Interaction mapping of a dynein heavy chain. Identification of dimerization and intermediate-chain binding domains

Patient-specific mutation of Dync1h1 in mice causes brain and behavioral deficits

AIMS

Cytoplasmic dynein heavy chain (DYNC1H1) is a multi-subunit protein complex that provides motor force for movement of cargo on microtubules and traffics them back to the soma. In humans, mutations along the DYNC1H1 gene result in intellectual disabilities, cognitive delays, and neurologic and motor deficits. The aim of the study was to generate a mouse model to a newly identified de novo heterozygous DYNC1H1 mutation, within a functional ATPase domain (c9052C > T(P3018S)), identified in a child with motor deficits, and intellectual disabilities.

RESULTS

P3018S heterozygous (HET) knockin mice are viable; homozygotes are lethal. Metabolic and EchoMRI™ testing show that HET mice have a higher metabolic rate, are more active, and have less body fat compared to wildtype mice. Neurobehavioral studies show that HET mice perform worse when traversing elevated balance beams, and on the negative geotaxis test. Immunofluorescent staining shows neuronal migration abnormalities in the dorsal and lateral neocortex with heterotopia in layer I. Neuron-subtype specific transcription factors CUX1 and CTGF identified neurons from layers II/III and VI respectively in cortical layer I, and abnormal pyramidal neurons with MAP2+ dendrites projecting downward from the pial surface.

CONCLUSION

The HET mice are a good model for the motor deficits seen in the child, and highlights the importance of cytoplasmic dynein in the maintenance of cortical function and dendritic orientation relative to the pial surface. Our results are discussed in the context of other dynein mutant mice and in relation to clinical presentation in humans with DYNC1H1 mutations.

Crystal structure of the stalk region of axonemal inner-arm dynein-d reveals unique features in the coiled-coil and microtubule-binding domain

Axonemal dynein is an ATP-dependent microtubular motor protein responsible for cilia and flagella beating, and its dysfunction can cause diseases such as primary ciliary dyskinesia and sperm dysmotility. Despite its biological importance, structure-based mechanisms underlying axonemal dynein motors remain unclear. Here, we determined the X-ray crystal structure of the human inner-arm dynein-d (DNAH1) stalk region, which contains a long antiparallel coiled-coil and a microtubule-binding domain (MTBD), at 2.7 Å resolution. Notably, differences in the relative orientation of the coiled-coil and MTBD in comparison with other dyneins, as well as the diverse orientations of the MTBD flap region among various isoforms, lead us to propose a 'spike shoe model' with an altered stepping angle for the interaction between IAD-d and microtubules. Based on these findings, we discuss isoform-specific functions of the axonemal dynein stalk MTBDs.

Acquired temozolomide resistance in MGMT-deficient glioblastoma cells is associated with regulation of DNA repair by DHC2

The acquisition of temozolomide resistance is a major clinical challenge for glioblastoma treatment. Chemoresistance in glioblastoma is largely attributed to repair of temozolomide-induced DNA lesions by O6-methylguanine-DNA methyltransferase (MGMT). However, some MGMT-deficient glioblastomas are still resistant to temozolomide, and the underlying molecular mechanisms remain unclear. We found that DYNC2H1 (DHC2) was expressed more in MGMT-deficient recurrent glioblastoma specimens and its expression strongly correlated to poor progression-free survival in MGMT promotor methylated glioblastoma patients. Furthermore, silencing DHC2, both in vitro and in vivo, enhanced temozolomide-induced DNA damage and significantly improved the efficiency of temozolomide treatment in MGMT-deficient glioblastoma. Using a combination of subcellular proteomics and in vitro analyses, we showed that DHC2 was involved in nuclear localization of the DNA repair proteins, namely XPC and CBX5, and knockdown of either XPC or CBX5 resulted in increased temozolomide-induced DNA damage. In summary, we identified the nuclear transportation of DNA repair proteins by DHC2 as a critical regulator of acquired temozolomide resistance in MGMT-deficient glioblastoma. Our study offers novel insights for improving therapeutic management of MGMT-deficient glioblastoma.

Setting the dynein motor in motion: New insights from electron tomography

Dyneins are ATP-fueled macromolecular machines that power all minus-end microtubule-based transport processes of molecular cargo within eukaryotic cells and play essential roles in a wide variety of cellular functions. These complex and fascinating motors have been the target of countless structural and biophysical studies. These investigations have elucidated the mechanism of ATP-driven force production and have helped unravel the conformational rearrangements associated with the dynein mechanochemical cycle. However, despite decades of research, it remains unknown how these molecular motions are harnessed to power massive cellular reorganization and what are the regulatory mechanisms that drive these processes. Recent advancements in electron tomography imaging have enabled researchers to visualize dynein motors in their transport environment with unprecedented detail and have led to exciting discoveries regarding dynein motor function and regulation. In this review, we will highlight how these recent structural studies have fundamentally propelled our understanding of the dynein motor and have revealed some unexpected, unifying mechanisms of regulation.

Whole-exome sequencing of familial cases of multiple morphological abnormalities of the sperm flagella (MMAF) reveals new DNAH1 mutations

STUDY QUESTION

Can whole-exome sequencing (WES) of patients with multiple morphological abnormalities of the sperm flagella (MMAF) identify causal mutations in new genes or mutations in the previously identified dynein axonemal heavy chain 1 (DNAH1) gene?

SUMMARY ANSWER

WES for six families with men affected by MMAF syndrome allowed the identification of DNAH1 mutations in four affected men distributed in two out of the six families but no new candidate genes were identified.

WHAT IS KNOWN ALREADY

Mutations in DNAH1, an axonemal inner dynein arm heavy chain gene, have been shown to be responsible for male infertility due to a characteristic form of asthenozoospermia called MMAF, defined by the presence in the ejaculate of spermatozoa with a mosaic of flagellar abnormalities including absent, coiled, bent, angulated, irregular and short flagella.

STUDY DESIGN, SIZE, DURATION

This was a retrospective genetics study of patients presenting a MMAF phenotype. Patients were recruited in Iran and Italy between 2008 and 2015.

PARTICIPANTS/MATERIALS, SETTING, METHODS

WES was performed for a total of 10 subjects. All identified variants were confirmed by Sanger sequencing. Two additional affected family members were analyzed by direct Sanger sequencing. To establish the prevalence of the DNAH1 mutation identified in an Iranian family, we carried out targeted sequencing on 38 additional MMAF patients of the same geographical origin. RT-PCR and immunochemistry were performed on sperm samples to assess the effect of the identified mutation on RNA and protein.

MAIN RESULTS AND THE ROLE OF CHANCE

WES in six families identified a causal mutations in two families. Two additional affected family members were confirmed to hold the same homozygous mutation as their sibling. In total, DNAH1 mutations were identified in 5 out of 12 analyzed subjects (41.7%). If we only include index cases, we detected two mutated subjects out of six (33%) tested MMAF individuals. Furthermore we sequenced one DNAH1 exon found to be mutated (c.8626-1G > A) in an Iranian family in an additional 38 MMAF patients from Iran. One of these patients carried the variant confirming that this variant is relatively frequent in the Iranian population. The effect of the c.8626-1G > A variant was confirmed by RT-PCR and immunochemistry as no RNA or protein could be observed in sperm from the affected men.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

WES allows the amplification of 80-90% of all coding exons. It is possible that some DNAH1 exons may not have been sequenced and that we may have missed some additional mutations. Also, WES cannot identify deep intronic mutations and it is not efficient for detection of large genomic events (deletions, insertions, inversions). We did not identify any causal mutations in DNAH1 or in other candidate genes in four out of the six tested families. This indicates that the technique and/or the analysis of our data can be improved to increase the diagnosis efficiency.

WIDER IMPLICATIONS OF THE FINDINGS

Our findings confirm that DNAH1 is one of the main genes involved in MMAF syndrome. It is a large gene with 78 exons making it challenging and expensive to sequence using the traditional Sanger sequencing methods. We show that WES sequencing is good alternative to Sanger sequencing to reach a genetic diagnosis in patients with severe male infertility phenotypes.

STUDY FUNDING/COMPETING INTERESTS

This work was supported by following grants: the 'MAS-Flagella' project financed by the French ANR and the DGOS for the program PRTS 2014 and the 'Whole genome sequencing of patients with Flagellar Growth Defects (FGD)' project financed by the Fondation Maladies Rares for the program Séquençage à haut débit 2012. The authors have no conflict of interest.

Live cell imaging reveals differential modifications to cytoplasmic dynein properties by phospho- and dephosphomimic mutations of the intermediate chain 2C S84

Cytoplasmic dynein is a multisubunit motor protein responsible for intracellular cargo transport toward microtubule minus ends. There are multiple isoforms of the dynein intermediate chain (DYNC1I, IC), which is encoded by two genes. One way to regulate cytoplasmic dynein is by IC phosphorylation. The IC-2C isoform is expressed in all cells, and the functional significance of phosphorylation on IC-2C serine 84 was investigated by using live cell imaging of fluorescent protein-tagged IC-2C wild type (WT) and phospho- and dephosphomimic mutant isoforms in axonal transport model systems. Both mutations modulated dynein functional properties. The dephosphomimic mutant IC-2C S84A had greater colocalization with mitochondria than the IC-2C WT or the phosphomimic mutant IC-2C S84D. The dephosphomimic mutant IC-2C S84A was also more likely to be motile than the phosphomimic mutant IC-2C S84D or the IC-2C WT. In contrast, the phosphomimic mutant IC-2C S84D mutant was more likely to move in the retrograde direction than was the IC-2C S84A mutant. The phosphomimic IC-2C S84D was also as likely as the IC-2C WT to colocalize with mitochondria. Both the S84D phospho- and the S84A dephosphomimic mutants were found to be capable of microtubule minus-end-directed (retrograde) movement in axons. They were also observed to be passively transported in the anterograde direction. These data suggest that the IC-2C S84 has a role in modulating dynein properties.

Axonal transport and neurodegenerative disease: vesicle-motor complex formation and their regulation

The process of axonal transport serves to move components over very long distances on microtubule tracks in order to maintain neuronal viability. Molecular motors - kinesin and dynein - are essential for the movement of neuronal cargoes along these tracks; defects in this pathway have been implicated in the initiation or progression of some neurodegenerative diseases, suggesting that this process may be a key contributor in neuronal dysfunction. Recent work has led to the identification of some of the motor-cargo complexes, adaptor proteins, and their regulatory elements in the context of disease proteins. In this review, we focus on the assembly of the amyloid precursor protein, huntingtin, mitochondria, and the RNA-motor complexes and discuss how these may be regulated during long-distance transport in the context of neurodegenerative disease. As knowledge of these motor-cargo complexes and their involvement in axonal transport expands, insight into how defects in this pathway contribute to the development of neurodegenerative diseases becomes evident. Therefore, a better understanding of how this pathway normally functions has important implications for early diagnosis and treatment of diseases before the onset of disease pathology or behavior.

Mutations in DNAH1, which encodes an inner arm heavy chain dynein, lead to male infertility from multiple morphological abnormalities of the sperm flagella

Ten to fifteen percent of couples are confronted with infertility and a male factor is involved in approximately half the cases. A genetic etiology is likely in most cases yet only few genes have been formally correlated with male infertility. Homozygosity mapping was carried out on a cohort of 20 North African individuals, including 18 index cases, presenting with primary infertility resulting from impaired sperm motility caused by a mosaic of multiple morphological abnormalities of the flagella (MMAF) including absent, short, coiled, bent, and irregular flagella. Five unrelated subjects out of 18 (28%) carried a homozygous variant in DNAH1, which encodes an inner dynein heavy chain and is expressed in testis. RT-PCR, immunostaining, and electronic microscopy were carried out on samples from one of the subjects with a mutation located on a donor splice site. Neither the transcript nor the protein was observed in this individual, confirming the pathogenicity of this variant. A general axonemal disorganization including mislocalization of the microtubule doublets and loss of the inner dynein arms was observed. Although DNAH1 is also expressed in other ciliated cells, infertility was the only symptom of primary ciliary dyskinesia observed in affected subjects, suggesting that DNAH1 function in cilium is not as critical as in sperm flagellum.

Cytoplasmic dynein heavy chain: the servant of many masters

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The cytoplasmic dynein complex is the main retrograde motor in all eukaryotic cells.

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This complex is built around a dimer of cytoplasmic dynein heavy chains (DYNC1H1).

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Mouse DYNC1H1 mutants have sensory defects, but motor defects have been controversial.

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Now human DYNC1H1 mutations with sensory, motor, and cognitive deficits are being found.

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The study of these mutations will give us new insight into DYNC1H1 function in the nervous system.

Cytoplasmic dynein is the main retrograde motor in all eukaryotic cells. This complex comprises different subunits assembled on a cytoplasmic dynein heavy chain 1 (DYNC1H1) dimer. Cytoplasmic dynein is particularly important for neurons because it carries essential signals and organelles from distal sites to the cell body. In the past decade, several mouse models have helped to dissect the numerous functions of DYNC1H1. Additionally, several DYNC1H1 mutations have recently been found in human patients that give rise to a broad spectrum of developmental and midlife-onset disorders. Here, we discuss the effects of mutations of mouse and human DYNC1H1 and how these studies are giving us new insight into the many critical roles DYNC1H1 plays in the nervous system.

Establishing a novel knock-in mouse line for studying neuronal cytoplasmic dynein under normal and pathologic conditions

Cytoplasmic dynein plays important roles in mitosis and the intracellular transport of organelles, proteins, and mRNAs. Dynein function is particularly critical for survival of neurons, as mutations in dynein are linked to neurodegenerative diseases. Dynein function is also implicated in neuronal regeneration, driving the active transport of signaling molecules following injury of peripheral neurons. To enhance our understanding of dynein function and regulation in neurons, we established a novel knock-in mouse line in which the neuron-specific cytoplasmic dynein 1 intermediate chain 1 (IC-1) is tagged with both GFP and a 3xFLAG tag at its C-terminus. The fusion gene is under the control of IC-1's endogenous promoter and is integrated at the endogenous locus of the IC-1-encoding gene Dync1i1. The IC-1-GFP-3xFLAG fusion protein is incorporated into the endogenous dynein complex, and movements of GFP-labeled dynein expressed at endogenous levels can be observed in cultured neurons for the first time. The knock-in mouse line also allows isolation and analysis of dynein-bound proteins specifically from neurons. Using this mouse line we have found proteins, including 14-3-3 zeta, which physically interact with dynein upon injury of the brain cortex. Thus, we have created a useful tool for studying dynein function in the central nervous system under normal and pathologic conditions.

Identification of a novel site in the tail of dynein heavy chain important for dynein function in vivo

The minus end-directed microtubule motor cytoplasmic dynein is responsible for the intracellular movements of many organelles, including nuclei and endosomes. The dynein heavy chain contains a C-terminal motor domain and an N-terminal tail domain. The tail binds other dynein subunits and the cargo-interacting dynactin complex but is dispensable for movement of single dynein molecules in vitro. Here, we identified a mutation in the Aspergillus nidulans heavy chain tail domain, nudA(F208V), which causes obvious defects in dynein-mediated nuclear positioning and early endosome movement. Astonishingly, the nudA(F208I) mutation in the same position does not cause the same defects, suggesting that a subtle difference in the size of the amino acid side chain at this position has a significant consequence. Importantly, our biochemical analyses indicate that the nudA(F208V) mutation does not affect dynein subunit interactions and the mutant dynein is also able to bind dynactin and another dynein regulator, NUDF/LIS1. The mutant dynein is able to physically interact with the early endosome cargo, but dynein-mediated early endosome movement away from the hyphal tip occurs at a significantly reduced frequency. Within the small group of early endosomes that move away from the hyphal tip in the mutant, the average speed of movement is lower than that in the wild type. Given the dispensability of the dynein tail in dynein motility in vitro, our results support the notion that the structural integrity of the dynein tail is critical in vivo for the coordination of dynein force production and movement when the motor is heavily loaded.

Coarse-grained modeling of the structural states and transition underlying the powerstroke of dynein motor domain

This study aims to model a minimal dynein motor domain capable of motor function, which consists of the linker domain, six AAA+ modules (AAA1-AAA6), coiled coil stalk, and C-terminus domain. To this end, we have used the newly solved X-ray structures of dynein motor domain to perform a coarse-grained modeling of dynein's post- and pre-powerstroke conformation and the conformational transition between them. First, we have used normal mode analysis to identify a single normal mode that captures the coupled motions of AAA1-AAA2 closing and linker domain rotation, which enables the ATP-driven recovery stroke of dynein. Second, based on the post-powerstroke conformation solved crystallographically, we have modeled dynein's pre-powerstroke conformation by computationally inducing AAA1-AAA2 closing and sliding of coiled coil stalk, and the resulting model features a linker domain near the pre-powerstroke position and a slightly tilted stalk. Third, we have modeled the conformational transition from pre- to post-powerstroke conformation, which predicts a clear sequence of structural events that couple microtubule binding, powerstroke and product release, and supports a signaling path from stalk to AAA1 via AAA3 and AAA4. Finally, we have found that a closed AAA3-AAA4 interface (compatible with nucleotide binding) is essential to the mechano-chemical coupling in dynein. Our modeling not only offers unprecedented structural insights to the motor function of dynein as described by past single-molecule, fluorescence resonance energy transfer, and electron microscopy studies, but also provides new predictions for future experiments to test.

A mouse neurodegenerative dynein heavy chain mutation alters dynein motility and localization in Neurospora crassa

Cytoplasmic dynein is responsible for the transport and delivery of cargoes in organisms ranging from humans to fungi. Dysfunction of dynein motor machinery due to mutations in dynein or its activating complex dynactin can result in one of several neurological diseases in mammals. The mouse Legs at odd angles (Loa) mutation in the tail domain of the dynein heavy chain has been shown to lead to progressive neurodegeneration in mice. The mechanism by which the Loa mutation affects dynein function is just beginning to be understood. In this work, we generated the dynein tail mutation observed in Loa mice into the Neurospora crassa genome and utilized cell biological and complementing biochemical approaches to characterize how that tail mutation affected dynein function. We determined that the Loa mutation exhibits several subtle defects upon dynein function in N. crassa that were not seen in mice, including alterations in dynein localization, impaired velocity of vesicle transport, and in the biochemical properties of purified motors. Our work provides new information on the role of the tail domain on dynein function and points out areas of future research that will be of interest to pursue in mammalian systems.

Analyses of dynein heavy chain mutations reveal complex interactions between dynein motor domains and cellular dynein functions

Cytoplasmic dynein transports cargoes for a variety of crucial cellular functions. However, since dynein is essential in most eukaryotic organisms, the in-depth study of the cellular function of dynein via genetic analysis of dynein mutations has not been practical. Here, we identify and characterize 34 different dynein heavy chain mutations using a genetic screen of the ascomycete fungus Neurospora crassa, in which dynein is nonessential. Interestingly, our studies show that these mutations segregate into five different classes based on the in vivo localization of the mutated dynein motors. Furthermore, we have determined that the different classes of dynein mutations alter vesicle trafficking, microtubule organization, and nuclear distribution in distinct ways and require dynactin to different extents. In addition, biochemical analyses of dynein from one mutant strain show a strong correlation between its in vitro biochemical properties and the aberrant intracellular function of that altered dynein. When the mutations were mapped to the published dynein crystal structure, we found that the three-dimensional structural locations of the heavy chain mutations were linked to particular classes of altered dynein functions observed in cells. Together, our data indicate that the five classes of dynein mutations represent the entrapment of dynein at five separate points in the dynein mechanochemical and transport cycles. We have developed N. crassa as a model system where we can dissect the complexities of dynein structure, function, and interaction with other proteins with genetic, biochemical, and cell biological studies.

DYNLT3 is required for chromosome alignment during mouse oocyte meiotic maturation

Dynein light chain, Tctex-type 3 (DYNLT3), is a member of the cytoplasmic dynein DYNLT light chain family and has been reported to have a potential role in chromosome congression in human mitosis. However, its role in mammalian meiosis is unclear. In this study, we examined its localization, expression, and functions in mouse oocyte meiosis. Immunofluorescent staining showed that DYNLT3 was restricted to the germinal vesicle and associated with kinetochores at the germinal vesicle breakdown stage, metaphase I and metaphase II. The expression level of DYNLT3 was similar at all meiotic stages. Depletion of DYNLT3 by antibody injection resulted in chromosome misalignment and decrease of the polar body extrusion rate. We further found that DYNLT3-depleted oocytes displayed kinetochore-microtubule detachments. Chromosome-spread experiments showed that depletion of DYNLT3 inhibited the metaphase-anaphase transition by preventing homologous chromosome segregation in meiosis I. Our data suggest that DYNLT3 is required for chromosome alignment and homologous chromosome segregation during mouse oocyte meiosis.

Functional interaction between dynein light chain and intermediate chain is required for mitotic spindle positioning

Cytoplasmic dynein is a large multisubunit complex involved in retrograde transport and the positioning of various organelles. Dynein light chain (LC) subunits are conserved across species; however, the molecular contribution of LCs to dynein function remains controversial. One model suggests that LCs act as cargo-binding scaffolds. Alternatively, LCs are proposed to stabilize the intermediate chains (ICs) of the dynein complex. To examine the role of LCs in dynein function, we used Saccharomyces cerevisiae, in which the sole function of dynein is to position the spindle during mitosis. We report that the LC8 homologue, Dyn2, localizes with the dynein complex at microtubule ends and interacts directly with the yeast IC, Pac11. We identify two Dyn2-binding sites in Pac11 that exert differential effects on Dyn2-binding and dynein function. Mutations disrupting Dyn2 elicit a partial loss-of-dynein phenotype and impair the recruitment of the dynein activator complex, dynactin. Together these results indicate that the dynein-based function of Dyn2 is via its interaction with the dynein IC and that this interaction is important for the interaction of dynein and dynactin. In addition, these data provide the first direct evidence that LC occupancy in the dynein motor complex is important for function.

Identification of the dynein light chains required for human papillomavirus infection

Human papillomaviruses (HPVs) are a family of small non-enveloped DNA viruses. Some genital HPV types, including HPV type 16 (HPV16), are the causative agent for the development of cancer at the site of infection. HPVs encode two capsid proteins, L1 and L2. After endocytic cell entry and egress from endosomes, L2 accompanies the viral DNA to the nucleus where replication is initiated. For cytoplasmic transport, L2 interacts with the microtubule network via the motor protein complex dynein. We have performed yeast two-hybrid screening and identified the dynein light chain DYNLT1 (previously called Tctex1) as interaction partner of HPV16 L2. Using co-immunoprecipitation and immunofluorescence colocalization studies we confirmed the L2-DYNLT1 interaction in mammalian cells. Further studies revealed that DYNLT3, the second member of the Tctex-light chain family, also interacts with L2 in vitro and in vivo, whereas other constituents of the dynein complex were not found to associate with L2. Depletion of DYNLT1 and DYNLT3 by specific siRNAs or cytosolic delivery of light chain-specific antibodies inhibited infection of HPV16. Therefore, this work identified two host cell proteins involved in HPV16 infection that are most likely required for transport purposes towards the nucleus.

Direct role of dynein motor in stable kinetochore-microtubule attachment, orientation, and alignment

Cytoplasmic dynein has been implicated in diverse mitotic functions, several involving its association with kinetochores. Much of the supporting evidence comes from inhibition of dynein regulatory factors. To obtain direct insight into kinetochore dynein function, we expressed a series of dynein tail fragments, which we find displace motor-containing dynein heavy chain (HC) from kinetochores without affecting other subunits, regulatory factors, or microtubule binding proteins. Cells with bipolar mitotic spindles progress to late prometaphase-metaphase at normal rates. However, the dynein tail, dynactin, Mad1, and BubR1 persist at the aligned kinetochores, which is consistent with a role for dynein in self-removal and spindle assembly checkpoint inactivation. Kinetochore pairs also show evidence of misorientation relative to the spindle equator and abnormal oscillatory behavior. Further, kinetochore microtubule bundles are severely destabilized at reduced temperatures. Dynein HC RNAi and injection of anti-dynein antibody in MG132-arrested metaphase cells produced similar effects. These results identify a novel function for the dynein motor in stable microtubule attachment and maintenance of kinetochore orientation during metaphase chromosome alignment.

Molecular mechanism of force generation by dynein, a molecular motor belonging to the AAA+ family

Dynein is an AAA+ (ATPase associated with various cellular activities)-type motor complex that utilizes ATP hydrolysis to actively drive microtubule sliding. The dynein heavy chain (molecular mass >500 kDa) contains six tandemly linked AAA+ modules and exhibits full motor activities. Detailed molecular dissection of this motor with unique architecture was hampered by the lack of an expression system for the recombinant heavy chain, as a result of its large size. However, the recent success of recombinant protein expression with full motor activities has provided a method for advances in structure-function studies in order to elucidate the molecular mechanism of force generation.

Interaction of the DYNLT (TCTEX1/RP3) light chains and the intermediate chains reveals novel intersubunit regulation during assembly of the dynein complex

The cytoplasmic dynein 1 cargo binding domain is formed by five subunits including the intermediate chain and the DYNLT, DYNLL, and DYNLRB light chain families. Six isoforms of the intermediate chain and two isoforms of each of the light chain families have been identified in mammals. There is evidence that different subunit isoforms are involved in regulating dynein function, in particular linking dynein to different cargoes. However, it is unclear how the subunit isoforms are assembled or if there is any specificity to their interactions. Co-immunoprecipitation using DYNLT-specific antibodies reveals that dynein complexes with DYNLT light chains also contain the DYNLL and DYNLRB light chains. The DYNLT light chains, but not DYNLL light chains, associate exclusively with the dynein complex. Yeast two-hybrid and co-immunoprecipitation assays demonstrate that both members of the DYNLT family are capable of forming homodimers and heterodimers. In addition, both homodimers of the DYNLT family bind all six intermediate chain isoforms. However, DYNLT heterodimers do not bind to the intermediate chain. Thus, whereas all combinations of DYNLT light chain dimers can be made, not all of the possible combinations of the isoforms are utilized during the assembly of the dynein complex.

The coordination of cyclic microtubule association/dissociation and tail swing of cytoplasmic dynein

The dynein motor domain is composed of a tail, head, and stalk and is thought to generate a force to microtubules by swinging the tail against the head during its ATPase cycle. For this "power stroke," dynein has to coordinate the tail swing with microtubule association/dissociation at the tip of the stalk. Although a detailed picture of the former process is emerging, the latter process remains to be elucidated. By using the single-headed recombinant motor domain of Dictyostelium cytoplasmic dynein, we address the questions of how the interaction of the motor domain with a microtubule is modulated by ATPase steps, how the two mechanical cycles (the microtubule association/dissociation and tail swing) are coordinated, and which ATPase site among the multiple sites in the motor domain regulates the coordination. Based on steady-state and pre-steady-state measurements, we demonstrate that the two mechanical cycles proceed synchronously at most of the intermediate states in the ATPase cycle: the motor domain in the poststroke state binds strongly to the microtubule with a K(d) of approximately 0.2 microM, whereas most of the motor domains in the prestroke state bind weakly to the microtubule with a K(d) of >10 microM. However, our results suggest that the timings of the microtubule affinity change and tail swing are staggered at the recovery stroke step in which the tail swings from the poststroke to the prestroke position. The ATPase site in the AAA1 module of the motor domain was found to be responsible for the coordination of these two mechanical processes.

AAA+ ATPases: Achieving Diversity of Function with Conserved Machinery

AAA+ adenosine triphosphatases (ATPases) are molecular machines that perform a wide variety of cellular functions. For instance, they can act in vesicle transport, organelle assembly, membrane dynamics and protein unfolding. In most cases, the ATPase domains of these proteins assemble into active ring-shaped hexamers. As AAA+ proteins have a common structure, a central issue is determining how they use conserved mechanistic principles to accomplish specific biological actions. Here, we review the features and motifs that partially define AAA+ domains, describe the cellular activities mediated by selected AAA+ proteins and discuss the recent work, suggesting that various AAA+ machines with very different activities employ a common core mechanism. The importance of this mechanism to human health is demonstrated by the number of genetic diseases caused by mutant AAA+ proteins.

Structural and thermodynamic characterization of a cytoplasmic dynein light chain-intermediate chain complex

Cytoplasmic dynein is a microtubule-based motor protein complex that plays important roles in a wide range of fundamental cellular processes, including vesicular transport, mitosis, and cell migration. A single major form of cytoplasmic dynein associates with membranous organelles, mitotic kinetochores, the mitotic and migratory cell cortex, centrosomes, and mRNA complexes. The ability of cytoplasmic dynein to recognize such diverse forms of cargo is thought to be associated with its several accessory subunits, which reside at the base of the molecule. The dynein light chains (LCs) LC8 and TcTex1 form a subcomplex with dynein intermediate chains, and they also interact with numerous protein and ribonucleoprotein partners. This observation has led to the hypothesis that these subunits serve to tether cargo to the dynein motor. Here, we present the structure and a thermodynamic analysis of a complex of LC8 and TcTex1 associated with their intermediate chain scaffold. The intermediate chains effectively block the major putative cargo binding sites within the light chains. These data suggest that, in the dynein complex, the LCs do not bind cargo, in apparent disagreement with a role for LCs in dynein cargo binding interactions.

Kinetic characterization of tail swing steps in the ATPase cycle of Dictyostelium cytoplasmic dynein

According to the power stroke model of dynein deduced from electron microscopic and fluorescence resonance energy transfer studies, the power stroke and the recovery stroke are expected to take place at the two isomerization steps of the ATPase cycle at the primary ATPase site. Here, we have conducted presteady-state kinetic analyses of these two isomerization steps with the single-headed motor domain of Dictyostelium cytoplasmic dynein by employing fluorescence resonance energy transfer to probe ATPase steps at the primary site and tail positions. Our results show that the recovery stroke at the first isomerization step proceeds quickly ( approximately 180 s(-1)), whereas the power stroke at the second isomerization step is very slow ( approximately 0.2 s(-1)) in the absence of microtubules, and that the presence of microtubules accelerates the second but not the first step. Moreover, a comparison of the microtubule-induced acceleration of the power stroke step and that of steady-state ATP hydrolysis implies the intriguing possibility that microtubules simultaneously accelerate the ATPase activity not only at the primary site but also at other site(s) in the motor domain.

Point mutations in the stem region and the fourth AAA domain of cytoplasmic dynein heavy chain partially suppress the phenotype of NUDF/LIS1 loss in Aspergillus nidulans

Cytoplasmic dynein performs multiple cellular tasks but its regulation remains unclear. The dynein heavy chain has a N-terminal stem that binds to other subunits and a C-terminal motor unit that contains six AAA (ATPase associated with cellular activities) domains and a microtubule-binding site located between AAA4 and AAA5. In Aspergillus nidulans, NUDF (a LIS1 homolog) functions in the dynein pathway, and two nudF6 partial suppressors were mapped to the nudA dynein heavy chain locus. Here we identified these two mutations. The nudAL1098F mutation resides in the stem region, and nudAR3086C is in the end of AAA4. These mutations partially suppress the phenotype of nudF deletion but do not suppress the phenotype exhibited by mutants of dynein intermediate chain and Arp1. Surprisingly, the stronger DeltanudF suppressor, nudAR3086C, causes an obvious decrease in the basal level of dynein's ATPase activity and an increase in dynein's distribution along microtubules. Thus, suppression of the DeltanudF phenotype may result from mechanisms other than simply the enhancement of dynein's ATPase activity. The fact that a mutation in the end of AAA4 negatively regulates dynein's ATPase activity but partially compensates for NUDF loss indicates the importance of the AAA4 domain in dynein regulation in vivo.

Two modes of microtubule sliding driven by cytoplasmic dynein

Dynein is a huge multisubunit microtubule (MT)-based motor, whose motor domain resides in the heavy chain. The heavy chain comprises a ring of six AAA (ATPases associated with diverse cellular activities) modules with two slender protruding domains, the tail and stalk. It has been proposed that during the ATP hydrolysis cycle, this tail domain swings against the AAA ring as a lever arm to generate the power stroke. However, there is currently no direct evidence to support the model that the tail swing is tightly linked to dynein motility. To address the question of whether the power stroke of the tail drives MT sliding, we devised an in vitro motility assay using genetically biotinylated cytoplasmic dyneins anchored on a glass surface in the desired orientation with a biotin-streptavidin linkage. Assays on the dyneins with the site-directed biotin tag at eight different locations provided evidence that robust MT sliding is driven by the power stroke of the tail. Furthermore, the assays revealed slow MT sliding independent of dynein orientation on the glass surface, which is mechanically distinct from the sliding driven by the power stroke of the tail.

Genetic analysis of the cytoplasmic dynein subunit families

Cytoplasmic dyneins, the principal microtubule minus-end-directed motor proteins of the cell, are involved in many essential cellular processes. The major form of this enzyme is a complex of at least six protein subunits, and in mammals all but one of the subunits are encoded by at least two genes. Here we review current knowledge concerning the subunits, their interactions, and their functional roles as derived from biochemical and genetic analyses. We also carried out extensive database searches to look for new genes and to clarify anomalies in the databases. Our analysis documents evolutionary relationships among the dynein subunits of mammals and other model organisms, and sheds new light on the role of this diverse group of proteins, highlighting the existence of two cytoplasmic dynein complexes with distinct cellular roles.

Head-head coordination is required for the processive motion of cytoplasmic dynein, an AAA+ molecular motor

Cytoplasmic dynein is an AAA(+)-type molecular motor whose major components are two identical heavy chains containing six AAA(+) modules in tandem. It moves along a single microtubule in multiple steps which are accompanied with multiple ATP hydrolysis. This processive sliding is crucial for cargo transports in vivo. To examine how cytoplasmic dynein exhibits this processivity, we performed in vitro motility assays of two-headed full-length or truncated single-headed heavy chains. The results indicated that four to five molecules of the single-headed heavy chain were required for continuous microtubule sliding, while approximately one molecule of the two-headed full-length heavy chain was enough for the continuous sliding. The ratio of the stroking time to the total ATPase cycle time, which is a quantitative indicator of the processivity, was approximately 0.2 for the single-headed heavy chain, while it was approximately 0.6 for the full-length molecule. When two single-headed heavy chains were artificially linked by a coiled-coil of myosin, the processivity was restored. These results suggest that the two heads of a single cytoplasmic dynein communicate with each other to take processive steps along a microtubule.

Identification of a Novel Region of the Cytoplasmic Dynein Intermediate Chain Important for Dimerization in the Absence of the Light Chains

Cytoplasmic dynein is the multisubunit protein complex responsible for many microtubule-based intracellular movements. Its cargo binding domain consists of dimers of five subunits: the intermediate chains, the light intermediate chains, and the Tctex1, Roadblock, and LC8 light chains. The intermediate chains have a key role in the dynein complex. They bind the three light chains and the heavy chains, which contain the motor domains, but little is known about how the two intermediate chains interact. There are six intermediate chain isoforms, and it has been hypothesized that different isoforms may regulate specific dynein functions. However, there are little data on the potential combinations of the intermediate chain isoforms in the dynein complexes. We used co-immunoprecipitation analyses to demonstrate that all combinations of homo- and heterodimers of the six intermediate chains are possible. Therefore the formation of dynein complexes with different combinations of isoforms is not limited by interaction between the various intermediate chains. We further sought to identify the domain necessary for the dimerization of the intermediate chains. Analysis of a series of truncation and deletion mutants showed that a 61-amino-acid region is necessary for dimerization of the intermediate chain. This region does not include the N-terminal coiled-coil, the C-terminal WD repeat domain, or the three different binding sites for the Tctex1, LC8, and Roadblock light chains. Analytical gel filtration and covalent cross-linking of purified recombinant polypeptides further demonstrated that the intermediate chains can dimerize in vitro in the absence of the light chains.

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.

ATP hydrolysis cycle-dependent tail motions in cytoplasmic dynein

The motor protein dynein is predicted to move the tail domain, a slender rod-like structure, relative to the catalytic head domain to carry out its power stroke. Here, we investigated ATP hydrolysis cycle-dependent conformational dynamics of dynein using fluorescence resonance energy transfer analysis of the dynein motor domain labeled with two fluorescent proteins. We show that dynein adopts at least two conformational states (states I and II), and the tail undergoes ATP-induced motions relative to the head domain during transitions between the two states. Our measurements also suggest that in the course of the ATP hydrolysis cycle of dynein, the tail motion from state I to state II takes place in the ATP-bound state, whereas the motion from state II to state I occurs in the ADP-bound state. The latter tail motion may correspond to the predicted power stroke of dynein.

Primary ciliary dyskinesia: clinical presentation, diagnosis and genetics

Primary ciliary dyskinesia (PCD) is a phenotypically and genetically heterogeneous disorder with an autosomal-recessive inheritance pattern. Only rarely other modes of inheritance such as X-linked transmission are observed. The disease phenotype is caused by defects of respiratory cilia, sperm tails and the cilia of the embryonic node. The lack of mucociliary clearance contributes to recurrent respiratory tract infections, that might progress to permanent lung damage (bronchiectasis). The goal of therapy is prevention of bronchiectasis. Male infertility due to sperm tail dysmotility is another frequent finding in PCD. Half of affected individuals have situs inversus (Kartagener's syndrome) due to randomization of left/right body asymmetry. Currently three genes (DNAI1, DNAH5, DNAH11) that encode for dynein proteins have been linked to recessive PCD. Mutations in RPGR located on the X chromosome have been identified in males with retinitis pigmentosa and PCD. As a screening test nasal nitric oxide (NO) measurement is widely used. Establishment of diagnosis currently relies on electron microscopy, direct evaluation of ciliary beat by light microscopy, and/or the novel method of high-resolution immunofluorecent analysis of respiratory cilia.

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.

Dynein: An ancient motor protein involved in multiple modes of transport

Cytoplasmic dynein has long been thought to be responsible for retrograde axonal transport. As the number of cellular roles for this multifunctional protein has expanded, the complexity of its contribution to axonal transport has increased. In this article the increasing evidence for a role for cytoplasmic dynein in anterograde as well as retrograde transport is discussed. The current status of the complex dynein cargo-binding mechanism is evaluated. Finally, recent genetic evidence supporting a role in axonal transport and revealing a role in neurodegenerative conditions is reviewed.

Analysis of the dynein-dynactin interaction in vitro and in vivo

Cytoplasmic dynein and dynactin are megadalton-sized multisubunit molecules that function together as a cytoskeletal motor. In the present study, we explore the mechanism of dynein-dynactin binding in vitro and then extend our findings to an in vivo context. Solution binding assays were used to define binding domains in the dynein intermediate chain (IC) and dynactin p150Glued subunit. Transient overexpression of a series of fragments of the dynein IC was used to determine the importance of this subunit for dynein function in mammalian tissue culture cells. Our results suggest that a functional dynein-dynactin interaction is required for proper microtubule organization and for the transport and localization of centrosomal components and endomembrane compartments. The dynein IC fragments have different effects on endomembrane localization, suggesting that different endomembranes may bind dynein via distinct mechanisms.

Novel role of cytoplasmic dynein motor in maintenance of the nuclear number in conidia through organized conidiation in Aspergillus oryzae

Cytoplasmic dynein is a minus-end-directed, microtubule-dependent motor protein complex. DhcA, cytoplasmic dynein heavy chain in Aspergillus oryzae, contained four P-loops involved in ATP binding which were conserved as in cytoplasmic dynein heavy chains of other organisms. The amino acid sequence of A. oryzae DhcA was similar to cytoplasmic dynein heavy chains from other organisms except for the N-terminus of Saccharomyces cerevisiae Dyn1. Disruption of dhcA gene in the region encoding four P-loop motifs resulted in a defective growth and perturbed distribution of nuclei and vacuoles. The dhcA disruptant exhibited an abnormal morphology of conidial heads and conidia with an increased nuclear number. The present study implicates a novel role of cytoplasmic dynein in maintenance of the nuclear number in conidia through an organized conidiation.

Mutations in dynein link motor neuron degeneration to defects in retrograde transport

Degenerative disorders of motor neurons include a range of progressive fatal diseases such as amyotrophic lateral sclerosis (ALS), spinal-bulbar muscular atrophy (SBMA), and spinal muscular atrophy (SMA). Although the causative genetic alterations are known for some cases, the molecular basis of many SMA and SBMA-like syndromes and most ALS cases is unknown. Here we show that missense point mutations in the cytoplasmic dynein heavy chain result in progressive motor neuron degeneration in heterozygous mice, and in homozygotes this is accompanied by the formation of Lewy-like inclusion bodies, thus resembling key features of human pathology. These mutations exclusively perturb neuron-specific functions of dynein.

The roadblock light chain binds a novel region of the cytoplasmic Dynein intermediate chain

Cytoplasmic dynein is the major minus-end directed microtubule-based motor in eukaryotic cells. It is composed of a number of different subunits including three light chain families: Tctex1, LC8, and roadblock. The incorporation of the roadblock light chains into the cytoplasmic dynein complex had not been determined. There are two roadblock genes in mammals, ROBL-1 and ROBL-2. We find that both members of the roadblock family bind directly to all of the intermediate chain isoforms of mammalian cytoplasmic dynein. This was determined with three complementary approaches. A yeast two-hybrid assay demonstrated that both roadblock light chains interact with intermediate chain isoforms from the IC74-1 and IC74-2 genes in vivo. This was confirmed in vitro with both a solid phase blot overlay assay and a solution-binding assay. The roadblock-binding domain on the intermediate chain was mapped to an approximately 72 residue region. The binding domain is downstream of each of the two alternative splice sites in the intermediate chains. This location is consistent with the finding that both roadblock-1 and roadblock-2 show no binding specificity for a single IC74-1 or IC74-2 intermediate chain isoform. In addition, this roadblock-binding domain is significantly downstream from both the Tctex1- and LC8-binding sites, supporting the hypothesis that multiple light chain family members can bind to the same intermediate chain.

Function of dynein and dynactin in herpes simplex virus capsid transport

After fusion of the viral envelope with the plasma membrane, herpes simplex virus type 1 (HSV1) capsids are transported along microtubules (MTs) from the cell periphery to the nucleus. The motor ATPase cytoplasmic dynein and its multisubunit cofactor dynactin mediate most transport processes directed toward the minus-ends of MTs. Immunofluorescence microscopy experiments demonstrated that HSV1 capsids colocalized with cytoplasmic dynein and dynactin. We blocked the function of dynein by overexpressing the dynactin subunit dynamitin, which leads to the disruption of the dynactin complex. We then infected such cells with HSV1 and measured the efficiency of particle binding, virus entry, capsid transport to the nucleus, and the expression of immediate-early viral genes. High concentrations of dynamitin and dynamitin-GFP reduced the number of viral capsids transported to the nucleus. Moreover, viral protein synthesis was inhibited, whereas virus binding to the plasma membrane, its internalization, and the organization of the MT network were not affected. We concluded that incoming HSV1 capsids are propelled along MTs by dynein and that dynein and dynactin are required for efficient viral capsid transport to the nucleus.

Dynein motors of the Chlamydomonas flagellum

Chlamydomonas is a biflagellate unicellular green alga that has proven especially amenable for the analysis of microtubule (MT)-based molecular motors, notably dyneins. These enzymes form the inner and outer arms of the flagellum and are also required for intraflagellar transport. Dyneins have masses of approximately 1-2 MDa and consist of up to 15 different polypeptides. Nucleotide binding/hydrolysis and MT motor activity are associated with the heavy chains, and we detail here our current model for the substructural organization of these approximately 520-kDa proteins. The remaining polypeptides play a variety of roles in dynein function, including attachment of the motor to cargo, regulation of motor activity in response to specific inputs, and their necessity for the assembly and/or stability of the entire complex. The combination of genetic, physiological, structural, and biochemical approaches has made the Chlamydomonas flagellum a very powerful model system in which to dissect the function of these fascinating molecular motors.

Role of dynein, dynactin, and CLIP-170 interactions in LIS1 kinetochore function

Mutations in the human LIS1 gene cause type I lissencephaly, a severe brain developmental disease involving gross disorganization of cortical neurons. In lower eukaryotes, LIS1 participates in cytoplasmic dynein-mediated nuclear migration. We previously reported that mammalian LIS1 functions in cell division and coimmunoprecipitates with cytoplasmic dynein and dynactin. We also localized LIS1 to the cell cortex and kinetochores of mitotic cells, known sites of dynein action. We now find that the COOH-terminal WD repeat region of LIS1 is sufficient for kinetochore targeting. Overexpression of this domain or full-length LIS1 displaces CLIP-170 from this site without affecting dynein and other kinetochore markers. The NH2-terminal self-association domain of LIS1 displaces endogenous LIS1 from the kinetochore, with no effect on CLIP-170, dynein, and dynactin. Displacement of the latter proteins by dynamitin overexpression, however, removes LIS1, suggesting that LIS1 binds to the kinetochore through the motor protein complexes and may interact with them directly. We find that of 12 distinct dynein and dynactin subunits, the dynein heavy and intermediate chains, as well as dynamitin, interact with the WD repeat region of LIS1 in coexpression/coimmunoprecipitation and two-hybrid assays. Within the heavy chain, interactions are with the first AAA repeat, a site strongly implicated in motor function, and the NH2-terminal cargo-binding region. Together, our data suggest a novel role for LIS1 in mediating CLIP-170-dynein interactions and in coordinating dynein cargo-binding and motor activities.

A split motor domain in a cytoplasmic dynein

The heavy chain of dynein forms a globular motor domain that tightly couples the ATP-cleavage region and the microtubule-binding site to transform chemical energy into motion along the cytoskeleton. Here we show that, in the fungus Ustilago maydis, two genes, dyn1 and dyn2, encode the dynein heavy chain. The putative ATPase region is provided by dyn1, while dyn2 includes the predicted microtubule-binding site. Both genes are located on different chromosomes, are transcribed into independent mRNAs and are translated into separate polypeptides. Both Dyn1 and Dyn2 co-immunoprecipitated and co-localized within growing cells, and Dyn1-Dyn2 fusion proteins partially rescued mutant phenotypes, suggesting that both polypeptides interact to form a complex. In cell extracts the Dyn1-Dyn2 complex dissociated, and microtubule affinity purification indicated that Dyn1 or associated polypeptides bind microtubules independently of Dyn2. Both Dyn1 and Dyn2 were essential for cell survival, and conditional mutants revealed a common role in nuclear migration, cell morphogenesis and microtubule organization, indicating that the Dyn1-Dyn2 complex serves multiple cellular functions.

Light chains of mammalian cytoplasmic dynein: identification and characterization of a family of LC8 light chains

Cytoplasmic dynein is a large multisubunit motor protein that moves various cargoes toward the minus ends of microtubules. In addition to the previously identified heavy, intermediate, and light intermediate chains, it has recently been recognized that cytoplasmic dynein also has several light chain subunits with apparent molecular weights between 8-20 kDa. To systematically identify the light chains of purified rat brain cytoplasmic dynein, peptide sequences were obtained from each light chain band resolved by gel electrophoresis. Both members of the tctex1 light chain family, tctex1 and rp3, were identified in a single band. Only one member of the roadblock family, roadblock-2, was found. Two members of the LC8 family were resolved as separate bands, the previously identified LC8 subunit, and a second novel cytoplasmic dynein family member, LC8b. The tissue distribution of these two dynein LC8 subunits differed, although LC8b was the major family member in brain. Database searches found that both LC8a and LC8b were also present in several mammalian species, and a third mammalian LC8 sequence, LC8c was found in the human database. The amino acid sequences of both LC8a and LC8b were completely conserved in mammals. LC8a and LC8b differ in only six of the 89 amino acids. The amino acid differences between LC8a and LC8b were located near the N-terminus of the molecules, and most were in the outward facing alpha-helices of the LC8 dimer. When the mammalian LC8a sequence was compared to the LC8 sequences found in six other animal species including Xenopus and Drosophila, there was, on average, 94% sequence identity. More variation was found in LC8 sequences obtained from plants, fungi, and parasites. LC8c differed from the other two human LC8 sequences in that it has amino acid substitutions in the intermediate chain binding domain at the C-terminal of the molecule. The position of amino acid substitutions of the three mammalian LC8 family members is consistent with the hypothesis that they bind to different proteins.

Phosphorylation by cdc2-CyclinB1 kinase releases cytoplasmic dynein from membranes

Movement of various cargoes toward microtubule minus ends is driven by the microtubule motor cytoplasmic dynein (CD). Many cargoes are motile only during certain cell cycle phases, suggesting that CD function may be under cell cycle control. Phosphorylation of the CD light intermediate chain (DLIC) has been suggested to play a crucial role in modulating CD function during the Xenopus embryonic cell cycle, where CD-driven organelle movement is active in interphase but greatly reduced in metaphase. This down-regulation correlates with hyperphosphorylation of DLIC and release of CD from the membrane. Here we investigate the role of the key mitotic kinase, cdc2-cyclinB1, in this process. We show that DLIC within the native Xenopus CD complex is an excellent substrate for purified Xenopus cdc2-glutathione S-transferase (GST) cyclinB1 (cdc2-GSTcyclinB1) kinase. Mass spectrometry of native DLIC revealed that a conserved cdc2 site (Ser-197) previously implicated in the metaphase modulation of CD remains phosphorylated in interphase and so is unlikely to be the key regulatory site. We also demonstrate that incubating interphase membranes with cdc2-GSTcyclinB1 kinase results in substantial release of CD from the membrane. These data suggest that phosphorylation of DLIC by cdc2 kinase leads directly to the loss of membrane-associated CD and an inhibition of organelle movement.

The dynein heavy chain: structure, mechanics and evolution

The translocation of dynein along microtubules is the basis for a variety of essential cellular movements. Despite a general domain organization that is found in all the cytoskeletal motors, there are structural features of dynein that set it apart from the other motors. These include a track-binding site that is located at the tip of a long projection, and six nucleotide-binding modules that together form the globular head of dynein. These unique features suggest that dynein produces movement by a mechanism that is different from that used by the other motors.

Antibodies to cytoplasmic dynein heavy chain map the surface and inhibit motility

Polyclonal antibodies have been raised against four 16 residue peptides with sequences taken from the C-terminal quarter of the human cytoplasmic dynein heavy chain. The sites are downstream from a known microtubule-binding domain associated with the "stalk" that protrudes from the motor domain. The antisera were assayed using bacterially expressed proteins with amino acid sequences taken from the human cytoplasmic dynein heavy chain. Every antiserum reacted specifically with the appropriate expressed protein and with pig brain cytoplasmic dynein, whether the protein molecules were denatured on Western blots or were in a folded state. But, whereas three of the four antisera recognized freshly purified cytoplasmic dynein, the fourth reacted only with dynein that had been allowed to denature a little. After affinity purification against the expressed domains, whole IgG molecules and Fab fragments were assayed for their effect on dynein activity in in vitro microtubule-sliding assays. Of the three anti-peptides that reacted with fresh dynein, one inhibited motility but the others did not. The way these peptides are exposed on the surface is compatible with a model whereby the dynein motor domain is constructed from a ring of AAA protein modules, with the C-terminal module positioned on the surface that interacts with microtubules. We have tentatively identified an additional AAA module in the dynein heavy chain sequence, which would be consistent with a heptameric ring.

Distinct but overlapping sites within the cytoplasmic dynein heavy chain for dimerization and for intermediate chain and light intermediate chain binding

Cytoplasmic dynein is a molecular motor complex consisting of four major classes of polypeptide: the catalytic heavy chains (HC), intermediate chains (IC), light intermediate chains (LIC), and light chains (LC). Previous studies have reported that the ICs bind near the N terminus of the HCs, which is thought to correspond to the base of the dynein complex. In this study, we co-overexpressed cytoplasmic dynein subunits in COS-7 cells to map HC binding sites for the ICs and LICs, as well as HC dimerization. We have found that the LICs bind directly to the N terminus of the HC, adjacent to and overlapping with the IC binding site, consistent with a role for the LICs in cargo binding. Mutation of the LIC P-loop had no detectable effect on HC binding. We detected no direct interaction between the ICs and LICs. Using triple overexpression of HC, IC and LIC, we found that both IC and LIC are present in the same complexes, a result verified by anti-IC immunoprecipitation of endogenous complexes and immunoblotting. Our results indicate that the LICs and ICs must be located on independent surfaces of cytoplasmic dynein to allow each to interact with other proteins without steric interference.