In vitro motility from recombinant dynein heavy chain

Fast and Easy Transient Mammalian Cell Expression and Purification of Cytoplasmic Dynein

Recombinant protein expression has been key to studying dynein's mechanochemistry and structure-function relationship. To gain further insight into the energy-converting mechanisms and interactions with an increasing variety of dynein cargos and regulators, rapid expression and purification of a variety of dynein proteins and fragments are important. Here we describe transient expression of cytoplasmic dynein in HEK293 cells and fast small-scale purification for high-throughput protein engineering. Mammalian cell expression might be generally considered to be a laborious process, but with recent technology and some simple inexpensive custom-built labware, dynein expression and purification from mammalian cells can be fast and easy.

Local inhibition of microtubule dynamics by dynein is required for neuronal cargo distribution

Abnormal axonal transport is associated with neuronal disease. We identified a role for DHC-1, the C. elegans dynein heavy chain, in maintaining neuronal cargo distribution. Surprisingly, this does not involve dynein's role as a retrograde motor in cargo transport, hinging instead on its ability to inhibit microtubule (MT) dynamics. Neuronal MTs are highly static, yet the mechanisms and functional significance of this property are not well understood. In disease-mimicking dhc-1 alleles, excessive MT growth and collapse occur at the dendrite tip, resulting in the formation of aberrant MT loops. These unstable MTs act as cargo traps, leading to ectopic accumulations of cargo and reduced availability of cargo at normal locations. Our data suggest that an anchored dynein pool interacts with plus-end-out MTs to stabilize MTs and allow efficient retrograde transport. These results identify functional significance for neuronal MT stability and suggest a mechanism for cellular dysfunction in dynein-linked disease.

JNK-interacting protein 3 mediates the retrograde transport of activated c-Jun N-terminal kinase and lysosomes

Retrograde axonal transport requires an intricate interaction between the dynein motor and its cargo. What mediates this interaction is largely unknown. Using forward genetics and a novel in vivo imaging approach, we identified JNK-interacting protein 3 (Jip3) as a direct mediator of dynein-based retrograde transport of activated (phosphorylated) c-Jun N-terminal Kinase (JNK) and lysosomes. Zebrafish jip3 mutants (jip3^nl7) displayed large axon terminal swellings that contained high levels of activated JNK and lysosomes, but not other retrograde cargos such as late endosomes and autophagosomes. Using in vivo analysis of axonal transport, we demonstrated that the terminal accumulations of activated JNK and lysosomes were due to a decreased frequency of retrograde movement of these cargos in jip3^nl7, whereas anterograde transport was largely unaffected. Through rescue experiments with Jip3 engineered to lack the JNK binding domain and exogenous expression of constitutively active JNK, we further showed that loss of Jip3–JNK interaction underlies deficits in pJNK retrograde transport, which subsequently caused axon terminal swellings but not lysosome accumulation. Lysosome accumulation, rather, resulted from loss of lysosome association with dynein light intermediate chain (dynein accessory protein) in jip3^nl7, as demonstrated by our co-transport analyses. Thus, our results demonstrate that Jip3 is necessary for the retrograde transport of two distinct cargos, active JNK and lysosomes. Furthermore, our data provide strong evidence that Jip3 in fact serves as an adapter protein linking these cargos to dynein.

To form and maintain connections, neurons require the active transport of proteins and organelles between the neuronal cell body and axon terminals. Inhibition of this “axonal” transport has been linked to neurodegenerative diseases. Despite the importance of this process, to date there was no vertebrate model system where axonal transport could be studied in an intact animal. Our study introduces zebrafish as such a model and demonstrates its power for the analysis of axonal transport. We used this system to 1) initiate a genetic screen to find novel mediators of axonal transport; 2) develop in vivo imaging strategies to visualize axonal transport in real time in the intact animal; and 3) discover, using these methods, that JNK interacting protein 3 (Jip3) is required for the transport of two cargos, a kinase and lysosomes, from axon terminals to the cell body (retrograde transport). In the absence of Jip3, these cargos accumulate and axon terminals become dysmorphic, though the retrograde transport of other cargos is normal. Interestingly, abnormal localization of these cargos has been linked to axonal disease states, but our work is the first to identify a specific adapter protein necessary for their transport from axon terminals.

Reconstitution of the human cytoplasmic dynein complex

Cytoplasmic dynein is the major motor protein responsible for microtubule minus-end-directed movements in most eukaryotic cells. It transports a variety of cargoes and has numerous functions during spindle assembly and chromosome segregation. It is a large complex of about 1.4 MDa composed of six different subunits, interacting with a multitude of different partners. Most biochemical studies have been performed either with the native mammalian cytoplasmic dynein complex purified from tissue or, more recently, with recombinant dynein fragments from budding yeast and Dictyostelium. Hardly any information exists about the properties of human dynein. Moreover, experiments with an entire human dynein complex prepared from recombinant subunits with a well-defined composition are lacking. Here, we reconstitute a complete cytoplasmic dynein complex using recombinant human subunits and characterize its biochemical and motile properties. Using analytical gel filtration, sedimentation-velocity ultracentrifugation, and negative-stain electron microscopy, we demonstrate that the smaller subunits of the complex have an important structural function for complex integrity. Fluorescence microscopy experiments reveal that while engaged in collective microtubule transport, the recombinant human cytoplasmic dynein complex is an active, microtubule minus-end-directed motor, as expected. However, in contrast to recombinant dynein of nonmetazoans, individual reconstituted human dynein complexes did not show robust processive motility, suggesting a more intricate mechanism of processivity regulation for the human dynein complex. In the future, the comparison of reconstituted dynein complexes from different species promises to provide molecular insight into the mechanisms regulating the various functions of these large molecular machines.

Stable kinesin and dynein assemblies drive the axonal transport of mammalian prion protein vesicles

Kinesin and dynein are opposite-polarity microtubule motors that drive the tightly regulated transport of a variety of cargoes. Both motors can bind to cargo, but their overall composition on axonal vesicles and whether this composition directly modulates transport activity are unknown. Here we characterize the intracellular transport and steady-state motor subunit composition of mammalian prion protein (PrP(C)) vesicles. We identify Kinesin-1 and cytoplasmic dynein as major PrP(C) vesicle motor complexes and show that their activities are tightly coupled. Regulation of normal retrograde transport by Kinesin-1 is independent of dynein-vesicle attachment and requires the vesicle association of a complete Kinesin-1 heavy and light chain holoenzyme. Furthermore, motor subunits remain stably associated with stationary as well as with moving vesicles. Our data suggest a coordination model wherein PrP(C) vesicles maintain a stable population of associated motors whose activity is modulated by regulatory factors instead of by structural changes to motor-cargo associations.

Towards the application of cytoskeletal motor proteins in molecular detection and diagnostic devices

Over the past ten years, great advancements have been made towards using biomolecular motors for nanotechnological applications. In particular, devices using cytoskeletal motor proteins for molecular transport are maturing. First efforts towards designing such devices used motor proteins attached to micro-structured substrates for the directed transport of microtubules and actin filaments. Soon thereafter, the specific capture, transport and detection of target analytes like viruses were demonstrated. Recently, spatial guiding of the gliding filaments was added to increase the sensitivity of detection and allow parallelization. Whereas molecular motor powered devices have not yet demonstrated performance beyond the level of existing detection techniques, the potential is great: Replacing microfluidics with transport powered by molecular motors allows integration of the energy source (ATP) into the assay solution. This opens up the opportunity to design highly integrated, miniaturized, autonomous detection devices. Such devices, in turn, may allow fast and cheap on-site diagnosis of diseases and detection of environmental pathogens and toxins.

Protein engineering approaches to study the dynein mechanism using a Dictyostelium expression system

Dyneins are microtubule-based motor complexes that power a wide variety of motile processes within eukaryotic cells, including the beating of cilia and flagella and intracellular trafficking along microtubules. Mechanistic studies on dynein have been hampered by their enormous size (molecular masses of 0.5-3MDa) and molecular complexity. However, the recent establishment of recombinant expression systems for cytoplasmic dynein, together with structural and functional analyses, has advanced our understanding of the molecular mechanisms of dynein motility. Here, we describe several protocols for protein engineering approaches to the dynein mechanism using a Dictyostelium discoideum expression system. We first describe the design and preparation of recombinant dynein suitable for mechanistic studies. We then discuss two distinct functional assays that take advantage of the recombinant dynein. One is for detection of dynein's conformational changes during the ATPase cycle. Another is an in vitro motility assay at multiple- and single-molecule levels for examination of the dynamic behavior of dynein moving on a microtubule.

MEMS technology for nanobio research

Micro- and nanotechnology have gathered 20 years of increasing research efforts. This research activity began and developed with the design and the fabrication of micro- and nanomechanisms, sensors and actuators, which range from 10nm to 100mum. More recent trends focus on the transfer of this technology know-how towards nanobiological topics and very wide range applications can be addressed. Among them, this review proposes various examples that include MEMS tweezers for molecular direct handling and characterization, single molecular characterization in femto-L chambers and dynamic microarray for cell positioning. The micromachined devices are described with bio-oriented experiences that are relevant to foresee their future contribution to drug discovery.

Setting up roadblocks for kinesin-1: mechanism for the selective speed control of cargo carrying microtubules

Motor-driven cytoskeletal filaments are versatile transport platforms for nanosized cargo in molecular sorting and nano-assembly devices. However, because cargo and motors share the filament lattice as a common substrate for their activity, it is important to understand the influence of cargo-loading on transport properties. By performing single-molecule stepping assays on biotinylated microtubules we found that individual kinesin-1 motors frequently stopped upon encounters with attached streptavidin molecules. Consequently, we attribute the deceleration of cargo-laden microtubules in gliding assays to an obstruction of kinesin-1 paths on the microtubule lattice rather than to 'frictional' cargo-surface interactions. We propose to apply this obstacle-caused slow-down of gliding microtubules in a novel molecular detection scheme: Using a mixture of two distinct microtubule populations that each bind a different kind of protein, the presence of these proteins can be detected via speed changes in the respective microtubule populations.

Towards single biomolecule handling and characterization by MEMS

Applications of microelectromechanical systems (MEMS) technology are widespread in both industrial and research fields providing miniaturized smart tools. In this review, we focus on MEMS applications aiming at manipulations and characterization of biomaterials at the single molecule level. Four topics are discussed in detail to show the advantages and impact of MEMS tools for biomolecular manipulations. They include the microthermodevice for rapid temperature alternation in real-time microscopic observation, a microchannel with microelectrodes for isolating and immobilizing a DNA molecule, and microtweezers to manipulate a bundle of DNA molecules directly for analyzing its conductivity. The feasibilities of each device have been shown by conducting specific biological experiments. Therefore, the development of MEMS devices for single molecule analysis holds promise to overcome the disadvantages of the conventional technique for biological experiments and acts as a powerful strategy in molecular biology.

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Molecular Motors: Strategies to Get Along

The majority of active transport in the cell is driven by three classes of molecular motors: the kinesin and dynein families that move toward the plus-end and minus-end of microtubules, respectively, and the unconventional myosin motors that move along actin filaments. Each class of motor has different properties, but in the cell they often function together. In this review we summarize what is known about their single-molecule properties and the possibilities for regulation of such properties. In view of new results on cytoplasmic dynein, we attempt to rationalize how these different classes of motors might work together as part of the intracellular transport machinery. We propose that kinesin and myosin are robust and highly efficient transporters, but with somewhat limited room for regulation of function. Because cytoplasmic dynein is less efficient and robust, to achieve function comparable to the other motors it requires a number of accessory proteins as well as multiple dyneins functioning together. This necessity for additional factors, as well as dynein's inherent complexity, in principle allows for greatly increased control of function by taking the factors away either singly or in combination. Thus, dynein's contribution relative to the other motors can be dynamically tuned, allowing the motors to function together differently in a variety of situations.

A single-headed recombinant fragment of Dictyostelium cytoplasmic dynein can drive the robust sliding of microtubules

A cytoplasmic dynein is a microtubule-based motor protein involved in diverse cellular functions, such as organelle transport and chromosome segregation. The dynein has two ring-shaped heads that contain six repeats of the AAA domain responsible for ATP hydrolysis. It has been proposed that the ATPase-dependent swing of a stalk and a stem emerging from each of the heads generates the power stroke (Burgess, S.A. (2003) Nature 421, 715-718). To understand the molecular mechanism of the dynein power stroke, it is essential to establish an easy and reproducible method to express and purify the recombinant dynein with full motor activities. Here we report the expression and purification of the C-terminal 380-kDa fragment of the Dictyostelium cytoplasmic dynein heavy-chain fused with an affinity tag and green fluorescent protein. The purified single-headed recombinant protein drove the robust minus-end-directed sliding of microtubules at a velocity of 1.2 microm/s. This recombinant protein had a high basal ATPase activity (approximately 4s(-1)), which was further activated by >15-fold on the addition of 40 microM microtubules. These results show that the 380-kDa recombinant fragment retains all the structures required for motor functions, i.e. the ATPase activity highly stimulated by microtubules and the robust motility.

Differential regulation of dynein-driven melanosome movement

Cytoplasmic dyneins are multisubunit minus-end-directed microtubule motors. Different isoforms of dynein are thought to provide a means for independent movement of different organelles. We investigated the differential regulation of dynein-driven transport of pigment organelles (melanosomes) in Xenopus melanophores. Aggregation of melanosomes to the cell center does not change the localization of mitochondria, nor does dispersion of melanosomes cause a change in the perinuclear localization of the Golgi complex, indicating that melanosomes bear a dedicated form of dynein. We examined the subcellular fractionation behavior of dynein light intermediate chains (LIC) and identified at least three forms immunologically, only one of which fractionated with melanosomes. Melanosome aggregation was specifically blocked after injection of an antibody recognizing this LIC. Our data indicate that melanosome-associated dynein is regulated independently of bulk cytoplasmic dynein and involves a subfraction of dynein with a distinct subunit composition.

Cytoplasmic dynein intermediate chain and heavy chain are dependent upon each other for microtubule end localization in Aspergillus nidulans

The multisubunit microtubule motor, cytoplasmic dynein, targets to various subcellular locations in eukaryotic cells for various functions. The cytoplasmic dynein heavy chain (HC) contains the microtubule binding and ATP binding sites for motor function, whereas the intermediate chain (IC) is implicated in the in vivo targeting of the HC. Concerning any targeting event, it is not known whether the IC has to form a complex with the HC for targeting or whether the IC can target to a site independently of the HC. In the filamentous fungus Aspergillus nidulans, the dynein HC is localized to the ends of microtubules near the hyphal tip. In this study, we demonstrate that our newly identified dynein IC in A. nidulans is also localized to microtubule ends and is required for HC's localization to microtubule ends in living cells. With the combination of two reagents, an HC loss-of function mutant and the green fluorescent protein (GFP)-fused IC that retains its function, we show that the IC's localization to microtubule ends also requires HC, suggesting that cytoplasmic dynein HC-IC complex formation is important for microtubule end targeting. In addition, we show that the HC localization is not apparently altered in the deletion mutant of NUDF, a LIS1-like protein that interacts directly with the ATP-binding domain of the HC. Our study suggests that, although HC-IC association is important for the targeting of dynein to microtubule ends, other essential components, such as NUDF, may interact with the targeted dynein complex to produce full motor activities in vivo.

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.

Identification of a novel light intermediate chain (D2LIC) for mammalian cytoplasmic dynein 2

The diversity of dynein's functions in mammalian cells is a manifestation of both the existence of multiple dynein heavy chain isoforms and an extensive set of associated protein subunits. In this study, we have identified and characterized a novel subunit of the mammalian cytoplasmic dynein 2 complex. The sequence similarity between this 33-kDa subunit and the light intermediate chains (LICs) of cytoplasmic dynein 1 suggests that this protein is a dynein 2 LIC (D2LIC). D2LIC contains a P-loop motif near its NH(2) terminus, and it shares a short region of similarity to the yeast GTPases Spg1p and Tem1p. The D2LIC subunit interacts specifically with DHC2 (or cDhc1b) in both reciprocal immunoprecipitations and sedimentation assays. The expression of D2LIC also mirrors that of DHC2 in a variety of tissues. D2LIC colocalizes with DHC2 at the Golgi apparatus throughout the cell cycle. On brefeldin A-induced Golgi fragmentation, a fraction of D2LIC redistributes to the cytoplasm, leaving behind a subset of D2LIC that is localized around the centrosome. Our results suggest that D2LIC is a bona fide subunit of cytoplasmic dynein 2 that may play a role in maintaining Golgi organization by binding cytoplasmic dynein 2 to its Golgi-associated cargo.

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.

ADP-dependent microtubule translocation by flagellar inner-arm dyneins

Previous studies have shown that the motility of flagellar and ciliary axonemes in many organisms are influenced by the concentration of both ATP and ADP. Detergent-extracted cell models of Chlamydomonas oda1, a mutant lacking flagellar outer-arm dynein, displayed slightly lower flagellar beating frequencies when reactivated with ATP in the presence of an ATP-regenerating system, composed of creatine phosphate and creatine phosphokinase, than when reactivated with ATP alone. Thus, presence of a low concentration of ADP may somehow stimulate axonemal motility. To see if this motility stimulation is due to a direct effect on dynein, we analyzed the effect of ADP on the in vitro microtubule translocation caused by isolated inner-arm dyneins in the presence of ATP. Of the seven inner-arm dyneins (species a-g) fractionated by ion-exchange chromatography, most species translocated microtubules at faster speed in the presence of 0.1 mM ATP and 0.1 mM ADP than in the presence of 0.1 mM ATP alone. Most notably, species a and e did not translocate microtubules at all in the presence of the ATP-regenerating system, indicating that a trace amount of ADP is necessary for their motility. This regulation may be effected through binding of ADP to some of the four nucleotide binding sites in each dynein heavy chain.

Direct interaction of pericentrin with cytoplasmic dynein light intermediate chain contributes to mitotic spindle organization

Pericentrin is a conserved protein of the centrosome involved in microtubule organization. To better understand pericentrin function, we overexpressed the protein in somatic cells and assayed for changes in the composition and function of mitotic spindles and spindle poles. Spindles in pericentrin-overexpressing cells were disorganized and mispositioned, and chromosomes were misaligned and missegregated during cell division, giving rise to aneuploid cells. We unexpectedly found that levels of the molecular motor cytoplasmic dynein were dramatically reduced at spindle poles. Cytoplasmic dynein was diminished at kinetochores also, and the dynein-mediated organization of the Golgi complex was disrupted. Dynein coimmunoprecipitated with overexpressed pericentrin, suggesting that the motor was sequestered in the cytoplasm and was prevented from associating with its cellular targets. Immunoprecipitation of endogenous pericentrin also pulled down cytoplasmic dynein in untransfected cells. To define the basis for this interaction, pericentrin was coexpressed with cytoplasmic dynein heavy (DHCs), intermediate (DICs), and light intermediate (LICs) chains, and the dynamitin and p150(Glued) subunits of dynactin. Only the LICs coimmunoprecipitated with pericentrin. These results provide the first physiological role for LIC, and they suggest that a pericentrin-dynein interaction in vivo contributes to the assembly, organization, and function of centrosomes and mitotic spindles.

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

Cytoplasmic dynein is a multisubunit microtubule-based motor protein that is involved in several eukaryotic cell motilities. Two dynein heavy chains each form a motor domain that connects to a common cargo-binding tail. Although this tail domain is composed of multiple polypeptides, subunit organization within this region is poorly understood. Here we present an in vitro dissection of the tail-forming region of the dynein heavy chain from Dictyostelium. Our work identifies a sequence important for dimerization and for binding the dynein intermediate chain. The core of this motif localizes within an approximately 150-amino acid region that is strongly conserved among other cytoplasmic dyneins. This level of conservation does not extend to the axonemal dynein heavy chains, suggesting functional differences between the two. Dimerization appears to occur through a different mechanism than the heavy chain-intermediate chain interaction. We corroborate the in vitro interactions with in vivo expression of heavy chain fragments in Dictyostelium. Fragments lacking the interaction domain express well, without an obvious phenotype. On the other hand, the region crucial for both interactions appears to be lethal when overexpressed.

Cytoplasmic dynein intermediate-chain isoforms with different targeting properties created by tissue-specific alternative splicing

The intermediate chains (ICs) are the subunits of the cytoplasmic dynein that provide binding of the complex to cargo organelles through interaction of their N termini with dynactin. We present evidence that in Drosophila, the IC subunits are represented by at least 10 structural isoforms, created by the alternative splicing of transcripts from a unique Cdic gene. The splicing pattern is tissue specific. A constitutive set of four IC isoforms is expressed in all tissues tested; in addition, tissue-specific isoforms are found in the ovaries and nervous tissue. The structural variations between isoforms are limited to the N terminus of the IC molecule, where the interaction with dynactin takes place. This suggests differences in the dynactin-mediated organelle binding by IC isoforms. Accordingly, when transiently expressed in Drosophila Schneider-3 cells, the IC isoforms differ in their intracellular targeting properties from each other. A mechanism is proposed for the regulation of dynein binding to organelles through the changes in the content of the IC isoform pool.

Localization of Tctex-1, a cytoplasmic dynein light chain, to the Golgi apparatus and evidence for dynein complex heterogeneity

To date, much attention has been focused on the heavy and intermediate chains of the multisubunit cytoplasmic dynein complex; however, little is known about the localization or function of dynein light chains. In this study, we find that Tctex-1, a light chain of cytoplasmic dynein, localizes predominantly to the Golgi apparatus in interphase fibroblasts. Immunofluorescent staining reveals striking juxtanuclear staining characteristic of the Golgi apparatus as well as nuclear envelope and punctate cytoplasmic staining that often decorates microtubules. Tctex-1 colocalization with Golgi compartment markers, its distribution upon treatment with various pharmacological agents, and the cofractionation of Tctex-1-associated membranes with Golgi membranes are all consistent with a Golgi localization. The distribution of Tctex-1 in interphase cells only partially overlaps with the dynein intermediate chain and p150(Glued) upon immunofluorescence, but most of Tctex-1 is redistributed onto mitotic spindles along with other dynein/dynactin subunits. Using sequential immunoprecipitations, we demonstrate that there is a subset of Tctex-1 not associated with the intermediate chain at steady state; the converse also appears to be true. Distinct populations of dynein complexes are likely to exist, and such diversity may occur in part at the level of their light chain compositions.

Evidence for four cytoplasmic dynein heavy chain isoforms in rat testis

Recent studies have revealed the expression of multiple putative cytoplasmic dynein heavy chain (DHC) genes in several organisms, with each gene encoding a separate protein isoform. This finding is consistent with the hypothesis that different isoforms do different things, as is the case for the axonemal dyneins. Furthermore, the large number of tasks ascribed to cytoplasmic dynein suggests that there may be additional isoforms not yet identified. Two of the mammalian cytoplasmic dynein heavy chains are DHC1a and DHC1b. DHC1a is conventional cytoplasmic dynein and is found in all organisms examined. DHC1b is expressed in organisms that have multiple dyneins, and has been implicated in the intracellular trafficking of molecules in unciliated and ciliated cells. In the present study, we examined the DHC1b protein from rat testis. Testis cytoplasmic dynein contains a large amount of dynein heavy chain reactive with an antibody raised against a peptide sequence of rat DHC1b. The testis anti-DHC1b immunoreactive protein is slightly smaller than testis DHC1a, as assessed by SDS-PAGE. In Northern blots, the DHC1b mRNA is smaller than the DHC1a mRNA. In sucrose gradients made in low ionic strength, DHC1a sedimented at approximately 20S, and the anti-1b immunoreactive heavy chains sedimented in a broad band centered at approximately 14S. The V1-photolysis reaction of individual sucrose gradient fractions revealed three distinct patterns of photolysis, suggesting that there are at least three separate 1b-like heavy chain isoforms in testis. Using a high-stringency Western blotting protocol, the anti-1b antibody and the anti-DHC2 antibody recognized the same heavy chain and specifically bound to one of the three 1b-like heavy chains. We conclude that rat testis contains three 1b-like dynein heavy chains, and one of these is the product of the DHC1b/DHC2 gene previously identified.

An extended microtubule-binding structure within the dynein motor domain

Flagellar dynein was discovered over 30 years ago as the first motor protein capable of generating force along microtubules. A cytoplasmic form of dynein has also been identified which is involved in mitosis and a wide range of other intracellular movements. Rapid progress has been made on understanding the mechanism of force production by kinesins and myosins. In contrast, progress in understanding the dyneins has been limited by their great size (relative molecular mass 1,000K-2,000K) and subunit complexity. We now report evidence that the entire carboxy-terminal two-thirds of the 532K force-producing heavy chain subunit is required for ATP-binding activity. We further identify a microtubule-binding domain, which, surprisingly, lies well downstream of the entire ATPase region and is predicted to form a hairpin-like stalk. Direct ultrastructural analysis of a recombinant fragment confirms this model, and suggests that the mechanism for dynein force production differs substantially from that of other motor proteins.