-related NUDF protein of Aspergillus nidulans interacts with the coiled-coil domain of the NUDE/RO11 protein

The nudF gene of the filamentous fungus Aspergillus nidulans acts in the cytoplasmic dynein/dynactin pathway and is required for distribution of nuclei. NUDF protein, the product of the nudF gene, displays 42% sequence identity with the human protein LIS1 required for neuronal migration. Haploinsufficiency of the LIS1 gene causes a malformation of the human brain known as lissencephaly. We screened for multicopy suppressors of a mutation in the nudF gene. The product of the nudE gene isolated in the screen, NUDE, is a homologue of the nuclear distribution protein RO11 of Neurospora crassa. The highly conserved NH(2)-terminal coiled-coil domain of the NUDE protein suffices for protein function when overexpressed. A similar coiled-coil domain is present in several putative human proteins and in the mitotic phosphoprotein 43 (MP43) of X. laevis. NUDF protein interacts with the Aspergillus NUDE coiled-coil in a yeast two-hybrid system, while human LIS1 interacts with the human homologue of the NUDE/RO11 coiled-coil and also the Xenopus MP43 coiled-coil. In addition, NUDF coprecipitates with an epitope-tagged NUDE. The fact that NUDF and LIS1 interact with the same protein domain strengthens the notion that these two proteins are functionally related.     

The Aspergillus cytoplasmic dynein heavy chain and NUDF localize to microtubule ends and affect microtubule dynamics

Cytoplasmic dynein is a multisubunit, minus end-directed microtubule motor that uses dynactin as an accessory complex to perform various in vivo functions including vesicle transport, spindle assembly, and nuclear distribution [1]. We previously showed that in the filamentous fungus Aspergillus nidulans, a GFP-tagged cytoplasmic dynein heavy chain (NUDA) forms comet-like structures that exhibited microtubule-dependent movement toward and back from the hyphal tip [2]. Here we demonstrate that another protein in the NUDA pathway, NUDF, which is homologous to the human LIS1 protein involved in brain development [3, 4], also exhibits such dynamic behavior. Both NUDA and NUDF are located at the ends of microtubules, and this observation suggests that the observed dynamic behavior is due to their association with the dynamic microtubule ends. To address whether NUDA and NUDF play a role in regulating microtubule dynamics in vivo, we constructed a GFP-labeled alpha-tubulin strain and used it to compare microtubule dynamics in vivo in wild-type A. nidulans versus temperature-sensitive loss-of-function mutants of nudA and nudF. The mutants showed a lower frequency of microtubule catastrophe, a lower rate of shrinkage during catastrophe, and a lower frequency of rescue. The microtubules in the mutant cells also paused longer at the hyphal tip than wild-type microtubules. These results indicate that cytoplasmic dynein and the LIS1 homolog NUDF affect microtubule dynamics in vivo.     

The LIS1-related protein NUDF of Aspergillus nidulans and its interaction partner NUDE bind directly to specific subunits of dynein and dynactin and to alpha- and gamma-tubulin

The NUDF protein of Aspergillus nidulans, which is required for nuclear migration through the fungal mycelium, closely resembles the LIS1 protein required for migration of neurons to the cerebral cortex in humans. Genetic experiments suggested that NUDF influences nuclear migration by affecting cytoplasmic dynein. NUDF interacts with another protein, NUDE, which also affects nuclear migration in A. nidulans. Interactions among LIS1, NUDE, dynein, and gamma-tubulin have been demonstrated in animal cells. In this paper we examine the interactions of the A. nidulans NUDF and NUDE proteins with components of dynein, dynactin, and with alpha- and gamma-tubulin. We show that NUDF binds directly to alpha- and gamma-tubulin and to the first P-loop of the cytoplasmic dynein heavy chain, whereas NUDE binds directly to alpha- and gamma-tubulin, to NUDK (actin-related protein 1), and to the NUDG dynein LC8 light chain. The data suggest a direct role for NUDF in regulation of the dynein heavy chain and an effect on other dynein/dynactin subunits via NUDE. The interactions between NUDE, NUDF, and gamma-tubulin suggest that this protein may also be involved in the regulation of dynein function. Additive interactions between NUDE and dynein and dynactin subunits suggest that NUDE acts as a scaffolding factor between components.     

Lis1 Acts as a "Clutch" between the ATPase and Microtubule-Binding Domains of the Dynein Motor

The lissencephaly protein Lis1 has been reported to regulate the mechanical behavior of cytoplasmic dynein, the primary minus-end-directed microtubule motor. However, the regulatory mechanism remains poorly understood. Here, we address this issue using purified proteins from Saccharomyces cerevisiae and a combination of techniques, including single-molecule imaging and single-particle electron microscopy. We show that rather than binding to the main ATPase site within dynein's AAA+ ring or its microtubule-binding stalk directly, Lis1 engages the interface between these elements. Lis1 causes individual dynein motors to remain attached to microtubules for extended periods, even during cycles of ATP hydrolysis that would canonically induce detachment. Thus, Lis1 operates like a "clutch" that prevents dynein's ATPase domain from transmitting a detachment signal to its track-binding domain. We discuss how these findings provide a conserved mechanism for dynein functions in living cells that require prolonged microtubule attachments.     

Mutations in cytoplasmic dynein lead to a Huntington's disease-like defect in energy metabolism of brown and white adipose tissues

The molecular motor dynein is regulated by the huntingtin protein, and Huntington's disease (HD) mutations of huntingtin disrupt dynein motor activity. Besides abnormalities in the central nervous system, HD animal models develop prominent peripheral pathology, with defective brown tissue thermogenesis and dysfunctional white adipocytes, but whether this peripheral phenotype is recapitulated by dynein dysfunction is unknown. Here, we observed prominently increased adiposity in mice harboring the legs at odd angles (Loa/+) or the Cramping mutations (Cra/+) in the dynein heavy chain gene. In Cra/+ mice, hyperadiposity occurred in the absence of energy imbalance and was the result of impaired norepinephrine-stimulated lipolysis. A similar phenotype was observed in 3T3L1 adipocytes upon chemical inhibition of dynein showing that loss of functional dynein leads to impairment of lipolysis. Ex vivo, dynein mutant adipose tissue displayed increased reactive oxygen species production that was, at least partially, responsible for the decreased cellular responses to norepinephrine and subsequent defect in stimulated lipolysis. Dynein mutation also affected norepinephrine efficacy to elicit a thermogenic response and led to morphological abnormalities in brown adipose tissue and cold intolerance in dynein mutant mice. Interestingly, protein levels of huntingtin were decreased in dynein mutant adipose tissue. Collectively, our results provide genetic evidence that dynein plays a key role in lipid metabolism and thermogenesis through a modulation of oxidative stress elicited by norepinephrine. This peripheral phenotype of dynein mutant mice is similar to that observed in various animal models of HD, lending further support for a functional link between huntingtin and dynein.     

Autoinhibitory and other autoregulatory elements within the dynein motor domain

The dyneins are a family of microtubule motor proteins. The motor domain, which represents the C-terminal 2/3 of the dynein heavy chain, exhibits homology to the AAA family of ATPases. It consists of a ring of six related but divergent AAA+ units, with two substantial sized protruding projections, the stem, or tail, which anchors the protein to diverse subcellular sites, and the stalk, which binds microtubules. This article reviews recent efforts to probe the mechanism by which the dyneins produce force, and work from the authors' lab regarding long-range conformational regulation of dynein enzymatic activity.     

The gene for the intermediate chain subunit of cytoplasmic dynein is essential in Drosophila

The microtubule motor cytoplasmic dynein powers a variety of intracellular transport events that are essential for cellular and developmental processes. A current hypothesis is that the accessory subunits of the dynein complex are important for the specialization of cytoplasmic dynein function. In a genetic approach to understanding the range of dynein functions and the contribution of the different subunits to dynein motor function and regulation, we have identified mutations in the gene for the cytoplasmic dynein intermediate chain, Dic19C. We used a functional Dic transgene in a genetic screen to recover X-linked lethal mutations that require this transgene for viability. Three Dic mutations were identified and characterized. All three Dic alleles result in larval lethality, demonstrating that the intermediate chain serves an essential function in Drosophila. Like a deficiency that removes Dic19C, the Dic mutations dominantly enhance the rough eye phenotype of Glued(1), a dominant mutation in the gene for the p150 subunit of the dynactin complex, a dynein activator. Additionally, we used complementation analysis to identify an existing mutation, shortwing (sw), as an allele of the dynein intermediate chain gene. Unlike the Dic alleles isolated de novo, shortwing is homozygous viable and exhibits recessive and temperature-sensitive defects in eye and wing development. These phenotypes are rescued by the wild-type Dic transgene, indicating that shortwing is a viable allele of the dynein intermediate chain gene and revealing a novel role for dynein function during wing development.     

Distinct functions of nucleotide-binding/hydrolysis sites in the four AAA modules of cytoplasmic dynein

Cytoplasmic dynein is a microtubule-based motor protein that is responsible for most intracellular retrograde transports along microtubule filaments. The motor domain of dynein contains six tandemly linked AAA (ATPases associated with diverse cellular activities) modules, with the first four containing predicted nucleotide-binding/hydrolysis sites (P1-P4). To dissect the functions of these multiple nucleotide-binding/hydrolysis sites, we expressed and purified Dictyostelium dynein motor domains in which mutations were introduced to block nucleotide binding at each of the four AAA modules, and then examined their detailed biochemical properties. The P1 mutant was trapped in a strong-binding state even in the presence of ATP and lost its motile activity. The P3 mutant also showed a high affinity for microtubules in the presence of ATP and lost most of the microtubule-activated ATPase activity, but retained microtubule sliding activity, although the sliding velocity of the mutant was more than 20-fold slower than that of the wild type. In contrast, mutation in the P2 or P4 site did not affect the apparent binding affinity of the mutant for microtubules in the presence of ATP, but reduced ATPase and microtubule sliding activities. These results indicate that ATP binding and its hydrolysis only at the P1 site are essential for the motor activities of cytoplasmic dynein, and suggest that the other nucleotide-binding/hydrolysis sites regulate the motor activities. Among them, nucleotide binding at the P3 site is not essential but is critical for microtubule-activated ATPase and motile activities of cytoplasmic dynein.     

Regulatory ATPase sites of cytoplasmic dynein affect processivity and force generation

The heavy chain of cytoplasmic dynein contains four nucleotide-binding domains referred to as AAA1-AAA4, with the first domain (AAA1) being the main ATP hydrolytic site. Although previous studies have proposed regulatory roles for AAA3 and AAA4, the role of ATP hydrolysis at these sites remains elusive. Here, we have analyzed the single molecule motility properties of yeast cytoplasmic dynein mutants bearing mutations that prevent ATP hydrolysis at AAA3 or AAA4. Both mutants remain processive, but the AAA4 mutant exhibits a surprising increase in processivity due to its tighter affinity for microtubules. In addition to changes in motility characteristics, AAA3 and AAA4 mutants produce less maximal force than wild-type dynein. These results indicate that the nucleotide binding state at AAA3 and AAA4 can allosterically modulate microtubule binding affinity and affect dynein processivity and force production.     

Dynein-mediated cargo transport in vivo. A switch controls travel distance

Cytoplasmic dynein is a microtubule-based motor with diverse cellular roles. Here, we use mutations in the dynein heavy chain gene to impair the motor's function, and employ biophysical measurements to demonstrate that cytoplasmic dynein is responsible for the minus end motion of bidirectionally moving lipid droplets in early Drosophila embryos. This analysis yields an estimate for the force that a single cytoplasmic dynein exerts in vivo (1.1 pN). It also allows us to quantitate dynein-mediated cargo motion in vivo, providing a framework for investigating how dynein's activity is controlled. We identify three distinct travel states whose general features also characterize plus end motion. These states are preserved in different developmental stages. We had previously provided evidence that for each travel direction, single droplets are moved by multiple motors of the same type (Welte et al. 1998). Droplet travel distances (runs) are much shorter than expected for multiple motors based on in vitro estimates of cytoplasmic dynein processivity. Therefore, we propose the existence of a process that ends runs before the motors fall off the microtubules. We find that this process acts with a constant probability per unit distance, and is typically coupled to a switch in travel direction. A process with similar properties governs plus end motion, and its regulation controls the net direction of transport.     

Redox-based control of the gamma heavy chain ATPase from Chlamydomonas outer arm dynein

The outer dynein arm from Chlamydomonas flagella contains two redox-active thioredoxin-related light chains associated with the alpha and beta heavy chains; these proteins belong to a distinct subgroup within the thioredoxin family. This observation suggested that some aspect of dynein activity might be modulated through redox poise. To test this, we have examined the effect of sulfhydryl oxidation on the ATPase activity of isolated dynein and axonemes from wildtype and mutant strains lacking various heavy chain combinations. The outer, but not inner, dynein arm ATPase was stimulated significantly following treatment with low concentrations of dithionitrobenzoic acid; this effect was readily reversible by dithiol, and to a lesser extent, monothiol reductants. Mutational and biochemical dissection of the outer arm revealed that ATPase activation in response to DTNB was an exclusive property of the gamma heavy chain, and that enzymatic enhancement was modulated by the presence of other dynein components. Furthermore, we demonstrate that the LC5 thioredoxin-like light chain binds to the N-terminal stem domain of the alpha heavy chain and that the beta heavy chain-associated LC3 protein also interacts with the gamma heavy chain. These data suggest the possibility of a dynein-associated redox cascade and further support the idea that the gamma heavy chain plays a key regulatory role within the outer arm.