Motor proteins of cytoplasmic microtubules

Modelling of transport processes: Theory and simulations

The transport processes, being a non-equilibrium system, have been a point of interest for physicists since many years revealing and explaining several unexpected effects. Such systems are often dealt with an archetypal model, known as totally asymmetric simple exclusion process, with two different types of boundary conditions: open and periodic. Moreover, these models are analyzed in two varieties of dynamics, random sequential and parallel updates, even at the micro level which play an important role in the global dynamics of the system. On contrary to the random sequential rule, the parallel updates introduce correlations in the system. Using theoretical and numerical methods in the framework based on mean-field approaches, the system properties are analyzed in both transient and steady state.•Both the updating rules are realized using Monte Carlo simulations.•In simplest form, mean-field approach ignores all the correlations and the results coincide with the random sequential update.•Correlations are induced in the system due to parallel update, therefore, a cluster mean-field theory is also discussed to handle them.

Apple hypanthium firmness: new insights from comparative proteomics

Fruit firmness constitutes an important textural property and is one of the key parameters for estimating ripening and shelf life, which has a major impact on commercialization. In order to decipher the mechanisms related to firmness of apples (Malus × domestica Borkh.), two-dimensional gel electrophoresis (2-DE) was used to compare the total proteome of high and low firmness phenotypes from apple hypanthia of a 'Golden Delicious' × 'Dietrich' population. A total of 36 differentially regulated protein spots were positively identified by matrix assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry (MS) and then validated against the Malus expressed sequence tags (EST) database. The findings of this study indicated a lower expression of ethylene biosynthesis related proteins in the high firmness phenotype, which could be linked to the slowing down of the ripening and softening processes. The reduced accumulation of proteins involved in ethylene biosynthesis juxtaposed to the upregulation of a transposase and a GTP-binding protein in the high firmness phenotype. The results also showed higher expression of cytoskeleton proteins in the high firmness phenotype compared to the low firmness phenotype, which play a role in maintaining cell structure and possibly fruit integrity. Finally, a number of proteins involved in detoxification and defense were expressed in fruit hypanthium. This proteomic study provides a contribution towards a better understanding of regulatory networks involved in fruit hypanthium firmness and/or softening, which could be instrumental in the development of improved fruit quality.

Characterization of inhibitors of glucocorticoid receptor nuclear translocation: a model of cytoplasmic dynein-mediated cargo transport

Agonist-induced glucocorticoid receptor [GR] transport from the cytoplasm to the nucleus was used as a model to identify dynein-mediated cargo transport inhibitors. Cell-based screening of the library of pharmacologically active compound (LOPAC)-1280 collection identified several small molecules that stalled the agonist-induced transport of GR-green fluorescent protein (GFP) in a concentration-dependent manner. Fluorescent images of microtubule organization, nuclear DNA staining, expression of GR-GFP, and its subcellular distribution were inspected and quantified by image analysis to evaluate the impact of compounds on cell morphology, toxicity, and GR transport. Given the complexity of the multi-protein complex involved in dynein-mediated cargo transport and the variety of potential mechanisms for interruption of that process, we therefore developed and validated a panel of biochemical assays to investigate some of the more likely intracellular target(s) of the GR transport inhibitors. Although the apomorphine enantiomers exhibited the most potency toward the ATPase activities of cytoplasmic dynein, myosin, and the heat-shock proteins (HSPs), their apparent lack of specificity made them unattractive for further study in our quest. Other molecules appeared to be nonspecific inhibitors that targeted reactive cysteines of proteins. Ideally, specific retrograde transport inhibitors would either target dynein itself or one of the other important proteins associated with the transport process. Although the hits from the cell-based screen of the LOPAC-1280 collection did not exhibit this desired profile, this screening platform provided a promising phenotypic system for the discovery of dynein/HSP modulators.

Bidirectional power stroke by ncd kinesin

Optical trapping experiments reveal details of molecular motor dynamics. In noisy data, temporal structure within the power stroke of motors can be analyzed by ensemble averaging, but this obscures infrequent subcategories of events. We have here developed an analysis method that uses Kalman filtering of measurements, model-based estimation of the power strokes produced by the motor head, and automatic event classification to discriminate between different types of motor events. This method was applied to optical trap measurements of power strokes of the Drosophila kinesin-14 ncd in a three-bead geometry. We found the majority of events to be consistent with the previously discovered minus-end directed power stroke of ncd, occurring with ATP binding. Unexpectedly, 30% of apparent power strokes were plus-directed and 6% of binding events did not terminate in a discernible stroke. Ensemble averaging for each event category revealed that plus- and minus-directed strokes have different size and occur at different instants within the ncd-MT attachment sequence.

Biochemical investigation of active intracellular transport of polymeric gene-delivery vectors

To design safe, efficient synthetic gene therapy vectors, it is desirable to understand the intracellular mechanisms that facilitate their delivery from the cell surface to the nucleus. Elements of the cytoskeleton and molecular motor proteins are known to play a pivotal role in most intracellular active transport processes. The actin depolymerizer cytochalasin D and microtubule effectors colchicine and paclitaxel were used to evaluate the function of these components of the cytoskeleton in the trafficking of polyethylenimine (PEI)-DNA complexes. In addition, ATPase inhibitors erythro-9[3-(2-hydroxynonyl)] adenine (EHNA), vanadate, adenylylimidodiphosphate (AMP-PNP), and rose bengal lactone (RBL), which have inhibitory activity against dynein and kinesin, were used to examine to the effects of these molecular motors on PEI-DNA delivery. Disruption of microfilaments decreased the delivery efficiency of PEI polyplexes 60-80%, though cytochalasin D did not significantly inhibit uptake. Depolymerization of microtubules by colchicine decreased transfection efficiency by 75%. Microtubule stabilization with paclitaxel, however, facilitated a 20-fold increase in gene expression. Treatment with EHNA and vanadate caused 50% and 80% decreases in transfection efficiency, respectively. Transfection efficiency was also decreased by RBL (80%) and AMP-PNP (98%). Our findings confirm the importance of microfilament- and microtubule-based active transport of PEI-DNA complexes. Further, the strong decrease in transfection efficiency caused by ATPase inhibitors that possess inhibitory activity against kinesin implies an unexpected role for these motors in gene delivery.

Intermolecular cross-linking of a novel rice Kinesin k16 motor domain with a photoreactive ATP derivative

A fluorescent photoreactive ATP derivative, 2'(3')-O-(4-benzoylbenzoyl)-1,N(6)-etheno-ATP (Bz(2)-epsilonATP), was synthesized and reacted with the rice kinesin K16 motor domain (K16MD). In the presence of ADP or ATP, UV irradiation of the K16MD solution containing Bz(2)-epsilonATP resulted in a new 100 kDa band, which was an intermolecular cross-linked product of motor domains. In contrast, no cross-linking was observed in the absence of nucleotides. For the motor domain of mouse brain kinesin and skeletal muscle myosin subfragment-1, no such intermolecular photo cross-linking by Bz(2)-epsilonATP was observed. Our results indicate that Bz(2)-epsilonATP acts unusually as a photoreactive crosslinker to detect conformational changes in K16MD induced by nucleotide binding resulting in the formation of dimers.

Preparation and characterization of a novel rice plant-specific kinesin

Kinesin is an ATP-driven motor protein that plays important physiological roles in intracellular transport, mitosis and meiosis, control of microtubule dynamics, and signal transduction. The kinesin family is classified into subfamilies. Kinesin species derived from vertebrates have been well characterized. In contrast, plant kinesins have yet to be adequately characterized. In this study, we expressed the motor domain of a novel rice plant-specific kinesin, K16, in Escherichia coli, and then determined its enzymatic characteristics and compared them with those of kinesin 1. Our findings demonstrated that the rice kinesin motor domain has different enzymatic properties from those of well known kinesin 1.

In search of membrane receptors for microtubule-based motors - is kinectin a kinesin receptor?

The past few years have seen an explosion in the number of molecular motors reported in the literature. By us the energy of hydrolysis, these motors move various organelles along cytoskeletal 'tracks' within the cell. It is thought that some of the specificity of movement resides in receptors on the surface of the cargo organelles, but, in general, little is known about these molecules. In this article, Janis Burkhardt discusses the evidence that the protein kinectin serves as a membrane receptor for kinesin, and describes how motor-receptor proteins may interact with other components of the motility machinery to generate regulated movement of membrane organelles.

Organelle transport along microtubules - the role of KIFs

Organelle transporters are very important for cellular morphogenesis and other cellular functions, conveying and targeting important materials to the correct destination, often at considerable velocities. One of the first proteins to be identified as a motor was kinesin, and recently at least 10 new kinesin superfamily proteins (KIFs) have been described. Characterization of some of them reveals that each member can convey a specific organelle or cargo, although there is some redundancy. It has also become clear that there are distinct subclasses of KIFs that form monomeric, heterodimeric and homodimeric motors. Here, Nobutaka Hirokawa reviews what is known about the kinesin superfamily and discusses how a study of the different types of motors is helping to elucidate the mechanism of mechanical force generation.

Visualising insulin secretion. The Minkowski Lecture 2004

Insulin secretion from pancreatic islet beta cells is a tightly regulated process, under the close control of blood glucose concentrations, neural inputs and circulating hormones. Defects in glucose-triggered insulin secretion, possibly exacerbated by a decrease in beta cell mass, are ultimately responsible for the development of type 2 diabetes. A full understanding of the mechanisms by which glucose and other nutrients trigger insulin secretion will probably be essential to allow for the development of new therapies of type 2 diabetes and for the derivation of "artificial" beta cells from embryonic stem cells as a treatment for type 1 diabetes. I focus here on recent developments in our understanding of beta cell glucose sensing, achieved in part through the development of recombinant targeted probes (luciferase, green fluorescent protein) that allow islet beta cell metabolism and Ca(2+) handling to be imaged in situ in the intact islet with single cell resolution. Combined with classical biochemistry, these techniques show that the beta cell is uniquely poised, thanks to the expression of low levels of lactate dehydrogenase and plasma membrane lactate/monocarboxylate transporters, to channel glucose carbons towards oxidative metabolism, ATP synthesis and inhibition of AMP-activated protein kinase, a newly defined regulator of insulin release. I also discuss the molecular basis of the recruitment of secretory vesicles to the cell surface, analysed by the use of new imaging techniques including total internal reflection of fluorescence, as well as the "nanomechanics" of the exocytotic event itself.

Motor proteins in cell division

The movements of eukaryotic cell division depend upon the conversion of chemical energy into mechanical work, which in turn involves the actions of motor proteins, molecular transducers that generate force and motion relative cytoskeletal elements. In animal cells, microtubule-based motor proteins of the mitotic apparatus are involved in segregating chromosomes and perhaps in organizing the mitotic apparatus itself, while microfilament-based motors in the contractile ring generate the forces that separate daughter cells during cytokinesis. This review outlines recent advances in our understanding of the roles of molecular motors in mitosis and cytokinesis.

Structure and molecular organization of the centromere-kinetochore complex

For over a century, the terms centromere and kinetochore have been used interchangeably to describe a complex locus on eukaryotic chromosomes that attaches chromosomes to spindle fibres and facilitates chromosome movement in mitosis and meiosis. This region has become the focus of research aimed at defining the mechanism of chromosome segregation. A variety of new molecular probes and vastly improved optical-imaging technology have provided much new information on the structure of this locus and raised new hopes that an understanding of its function may soon be at hand.

Integrin clustering induces kinectin accumulation

Integrin receptors mediate the formation of adhesion complexes and play important roles in signal transduction from the extracellular matrix. Integrin-based adhesion complexes (IAC) contain proteins that link integrins to the cytoskeleton and recruit signaling molecules, including vinculin,paxillin, focal adhesion kinase, talin and α-actinin. In this study, we describe a ∼160 kDa protein that is markedly enriched at IAC induced by clustering integrins with fibronectin-coated beads. Protein sequence analysis reveals that this ∼160 kDa protein is kinectin. Kinectin is an integral membrane protein found in endoplasmic reticulum, and it serves as a receptor for the motor protein kinesin. Fibronectin-induced IAC sequestered over half of the total cellular content of kinectin within 20 minutes. In addition, two other ER-resident proteins, RAP [low-density lipoprotein receptor-related protein (LRP) receptor-associated protein] and calreticulin, were found to be clustered at IAC, whereas kinesin was not. Our results identify a novel class of constituents of IAC.

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.

Molecular structure of dynein and motility of a giant sperm axoneme provided with only the outer dynein arm

The peculiar sperm axoneme of the dipteran Asphondylia ruebsaameni is characterized by an extraordinarily high number of microtubule doublets (up to 2,500) arranged in double parallel spirals. Doublets of the inner row of each spiral are tilted, so that their outer arms point towards the B-tubule of the next doublet in the outer row. Doublets are provided with only the outer arm, and no structure related to the central pair/radial spoke complex is present. When analyzed by quick-freeze, deep-etch electron microscopy, the structure of the dynein arms was shown to share the same organization described in other organisms; however, it appears to be somewhat more complex than that previously found in a related dipteran species, Monarthropalpus flavus, since the foot region of the arms displays a globular extra-domain that is intercalated between adjacent arms. Treatment of demembranated sperm with ATP and vanadate induced conformational changes in the dynein arms. SDS-page suggested the presence of a single dynein high molecular weight band or, in the gels with the best electrophoretic resolution, of two very closely spaced bands. This polypeptide positively reacted with a polyclonal antibody raised against a specific amino acid sequence located in the phosphate-binding loop of the dynein catalytic site. Dynein heavy chain-related DNA sequences corresponding to the catalytic phosphate-binding region were amplified by RT-PCR. Two distinct fragments (Asph-ax1 and Asph-ax2) encoding axonemal dynein sequences were identified. Southern blot analysis performed on genomic DNA using these sequences as a probe showed that they are part of different genes. An intron was identified in the Asph-ax1 fragment at a position corresponding to the site of a nucleotide deletion in the putative pseudogene of Monarthropalpus. Asphondylia spermatozoa exhibited in vivo a whirling movement both in the deferent duct and in the spermatheca, but they were unable to undergo processive movement in vitro. They propagated a three-dimensional wave only when constrained in a bent configuration by some mechanical means. The phylogenetic relationships between the two dipteran species, Monarthopalpus and Asphondylia, based on these biochemical and molecular data are also discussed.

Subunit heterogeneity of cytoplasmic dynein: Differential expression of 14 kDa dynein light chains in rat hippocampus

Cytoplasmic dynein is a multi-subunit protein complex in which each subunit is encoded by a few genes. How these subunit isoforms are assembled and regulated to mediate the diverse functions of cytoplasmic dynein is unknown. We previously have shown that two highly conserved 14 kDa dynein light chains, Tctex-1 and RP3, have different cargo-binding abilities. In this report, coimmunoprecipitation revealed that Tctex-1 and RP3 were present in mutually exclusive dynein complexes of brain. Two specific antibodies were used to examine the localization of these two dynein light chains in adult rat hippocampal formation and cerebral cortex. By light microscopy, Tctex-1 and RP3 immunoreactivities exhibited distinct and almost complementary distribution patterns in both brain regions. In hippocampal formation, Tctex-1 immunoreactivity was most enriched in somata of newly generated granule cells and scant in the mature granule and pyramidal cell somata. In contrast, RP3 immunoreactivity was abundant in pyramidal and granule cell somata. Ultrastructural analysis of the dentate gyrus revealed both dynein light chains were associated with various membranous organelles that often were affiliated with microtubules. In addition, Tctex-1 and RP3 immunoreactivities were preferentially and highly enriched on membranous organelles and/or vesicles of axon terminals and dendritic spines, respectively. These results suggest that dynein complexes with different subunit composition, and possibly function, are expressed differentially in a spatially and temporally regulated manner. Furthermore, Tctex-1 and RP3 may play important roles in synaptic functions.

A labile component of AMPA receptor-mediated synaptic transmission is dependent on microtubule motors, actin, and N-ethylmaleimide-sensitive factor

Glutamate receptor channels are synthesized in the cell body, are inserted into intracellular vesicles, and move to dendrites where they become incorporated into synapses. Dendrites contain abundant microtubules that have been implicated in the vesicle-mediated transport of ion channels. We have examined how the inhibition of microtubule motors affects synaptic transmission. Monoclonal antibodies that inactivate the function of dynein or kinesin were introduced into hippocampal CA1 pyramidal cells through a patch pipette. Both antibodies substantially reduced the AMPA receptor-mediated responses within 1 hr but had no effect on the NMDA receptor-mediated response. Heat-inactivated antibody or control antibodies had a much smaller effect. A component of transmission appeared to be resistant even to the combination of these inhibitors, and we therefore explored whether other agents also produce only a partial inhibition of transmission. A similar resistant component was found by using an actin inhibitor (phalloidin) or an inhibitor of NSF (N-ethylmaleimide-sensitive fusion protein)/GluR2 interaction. We then examined whether these effects were independent or occluded each other. We found that a combination of phalloidin and NSF/GluR2 inhibitor reduced the response to approximately 30% of baseline level, an effect only slightly larger than that produced by each agent alone. The addition of microtubule motor inhibitors to this combination produced no further inhibition. We conclude that there are two components of AMPA receptor-mediated transmission; one is a labile pool sensitive to NSF/GluR2 inhibitors, actin inhibitors, and microtubule motor inhibitors. A second, nonlabile pool resembles NMDA receptor channels in being nearly insensitive to any of these agents on the hour time scale of our experiments.

Analysis of the myosins encoded in the recently completed Arabidopsis thaliana genome sequence

BACKGROUND

Three types of molecular motors play an important role in the organization, dynamics and transport processes associated with the cytoskeleton. The myosin family of molecular motors move cargo on actin filaments, whereas kinesin and dynein motors move cargo along microtubules. These motors have been highly characterized in non-plant systems and information is becoming available about plant motors. The actin cytoskeleton in plants has been shown to be involved in processes such as transportation, signaling, cell division, cytoplasmic streaming and morphogenesis. The role of myosin in these processes has been established in a few cases but many questions remain to be answered about the number, types and roles of myosins in plants.

RESULTS

Using the motor domain of an Arabidopsis myosin we identified 17 myosin sequences in the Arabidopsis genome. Phylogenetic analysis of the Arabidopsis myosins with non-plant and plant myosins revealed that all the Arabidopsis myosins and other plant myosins fall into two groups - class VIII and class XI. These groups contain exclusively plant or algal myosins with no animal or fungal myosins. Exon/intron data suggest that the myosins are highly conserved and that some may be a result of gene duplication.

CONCLUSIONS

Plant myosins are unlike myosins from any other organisms except algae. As a percentage of the total gene number, the number of myosins is small overall in Arabidopsis compared with the other sequenced eukaryotic genomes. There are, however, a large number of class XI myosins. The function of each myosin has yet to be determined.

A LIS1/NUDEL/cytoplasmic dynein heavy chain complex in the developing and adult nervous system

Mutations in mammalian Lis1 (Pafah1b1) result in neuronal migration defects. Several lines of evidence suggest that LIS1 participates in pathways regulating microtubule function, but the molecular mechanisms are unknown. Here, we demonstrate that LIS1 directly interacts with the cytoplasmic dynein heavy chain (CDHC) and NUDEL, a murine homolog of the Aspergillus nidulans nuclear migration mutant NudE. LIS1 and NUDEL colocalize predominantly at the centrosome in early neuroblasts but redistribute to axons in association with retrograde dynein motor proteins. NUDEL is phosphorylated by Cdk5/p35, a complex essential for neuronal migration. NUDEL and LIS1 regulate the distribution of CDHC along microtubules, and establish a direct functional link between LIS1, NUDEL, and microtubule motors. These results suggest that LIS1 and NUDEL regulate CDHC activity during neuronal migration and axonal retrograde transport in a Cdk5/p35-dependent fashion.

Unipolar microtubule array is directly involved in nurse cell-oocyte transport

The telotrophic ovariole of Rhodnius prolixus is richly endowed with microtubules (MTs). An extensive, stable array of MTs packs the trophic core and trophic cords which link the nurse cell compartments to the growing oocytes. This system is excellent to study MT-based transport as the MTs are believed to play a role in transport of nurse cell-produced mitochondria, ribosomes, and mRNAs to the oocytes. We investigated MT polarity and molecular MT motors in this unidirectional transport system. Hook decoration revealed that the MTs of the trophic core and cords have their plus (+) ends in the tropharium and minus (-) ends in the oocytes. Video differential interference optics (DIC) microscopy showed that vesicle transport was saltatory, ATP-dependent, and had an average velocity of 0.77 micron/sec toward the oocyte. Transport was sensitive to 2 mM N-ethylmaleimide (NEM) and 50 microM vanadate and resistant to 1 mM 5'-adenylylimidodiphosphate (AMP-PNP) and 5 microM vanadate. We report that the unipolar, acetylated trophic cord MTs play a direct role in nurse cell-oocyte transport via a cytoplasmic dynein-like retrograde motor.

Poly(A) mRNA is attached to insect ovarian microtubules in vivo in a nucleotide-sensitive manner

In ovarioles of hemipteran insects, RNA passes from anteriorly positioned nurse cells to the chain of developing oocytes via extended nutritive tubes. These intercellular connections may reach several millimeters in length. Each nutritive tube is comprised of many thousands of parallel microtubules. We have extracted microtubule bundles from isolated nutritive tubes of Notonecta glauca and, using hybridization techniques, provide evidence of poly(A) mRNA attachment to microtubules in vivo. We also show this attachment to be nucleotide-sensitive, which is typical of a motor protein-mediated interaction. The pattern of nucleotide sensistivity is indicative of a kinesin motor mechanism. We provide evidence that a kinesin is present in the nutritive tube translocation channels and is a component of the mRNA/microtubule bundles isolated and extracted from them. Our findings are consistent with kinesin-driven transport of mRNA along the nutritive tube microtubules.

Three-step chromatographic purification procedure for the production of a his-tag recombinant kinesin overexpressed in E. coli

A kinesin gene has been cloned by RT-PCR (reverse transcription polymerase chain reaction) from Trypanosoma brucei and the corresponding protein overexpressed as a recombinant His-tag (histidine-tag) kinesin in E. coli in order to study its biochemical properties and to determine its three-dimensional structure by X-ray crystallography. Starting from several liters of culture, an ultrasonic homogenizer was used for cell disruption and an unclarified feedstock was obtained. From this homogenate, a protein was then purified by immobilized metal affinity chromatography (IMAC) using expanded bed adsorption (EBA) technology (Streamline chelating). For this capture step, 100% of the recombinant protein was purified with more than 90% of purity. This step was followed by ion-exchange chromatography (Q Sepharose Fast Flow) for intermediate purification (96% purity, 53% recovery) and by size-exclusion chromatography with Superdex 75 as a polishing step (99% purity, 93% recovery). We then separated two forms of kinesin, a dimer (70%) and a monomer (30%). It was then possible to purify His-tag recombinant protein directly from feedstock in a rapid and efficient way and to isolate two forms of kinesin.

Rhodopsin trafficking and its role in retinal dystrophies

We review the sorting/targeting steps involved in the delivery of rhodopsin to the outer segment compartment of highly polarized photoreceptor cells. The transport of rhodopsin includes (1) the sorting/budding of rhodopsin-containing vesicles at the trans-Golgi network, (2) the directional translocation of rhodopsin-bearing vesicles through the inner segment, and (3) the delivery of rhodopsin across the connecting cilium to the outer segment. Several independent lines of evidence suggest that the carboxyl-terminal, cytoplasmic tail of rhodopsin is involved in the post-Golgi trafficking of rhodopsin. Inappropriate subcellular targeting of naturally occurring rhodopsin mutants in vivo leads to photoreceptor cell death. Thus, the genes encoding mutations in the cellular components involved in photoreceptor protein transport are likely candidate genes for retinal dystrophies.

Nuclear movement in filamentous fungi

One of the most striking features of eukaryotic cells is the organization of specific functions into organelles such as nuclei, mitochondria, chloroplasts, the endoplasmic reticulum, vacuoles, peroxisomes or the Golgi apparatus. These membrane-surrounded compartments are not synthesized de novo but are bequeathed to daughter cells during cell division. The successful transmittance of organelles to daughter cells requires the growth, division and separation of these compartments and involves a complex machinery consisting of cytoskeletal components, mechanochemical motor proteins and regulatory factors. Organelles such as nuclei, which are present in most cells in a single copy, must be precisely positioned prior to cytokinesis. In many eukaryotic cells the cleavage plane for cell division is defined by the location of the nucleus prior to mitosis. Nuclear positioning is thus absolutely crucial in the unequal cell divisions that occur during development and embryogenesis. Yeast and filamentous fungi are excellent organisms for the molecular analysis of nuclear migration because of their amenability to a broad variety of powerful analytical methods unavailable in higher eukaryotes. Filamentous fungi are especially attractive models because the longitudinally elongated cells grow by apical tip extension and the organelles are often required to migrate long distances. This review describes nuclear migration in filamentous fungi, the approaches used for and the results of its molecular analysis and the projection of the results to other organisms.

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.

Role of fungal dynein in hyphal growth, microtubule organization, spindle pole body motility and nuclear migration

Cytoplasmic dynein is a microtubule-associated motor protein with several putative subcellular functions. Sequencing of the gene (DHC1) for cytoplasmic dynein heavy chain of the filamentous ascomycete, Nectria haematococca, revealed a 4,349-codon open reading frame (interrupted by two introns) with four highly conserved P-loop motifs, typical of cytoplasmic dynein heavy chains. The predicted amino acid sequence is 78.0% identical to the cytoplasmic dynein heavy chain of Neurospora crassa, 70.2% identical to that of Aspergillus nidulans and 24.8% identical to that of Saccharomyces cerevisiae. The genomic copy of DHC1 in N. haematococca wild-type strain T213 was disrupted by inserting a selectable marker into the central motor domain. Mutants grew at 33% of the wild-type rate, forming dense compact colonies composed of spiral and highly branched hyphae. Major cytological phenotypes included (1) absence of aster-like arrays of cytoplasmic microtubules focused at the spindle pole bodies of post-mitotic and interphase nuclei, (2) limited post-mitotic nuclear migration, (3) lack of spindle pole body motility at interphase, (4) failure of spindle pole bodies to anchor interphase nuclei, (5) nonuniform distribution of interphase nuclei and (6) small or ephemeral Spitzenkorper at the apices of hyphal tip cells. Microtubule distribution in the apical region of tip cells of the mutant was essentially normal. The nonuniform distribution of nuclei in hyphae resulted primarily from a lack of both post-mitotic nuclear migration and anchoring of interphase nuclei by the spindle pole bodies. The results support the hypothesis that DHC1 is required for the motility and functions of spindle pole bodies, normal secretory vesicle transport to the hyphal apex and normal hyphal tip cell morphogenesis.

Cloning and expression of human mitotic centromere-associated kinesin gene

The human homologue of the hamster mitotic centromere-associated kinesin (HsMCAK) gene containing a central type motor domain was isolated from a Jurkat T-cell derived cDNA library. The HsMCAK gene has a predicted 723 amino acid open reading frame, encoding a 81 kDa protein that shares 79.2% homology with hamster MCAK. Unstimulated T lymphocytes contained no detectable HsMCAK-specific mRNA. Activation of resting T-cells by immobilized anti-CD3 resulted in the expression of a 2.9-kb transcript during the S phase of the cell cycle. The TPA-induced monocytic differentiation of U937 which also results in growth-arrest abruptly downregulates the expression of HsMCAK. Removal of TPA restored the growth of the cell through the retrodifferentiation process and the subsequent expression of HsMCAK. HsMCAK is expressed in tissues containing dividing cells, such as thymus, testis, small intestine, colon (mucosal lining), and placenta. These results suggest that the expression of HsMCAK is first detected in early S phase to support the proliferative response and is strictly regulated at the transcriptional level.

Acrylamide and carbon disulfide treatments increase the rate of rat brain tubulin polymerization

Acrylamide and carbon disulfide produce central-peripheral distal axonopathy in experimental animals and humans. The main feature of this disease is the focal swellings containing neurofilaments in distal axons, followed by nerve degeneration beyond these swellings. We studied the possible role of tubulin assembly kinetics in this disease. The rats were either administered acrylamide (50 mg/kg, ip, saline) or exposed to carbon disulfide (700 ppm, 9 h) via inhalation for 12 and 15 d, respectively. Tubulin, purified from both acrylamide-(10.37 +/- 0.3 vs 11.3 +/- 0.15) and carbon disulfide-treated (9.72 +/- 0.5 vs 11.18 +/- 0.25) rat brains showed increase in Vmax (OD/min x 10(3)) of its polymerization. However, only acrylamide treatment showed a decrease in time to Vmax, when brain supernatant was used for tubulin polymerization. In vitro addition of acrylamide (0.1-1 mM) to bovine brain tubulin also showed a decrease in time to Vmax (16-21%) of its polymerization. Carbon disulfide treatment of rats, on the other hand, showed a decrease in MAP-2 and an increase in a 120-kDa peptide concentration. The latter showed immunoreactivity with anti-MAP-2. The increase in the rate of tubulin polymerization by acrylamide and carbon disulfide treatment may alter the rate of transport of axonal constituents, including neurofilament, and contribute toward their accumulation in the focal swellings observed in this neuropathy.

Characterization of KIFC2, a neuronal kinesin superfamily member in mouse

Members of the kinesin superfamily of microtubule-associated proteins are involved in a variety of intracellular processes including cell division and organelle transport. In the case of axonal transport, all kinesin superfamily members reported thus far appear to play a role in anterograde transport, while a different type of microtubule motor, dynein, appears to function in retrograde transport. To better understand the role of kinesins in axonal transport, we cloned and characterized KIFC2, a novel kinesin superfamily member from mouse brain. KIFC2 encodes a 792 amino acid protein, which contains the conserved motor domain at the C-terminal end of the protein and is most similar to members of the KAR3 family involved in cell division. However, expression analysis localized KIFC2 mRNA to nonproliferative neuronal cells in the central nervous system, and immunolocalization studies demonstrated that KIFC2 is present in axons and dendrites of neurons in the central and peripheral nervous systems. Immunolocalization and biochemical fractionation studies suggest that KIFC2 localizes with some, but not all, axonally transported organelles. Finally, ligation of mouse peripheral nerves showed that KIFC2 accumulates at the proximal and distal sides of an axonal ligature. Taken together, the data suggest that, unlike other C-terminal motor proteins that appear to be involved in cell division, KIFC2 may play a role in retrograde axonal transport.

Structure within eukaryotic cytoplasm and its relationship to glycolytic metabolism

Taken together, the results reviewed here indicate that both structural proteins and enzymes exist in a relatively mobile, uncomplexed form and in a relatively immobile form, complexed with the matrix. The relative amounts of free and complexed forms of each protein are dependent upon the local concentrations of both small molecules and other macromolecules and hence may vary in time and space throughout the cell. Free and cytomatrix-bound enzymes exchange rapidly, while free and cytomatrix-bound structural proteins exchange more slowly. These two distinct time scales suggest that the slowly exchanging structural proteins form the core of fibrous structural elements--having many stabilizing intermolecular contacts with near neighbours--whereas the more rapidly exchanging enzymes adsorb to the surface of the structural elements and have fewer near neighbour contacts. The hierarchical nature of these associations is depicted schematically in Figure 3. Metabolism is proposed to proceed primarily via transport of small metabolites rather than by transport of enzymes, which may be organized in functional clusters to facilitate, metabolic regulation.

Human kinesin light (beta) chain gene: DNA sequence and functional characterization of its promoter and first exon

Kinesins are tubulin molecular motors whose function is to transport organelles within cells. Very little is known about the regulation of expression of these proteins. We have characterized the gene product of one differentially spliced mRNA of the human light chain kinesin and cloned its promoter region. A full-length kinesin cDNA was translated in vitro in a cell-free system, producing a 70-kDa protein. Using this cDNA as a probe, we isolated and sequenced the promoter, first exon, and part of the first intron of this gene from a genomic lambda EMBL3 human placental DNA library. The whole gene spans more than 90 kb. The beta kinesin promoter region confers only constitutive transcription to the bacterial chloramphenicol acetyltransferase (CAT) reporter gene. In permanently transfected human HeLa and NB100 neuroblastoma cells, a reporter gene containing the promoter and part of the first exon of beta kinesin was 75-fold more active than the HSV-tk promoter. The first exon contains the 5'-untranslated sequence capable of forming a stable double-hairpin loop, which functions as a translational enhancer. Its deletion decreases the efficiency of in vitro translation of beta kinesin mRNA and confers increased translation to a CAT reporter gene.

Equilibrium studies of kinesin-nucleotide intermediates

We have examined the energetics of the interactions of two kinesin constructs with nucleotide and microtubules to develop a structural model of kinesin-dependent motility. Dimerization of the constructs was found to reduce the maximum rate of the microtubule-activated kinesin ATPase 5-fold. Beryllium fluoride and aluminum fluoride also reduce this rate, and they increase the affinity of kinesin for microtubules. By contrast, inorganic phosphate reduces the affinity of a dimeric kinesin construct for microtubules. These findings are consistent with a model in which the kinesin head can assume one of two conformations, "strong" or "weak" binding, determined by the nature of the nucleotide that occupies the active site. Data for dimeric kinesin are consistent with a model in which kinesin.ATP binds to the microtubule in a strong state with positive cooperativity; hydrolysis of ATP to ADP+P(i) leads to dissociation of one of the attached heads and converts the second, attached head to a weak state; and dissociation of phosphate allows the second head to reattach. These results also argue that a large free energy change is associated with formation of kinesin.ADP.P(i) and that this step is the major pathway for dissociation of kinesin from the microtubule.

The human CAS protein which is homologous to the CSE1 yeast chromosome segregation gene product is associated with microtubules and mitotic spindle

Human CAS cDNA contains a 971-aa open reading frame that is homologous to the essential yeast gene CSE1. CSE1 is involved in chromosome segregation and is necessary for B-type cyclin degradation in mitosis. Using antibodies to CAS, it was shown that CAS levels are high in proliferating and low in nonproliferating cells. Here we describe the distribution of CAS in cells and tissues analyzed with antibodies against CAS. CAS is an approximately 100-kDa protein present in the cytoplasm of proliferating cells at levels between 2 x 10(5) and 1 x 10(6) molecules per cell. The intracellular distribution of CAS resembles that of tubulin. In interphase cells, anti-CAS antibody shows microtubule-like patterns and in mitotic cells it labels the mitotic spindle. CAS is removed from microtubules by mild detergent treatment (cytoskeleton preparations) and in vincristine- or taxol-treated cells. CAS is diffusely distributed in the cytoplasm with only traces present in tubulin paracrystals or bundles. Thus, CAS appears to be associated with but not to be an integral part of microtubules. Immunohistochemical staining of frozen tissues shows elevated amounts of CAS in proliferating cells such as testicular spermatogonia and cells in the basal layer cells of the colon. CAS was also concentrated in the respiratory epithelium of the trachea and in axons and Purkinje cells in the cerebellum. These cells contain many microtubules. The cellular location of CAS is consistent with an important role in cell division as well as in ciliary movement and vesicular transport.

A novel plant calmodulin-binding protein with a kinesin heavy chain motor domain

Calmodulin, a ubiquitous calcium-binding protein, regulates many diverse cellular functions by modulating the activity of the proteins that interact with it. Here, we report isolation of a cDNA encoding a novel kinesin-like calmodulin-binding protein (KCBP) from Arabidopsis using biotinylated calmodulin as a probe. Calcium-dependent binding of the cDNA-encoded protein to calmodulin is confirmed by 35S-labeled calmodulin. Sequence analysis of a full-length cDNA indicates that it codes for a protein of 1261 amino acids. The predicted amino acid sequence of the KCBP has a domain of about 340 amino acids in the COOH terminus that shows significant sequence similarity with the motor domain of kinesin heavy chains and kinesin-like proteins and contains ATP and microtubule binding sites typical of these proteins. Outside the motor domain, the KCBP has no sequence similarity with any of the known kinesins, but contains a globular domain in the NH2 terminus and a putative coiled-coil region in the middle. By analyzing the calmodulin binding activity of truncated proteins expressed in Escherichia coli, the calmodulin binding region is mapped to a stretch of about 50 amino acid residues in the COOH terminus region of the protein. Using a synthetic peptide, the calmodulin binding domain is further narrowed down to a 23-amino acid stretch. The synthetic peptide binds to calmodulin with high affinity in a calcium-dependent manner as judged by electrophoretic mobility shift assay of calmodulin-peptide complex. The KCBP is coded by a single gene and is highly expressed in developing flowers and suspension cultured cells. Although many kinesin heavy chains and kinesin-like proteins have been extensively characterized at the biochemical and molecular level in evolutionarily distant organisms, none of them is known to bind calmodulin. The plant kinesin-like protein with a calmodulin binding domain and a unique amino-terminal region is a new member of the kinesin superfamily. The presence of a calmodulin-binding motif in a kinesin heavy chain-like protein suggests a role for calcium and calmodulin in kinesin-driven motor function(s) in plants.

Effect of a dynein inhibitor on vasopressin action in toad urinary bladder

1. The effect of the dynein inhibitor erythro-9-[3-(2-hydroxynonyl)] adenine (EHNA) on the osmotic water flow response to vasopressin or exogenous cAMP has been investigated in isolated toad urinary bladders. 2. Pretreatment with serosal EHNA had no effect on basal water flow, but inhibited the development and maintenance of the hydrosmotic response to vasopressin (20 mU ml-1) or 8-(4-parachlorophenylthio)-adenosine 3',5'-cyclic monophosphate (8 CPT-cAMP; 0.1 mM). 3. The inhibitory effect of EHNA on vasopressin-induced water flow was dose dependent. Inhibition occurred in the dose range in which EHNA inhibits the ATPase and motor activities of dynein in vitro. 4. EHNA also inhibited the maintenance of the high rate of water flow established by prior exposure to vasopressin. 5. The inhibitory effect of EHNA on the onset phase of the vasopressin response was attenuated after exposure of the tissue to the microtubule-disruptive drug nocodazole but was fully additive with that of cytochalasin B. 6. EHNA inhibited basal and vasopressin-stimulated transepithelial sodium transport. 7. The findings support the view that EHNA inhibits hormone-induced water flow through an action on a cytoplasmic dynein. The results are consistent with the hypothesis that dynein is involved in the microtubule-based delivery of water channel-containing vesicles to the apical membrane of the granular epithelial cells during both the onset and maintenance of the water permeability response to vasopressin.

Yeast motor proteins

Yeast accomplish a variety of intracellular motile events with the aid of mechanochemical enzymes known as motor proteins. This review covers the current state of knowledge on myosins, kinesins, dyneins, dynamins and SMC proteins present in yeast cells, and the most important developments in the study of yeast mitosis. Both topics have seen rapid progress over the past few years.

Mechanics of motility: distinct dynein binding domains on alpha- and beta-tubulin

Microtubules (MTs) interact with force-generating proteins to generate a variety of intracellular movements, including intracellular particle transport, ciliary-flagellar beating, and chromosome-spindle movements during mitosis-meiosis. Relatively little is known about the mechanics of these motor-MT interactions, in part because the motor binding domains of the MT and the corresponding MT binding domains of the motor have not been well characterized. Using a flagellar motility assay, we report that the MT subunits, alpha- and beta-tubulin, each contain a dynein binding domain located near the C-termini of their respective tubulin subunits. Blocking either alpha- or beta-tubulin binding domains of dynein attenuates motility in demembranated sea urchin sperm up to 50%. Interestingly, blocking both alpha- and beta-tubulin binding domains on dynein produces much greater decreases in motility. These data suggest that flagellar dynein binds to both subunits of the MT polymer, alpha- and beta-tublin. In addition, the two subunits appear to contribute equivalent, but functionally separate, roles to flagellar motility.

Three-dimensional structure of a tubulin-motor-protein complex

The kinesin superfamily is a class of microtubule-based mechano-enzymes involved in intracellular transport and chromosome movements. Molecules that move towards either the plus end or the minus end of microtubules are represented within the family. The motor domains of these molecules exhibit considerable sequence homology and contain both the ATP- and microtubule-binding sites (reviewed in refs 1, 2). Here we focus on non-claret disjunctional (ncd), a minus-end-directed motor involved in chromosome segregation in meiosis and early mitosis in Drosophila. We have calculated a three-dimensional map of tubulin sheets decorated with monomeric recombinant ncd motor domains by negative-stain electron microscopy and image analysis. Comparisons with a control structure of tubulin alone reveal that each motor domain binds to the crest of a single protofilament, making extensive contacts with both the alpha and beta tubulin monomers. Binding of the motor domain results in significant conformational changes in both of the tubulin monomers.

Kinectin, an essential anchor for kinesin-driven vesicle motility

The membrane anchor for the molecular motor kinesin is a critical site involved in intracellular membrane trafficking. Monoclonal antibodies specific for the cytoplasmic surface of chick brain microsomes were used to define proteins involved in microtubule-dependent transport. One of four antibodies tested inhibited plus-end-directed vesicle motility by approximately 90 percent even as a monovalent Fab fragment and reduced kinesin binding to vesicles. This antibody bound to the cytoplasmic domain of kinectin, an integral membrane protein of the endoplasmic reticulum that binds to kinesin. Thus, kinectin acted as a membrane anchor protein for kinesin-driven vesicle motility.

Isolation of the microtubule-vesicle motor kinesin from rat liver: selective inhibition by cholestatic bile acids

BACKGROUND/AIMS

Vesicular transport is supported by microtubule-based, force-transducing adenosine triphosphatases (ATPases), such as kinesin, a ubiquitous motor enzyme that has been well studied in neuronal tissues. Although vesicular transport is important for hepatocellular secretory and clearance activities, the role of kinesin in liver function is poorly understood. Furthermore, the effects of bile acids on kinesin are unknown.

METHODS

Kinesin was purified from rat liver cytosol by conventional chromatography and microtubule affinity binding and was characterized by immunoblotting with domain-specific kinesin antibodies and amino acid sequencing of tryptic fragments. Kinesin activity was measured with and without bile acids using an in vitro motility assay and ATPase assays.

RESULTS

Immunoblot analysis and partial amino acid sequencing of purified kinesin showed that the sequence at the heavy chain of hepatic kinesin is nearly identical to that of brain kinesin. Purified kinesin transported microtubules in vitro with a velocity of approximately 0.5 microns/s; this activity was significantly inhibited by 0.5-1 mmol/L taurochenodeoxycholate but not by tauroursodeoxycholate. At a dose of 1 mmol/L, chenodeoxycholate conjugates, but not ursodeoxycholate or cholate conjugates, directly inhibited the ATPase activities of kinesin and another microtubule motor, cytoplasmic dynein.

CONCLUSIONS

Cholestatic concentrations of chenodeoxycholate conjugates directly inhibit the activity of microtubule motors, suggesting a possible mechanism for impairment of vesicular transport in cholestasis.

Aspergillus nidulans apsA (anucleate primary sterigmata) encodes a coiled-coil protein required for nuclear positioning and completion of asexual development

Many fungi are capable of growing by polarized cellular extension to form hyphae or by isotropic expansion to form buds. Aspergillus nidulans anucleate primary sterigmata (apsA) mutants are defective in nuclear distribution in both hyphae and in specialized, multicellular reproductive structures, called conidiophores. apsA mutations have a negligible effect on hyphal growth, unlike another class of nuclear distribution (nud) mutants. By contrast, they almost completely block entry of nuclei into primary buds, or sterigmata (bud nucleation), produced during development of conidiophores. Failure of the primary sterigmata to become nucleated results in developmental arrest and a failure to activate the transcriptional program associated with downstream developmental steps. However, occasionally in mutants a nucleus enters a primary bud and this event relieves the developmental blockage. Thus, there is a stringent developmental requirement for apsA function, but only at the stage of primary bud formation. apsA encodes a 183-kD coiled-coil protein with similarity to Saccharomyces cerevisiae NUM1p, required for nuclear migration in the budding process.

Characterization of kinectin, a kinesin-binding protein: primary sequence and N-terminal topogenic signal analysis

Kinectin is a kinesin-binding protein (Toyoshima et al., 1992) that is required for kinesin-based motility (Kumar et al., 1995). A kinectin cDNA clone containing a 4.7-kilobase insert was isolated from an embryonic chick brain cDNA library by immunoscreening with a panel of monoclonal antibodies. The cDNA contained an open reading frame of 1364 amino acids encoding a protein of 156 kDa. A bacterially expressed product of the full length cDNA bound purified kinesin. Transient expression in CV-1 cells gave an endoplasmic reticulum distribution that depended upon the N-terminal domain. Analysis of the predicted amino acid sequence indicated a highly hydrophobic near N-terminal stretch of 28 amino acids and a large portion (326-1248) of predicted alpha helical coiled coils. The 30-kDa fragment containing the N-terminal hydrophobic region was produced by cell-free in vitro translation and found to assemble with canine pancreas rough microsomes. Cleavage of the N terminus was not observed confirming its role as a potential transmembrane domain. Thus, the kinectin cDNA encodes a cytoplasmic-oriented integral membrane protein that binds kinesin and is likely to be a coiled-coil dimer.

Cytoplasmic dynein undergoes intracellular redistribution concomitant with phosphorylation of the heavy chain in response to serum starvation and okadaic acid

Cytoplasmic dynein is a microtubule-binding protein which is considered to serve as a motor for retrograde organelle movement. In cultured fibroblasts, cytoplasmic dynein localizes primarily to lysosomes, membranous organelles whose movement and distribution in the cytoplasm have been shown to be dependent on the integrity of the microtubule cytoskeleton. We have recently identified conditions which lead to an apparent dissociation of dynein from lysosomes in vivo, indicating that alterations in membrane binding may be involved in the regulation of retrograde organelle movement (Lin, S. X. H., and C. A. Collins. 1993. J. Cell Sci. 105:579-588). Both brief serum withdrawal and low extracellular calcium levels induced this alteration, and the effect was reversed upon addition of serum or additional calcium. Here we demonstrate that the phosphorylation state of the dynein molecule is correlated with changes in its intracellular distribution in normal rat kidney fibroblasts. Dynein heavy chain phosphorylation level increased during serum starvation, and decreased back to control levels upon subsequent addition of serum. We found that okadaic acid, a phosphoprotein phosphatase inhibitor, mimicked the effects of serum starvation on both phosphorylation and the intracellular redistribution of dynein from a membrane-associated pool to one that was more soluble, with similar dose dependence for both phenomena. Cell fractionation by differential detergent extraction revealed that a higher proportion of dynein was present in a soluble pool after serum starvation than was found in comparable fractions from control cells. Our data indicate that cytoplasmic dynein is phosphorylated in vivo, and changes in phosphorylation state may be involved in a regulatory mechanism affecting the distribution of this protein among intracellular compartments.

Drosophila cytoplasmic dynein, a microtubule motor that is asymmetrically localized in the oocyte

The unidirectional movements of the microtubule-associated motors, dyneins, and kinesins, provide an important mechanism for the positioning of cellular organelles and molecules. An intriguing possibility is that this mechanism may underlie the directed transport and asymmetric positioning of morphogens that influence the development of multicellular embryos. In this report, we characterize the Drosophila gene, Dhc64C, that encodes a cytoplasmic dynein heavy chain polypeptide. The primary structure of the Drosophila cytoplasmic dynein heavy chain polypeptide has been determined by the isolation and sequence analysis of overlapping cDNA clones. Drosophila cytoplasmic dynein is highly similar in sequence and structure to cytoplasmic dynein isoforms reported for other organisms. The Dhc64C dynein transcript is differentially expressed during development with the highest levels being detected in the ovaries of adult females. Within the developing egg chambers of the ovary, the dynein gene is predominantly transcribed in the nurse cell complex. In contrast, the encoded dynein motor protein displays a striking accumulation in the single cell that will develop as the oocyte. The temporal and spatial pattern of dynein accumulation in the oocyte is remarkably similar to that of several maternal effect gene products that are essential for oocyte differentiation and axis specification. This distribution and its disruption by specific maternal effect mutations lends support to recent models suggesting that microtubule motors participate in the transport of these morphogens from the nurse cell cytoplasm to the oocyte.