Centractin (ARP1) associates with spectrin revealing a potential mechanism to link dynactin to intracellular organelles

Cytoplasmic dynein and dynactin as likely candidates for microtubule-dependent apical targeting of pancreatic zymogen granules

The critical role of microtubules in vectorial delivery of post-Golgi carrier vesicles to the apical cell surface has been established for various polarized epithelial cell types. In the present study we used secretory granules of the rat and chicken pancreas, termed zymogen granules, as model system for apically bound post-Golgi carrier vesicles that underlie the regulated exocytotic pathway. We found that targeting of zymogen granules to the apical cell surface requires an intact microtubule system which contains its colchicine-resistant organizing center and, thus, the microtubular minus ends close to the apical membrane domain. Purified zymogen granules and their membranes were found to be associated with cytoplasmic dynein intermediate and heavy chain and to contain the major components of the dynein activator complex, dynactin, i.e. p150Glued, p62, p50, Arp1, and beta-actin. Kinesin heavy chain and the kinesin receptor, 160 kD kinectin, were not detected as components of zymogen granules. Immunofluorescence staining showed a zymogen granule-like distribution for dynein and dynactin (p150Glued, p62, p50, Arpl) in the apical cytoplasm, whereas kinesin and kinectin were largely concentrated in the basal half of the cells in a pattern similar to the distribution of calreticulin, a component of the endoplasmic reticulum. Secretory granules of non-polarized chromaffin cells of the bovine adrenal medulla, that are assumed to underlie microtubular plus end targeting from the Golgi apparatus to the cell periphery, were not found to be associated with dynein or dynactin. To our knowledge, this is the first demonstration of major components of the dynein-dynactin complex associated with the membrane of a biochemically and functionally well-defined organelle which is considered to underlie a vectorial minus end-driven microtubular transport critically involved in precise delivery of digestive enzymes to the apically located acinar lumen.

Gene knockouts reveal separate functions for two cytoplasmic dyneins in Tetrahymena thermophila

In many organisms, there are multiple isoforms of cytoplasmic dynein heavy chains, and division of labor among the isoforms would provide a mechanism to regulate dynein function. The targeted disruption of somatic genes in Tetrahymena thermophila presents the opportunity to determine the contributions of individual dynein isoforms in a single cell that expresses multiple dynein heavy chain genes. Substantial portions of two Tetrahymena cytoplasmic dynein heavy chain genes were cloned, and their motor domains were sequenced. Tetrahymena DYH1 encodes the ubiquitous cytoplasmic dynein Dyh1, and DYH2 encodes a second cytoplasmic dynein isoform, Dyh2. The disruption of DYH1, but not DYH2, resulted in cells with two detectable defects: 1) phagocytic activity was inhibited, and 2) the cells failed to distribute their chromosomes correctly during micronuclear mitosis. In contrast, the disruption of DYH2 resulted in a loss of regulation of cell size and cell shape and in the apparent inability of the cells to repair their cortical cytoskeletons. We conclude that the two dyneins perform separate tasks in Tetrahymena.

In vitro approaches to study actin and microtubule dependent cell processes

Actin and microtubules represent complex polymer systems that play essential roles during many cellular processes including chromosome segregation, cytokinesis and motility. The dynamic nature of actin and microtubules together with their regulation by a myriad of proteins makes their study both fascinating and challenging. Over the past few years there has been an increasing move towards development of in vitro systems to facilitate the elucidation of the molecular basis of actin and microtubule dependent cell processes. This review focuses on some of the recent developments using in vitro assays to dissect the cellular role of the actin and microtubule cytoskeleton.

Cytoplasmic dynein and dynactin in cell division and intracellular transport

Since the initial discovery of cytoplasmic dynein, it has become apparent that this microtubule-based motor is involved in several cellular functions including cell division and intracellular transport. Another multisubunit complex, dynactin, may be required for most, if not all, cytoplasmic dynein-driven activities and may provide clues to dynein's functional diversity. Recent genetic and biochemical findings have illuminated the cellular roles of dynein and dynactin and provided insight into the functional mechanism of this complex motor.

How to get to the right place at the right time: Rab/Ypt small GTPases and vesicle transport

Vesicles often must be transported over long distances in a very crowded cytoplasmic environment encumbered by the cytoskeleton and membranes of different origin that provide an important barrier to their free diffusion. In animal cells with specialised tasks, such as neurons or endothelial cells, vesicles that are directed to the cell periphery are linked to the microtubular cytoskeleton tracks via association with motor proteins that allow their vectorial movement. In lower eukaryotes the actin cytoskeleton plays a prominent role in organising vesicle movement during polarised growth and mating. The Ras-like small GTPases of the Rab/Ypt family play an essential role in vesicle trafficking and due to their diversity and specific localisation have long been implicated in the selective delivery of vesicles. Recent evidence has cast doubt on the classical point of view of how this class of proteins acts in vesicle transport and suggests their involvement also in the events that permit vesicle anchoring to the cytoskeleton. Therefore, after a brief review of what is known about how vesicle movement is achieved in mammalian and yeast systems, and how Rab/Ypt proteins regulate the vesicle predocking events, it is discussed how these proteins might participate in the events that lead to vesicle movement through association with the cytoskeleton machinery.

A widely expressed betaIII spectrin associated with Golgi and cytoplasmic vesicles

Spectrin is an important structural component of the plasma membrane skeleton. Heretofore-unidentified isoforms of spectrin also associate with Golgi and other organelles. We have discovered another member of the beta-spectrin gene family by homology searches of the GenBank databases and by 5' rapid amplification of cDNA ends of human brain cDNAs. Collectively, 7,938 nucleotides of contiguous clones are predicted to encode a 271,294-Da protein, called betaIII spectrin, with conserved actin-, protein 4.1-, and ankyrin-binding domains, membrane association domains 1 and 2, a spectrin dimer self-association site, and a pleckstrin-homology domain. betaIII spectrin transcripts are concentrated in the brain and present in the kidneys, liver, and testes and the prostate, pituitary, adrenal, and salivary glands. All of the tested tissues contain major 9.0-kb and minor 11.3-kb transcripts. The human betaIII spectrin gene (SPTBN2) maps to chromosome 11q13 and the mouse gene (Spnb3) maps to a syntenic region close to the centromere on chromosome 19. Indirect immunofluorescence studies of cultured cells using antisera specific to human betaIII spectrin reveal a Golgi-associated and punctate cytoplasmic vesicle-like distribution, suggesting that betaIII spectrin associates with intracellular organelles. This distribution overlaps that of several Golgi and vesicle markers, including mannosidase II, p58, trans-Golgi network (TGN)38, and beta-COP and is distinct from the endoplasmic reticulum markers calnexin and Bip. Liver Golgi membranes and other vesicular compartment markers cosediment in vitro with betaIII spectrin. betaIII spectrin thus constitutes a major component of the Golgi and vesicular membrane skeletons.

Microtubule-dependent vesicle transport: modulation of channel and transporter activity in liver and kidney

Microtubule-based vesicle transport driven by kinesin and cytoplasmic dynein motor proteins facilitates several membrane-trafficking steps including elements of endocytosis and exocytosis in many different cell types. Most early studies on the role of microtubule-dependent vesicle transport in membrane trafficking focused either on neurons or on simple cell lines. More recently, other work has considered the role of microtubule-based vesicle transport in other physiological systems, including kidney and liver. Investigation of the role of microtubule-based vesicle transport in membrane trafficking in cells of the kidney and liver suggests a major role for microtubule-based vesicle transport in the rapid and directed movement of ion channels and transporters to and from the apical plasma membranes, events essential for kidney and liver function and homeostasis. This review discusses the evidence supporting a role for microtubule-based vesicle transport and the motor proteins, kinesin and cytoplasmic dynein, in different aspects of membrane trafficking in cells of the kidney and liver, with emphasis on those functions such as maintenance of ion channel and transporter composition in apical membranes that are specialized functions of these organs. Evidence that defects in microtubule-based transport contribute to diseases of the kidney and liver is also discussed.

Characterization of the p22 subunit of dynactin reveals the localization of cytoplasmic dynein and dynactin to the midbody of dividing cells

Dynactin, a multisubunit complex that binds to the microtubule motor cytoplasmic dynein, may provide a link between dynein and its cargo. Many subunits of dynactin have been characterized, elucidating the multifunctional nature of this complex. Using a dynein affinity column, p22, the smallest dynactin subunit, was isolated and microsequenced. The peptide sequences were used to clone a full-length human cDNA. Database searches with the predicted amino acid sequence of p22 indicate that this polypeptide is novel. We have characterized p22 as an integral component of dynactin by biochemical and immunocytochemical methods. Affinity chromatography experiments indicate that p22 binds directly to the p150(Glued) subunit of dynactin. Immunocytochemistry with antibodies to p22 demonstrates that this polypeptide localizes to punctate cytoplasmic structures and to the centrosome during interphase, and to kinetochores and to spindle poles throughout mitosis. Antibodies to p22, as well as to other dynactin subunits, also revealed a novel localization for dynactin to the cleavage furrow and to the midbodies of dividing cells; cytoplasmic dynein was also localized to these structures. We therefore propose that dynein/dynactin complexes may have a novel function during cytokinesis.

Annexin VI-mediated loss of spectrin during coated pit budding is coupled to delivery of LDL to lysosomes

Previously we reported that annexin VI is required for the budding of clathrin-coated pits from human fibroblast plasma membranes in vitro. Here we show that annexin VI bound to the NH2-terminal 28-kD portion of membrane spectrin is as effective as cytosolic annexin VI in supporting coated pit budding. Annexin VI-dependent budding is accompanied by the loss of approximately 50% of the spectrin from the membrane and is blocked by the cysteine protease inhibitor N-acetyl-leucyl-leucyl-norleucinal (ALLN). Incubation of fibroblasts in the presence of ALLN initially blocks the uptake of low density lipoprotein (LDL), but the cells recover after 1 h and internalize LDL with normal kinetics. The LDL internalized under these conditions, however, fails to migrate to the center of the cell and is not degraded. ALLN-treated cells have twice as many coated pits and twofold more membrane clathrin, suggesting that new coated pits have assembled. Annexin VI is not required for the budding of these new coated pits and ALLN does not inhibit. Finally, microinjection of a truncated annexin VI that inhibits budding in vitro has the same effect on LDL internalization as ALLN. These findings suggest that fibroblasts are able to make at least two types of coated pits, one of which requires the annexin VI-dependent activation of a cysteine protease to disconnect the clathrin lattice from the spectrin membrane cytoskeleton during the final stages of budding.

The role of microtubule-based motor proteins in maintaining the structure and function of the Golgi complex

The intimate association between the Golgi complex and the microtubule cytoskeleton plays an important role in Golgi structure and function. Recent evidence indicates that the dynamic flow of material from the ER to the Golgi is crucial to maintaining the integrity of the Golgi complex and its characteristic location within the cell, and it is now clear that this flow is dependent on the ongoing activity of microtubule motor proteins. This review focuses primarily on recent microinjection and expression studies which have explored the role of individual microtubule motor proteins in controlling Golgi dynamics. The collective evidence shows that one or more isoforms of cytoplasmic dynein, together with its cofactor the dynactin complex, are required to maintain a juxtanuclear Golgi complex in fibroblasts. Although questions remain about how dynein and dynactin are linked to the Golgi, there is evidence that the Golgi-spectrin lattice is involved. Kinesin and kinesin-like proteins appear to play a smaller role in Golgi dynamics, though this may be very cell-type specific. Moreover, new evidence about the role of kinesin family members continues to emerge. Thanks in part to recent advances in our understanding of these molecular motors, our current view of the Golgi complex is of an organelle in flux, undergoing constant renewal. Future research will be aimed at elucidating how and to what extent these motor proteins function as regulators of Golgi function.

Vesicle budding on Golgi membranes: regulation by G proteins and myosin motors

One of the main functions of the Golgi complex is to generate transport vesicles for the post-Golgi trafficking of proteins in secretory pathways. Many different populations of vesicles are distinguished by unique sets of structural and regulatory proteins which participate in vesicle budding and fusion. Monomeric and heterotrimeric G proteins regulate vesicle budding and secretory traffic into and out of the Golgi complex. An inventory of G protein alpha subunits associated with Golgi membranes highlights their diverse involvement and potential for coupling Golgi trafficking, through various signal transduction pathways, to cell growth or other more specialized cell functions. Cytoskeletal proteins are now also known to associate specifically with the Golgi complex and Golgi-derived vesicles. Amongst these, conventional and unconventional myosins are recruited to vesicle membranes. Several roles in vesicle budding and vesicle trafficking can be proposed for these actin-based motors.

A spectrin membrane skeleton of the Golgi complex

The existence of a Golgi-localized membrane cytoskeleton has been revealed by the identification of two major components of the spectrin membrane skeleton, spectrin and ankyrin, that associate with the Golgi complex. Golgi spectrin was identified with an antibody specific for the beta-subunit of the erythroid isoform of spectrin (beta1Sigma1). This antibody recognizes a 220 kDa polypeptide that localizes to discrete regions of the Golgi complex and associates with Golgi membranes in a Brefeldin A sensitive manner. Two isoforms of Golgi ankyrin have been identified: a 119 kDa form (AnkG119) which represents a truncated, alternatively spliced isoform of a recently cloned novel ankyrin of the nervous system AnkG, and a larger 195 kDa ankyrin (Ank195) that cross-reacts with antibodies to erythrocyte ankyrin. A Golgi localized membrane skeleton composed of these unique membrane skeleton isoforms could serve a variety of important functions, including the maintenance of Golgi structural organization and the formation of discrete membrane domains within Golgi compartments.

Mechanisms of trafficking in axons and dendrites: implications for development and neurodegeneration

In the area of routing and sorting of dendritic traffic, the current phenomenological data beg questions about the cellular mechanisms utilized not only to transport material but also to modulate activity in a process, even apoptosis. To aid in formulating testable hypotheses, many plausible models are developed here and linked with some of the preliminary data that supports them. We first assume that in long dendrites the sorting of membranous proteins into transport vesicles also involves the linkage of motor proteins to the vesicles. Second, we assume that the cytoskeleton in dendrites is altered from the cytoskeleton in axons and the cell body. Viral glycoproteins, MAP2 and specific mRNA sorting into dendrites provide the simplest models for analyzing vesicular, cytoskeletal and RNA sorting. In the case of viral glycoproteins, initial sorting appears to occur at the Golgi but additional routing steps involve further complexities that could best be served by an additional sorting step at the junction of the cell body and the process. Transport of the specialized cytoskeletal proteins and specific mRNAs as well as vesicular material could be controlled by a similar gatekeeper at the mouth of a process. Studies of the microtubule-organelle motor complex, regulation of microtubule-based motility by microtubule-associated proteins, and slow axonal transport all provide insights into important aspects of the routing and sorting. These processes are in turn controlled by extracellular signals such as those generated by matrix molecules or their hydrolysis products in the case of amyloid precursor protein (APP). Routing and sorting mechanisms may be central to the development of Alzheimer's disease in view of evidence that APP processing is affected, transport is disturbed, and intracellular vesicles (early endosomes) hypertrophied. Further it is possible that routing mechanisms play a role in cell-cell interactions as, for example, the possibility that pathogenic/cellular stress signals may be passed along circuits transsynaptically.

Adducin is an in vivo substrate for protein kinase C: phosphorylation in the MARCKS-related domain inhibits activity in promoting spectrin-actin complexes and occurs in many cells, including dendritic spines of neurons

Adducin is a heteromeric protein with subunits containing a COOH-terminal myristoylated alanine-rich C kinase substrate (MARCKS)-related domain that caps and preferentially recruits spectrin to the fast-growing ends of actin filaments. The basic MARCKS-related domain, present in alpha, beta, and gamma adducin subunits, binds calmodulin and contains the major phosphorylation site for protein kinase C (PKC). This report presents the first evidence that phosphorylation of the MARCKS-related domain modifies in vitro and in vivo activities of adducin involving actin and spectrin, and we demonstrate that adducin is a prominent in vivo substrate for PKC or other phorbol 12-myristate 13-acetate (PMA)-activated kinases in multiple cell types, including neurons. PKC phosphorylation of native and recombinant adducin inhibited actin capping measured using pyrene-actin polymerization and abolished activity of adducin in recruiting spectrin to ends and sides of actin filaments. A polyclonal antibody specific to the phosphorylated state of the RTPS-serine, which is the major PKC phosphorylation site in the MARCKS-related domain, was used to evaluate phosphorylation of adducin in cells. Reactivity with phosphoadducin antibody in immunoblots increased twofold in rat hippocampal slices, eight- to ninefold in human embryonal kidney (HEK 293) cells, threefold in MDCK cells, and greater than 10-fold in human erythrocytes after treatments with PMA, but not with forskolin. Thus, the RTPS-serine of adducin is an in vivo phosphorylation site for PKC or other PMA-activated kinases but not for cAMP-dependent protein kinase in a variety of cell types. Physiological consequences of the two PKC phosphorylation sites in the MARCKS-related domain were investigated by stably transfecting MDCK cells with either wild-type or PKC-unphosphorylatable S716A/S726A mutant alpha adducin. The mutant alpha adducin was no longer concentrated at the cell membrane at sites of cell-cell contact, and instead it was distributed as a cytoplasmic punctate pattern. Moreover, the cells expressing the mutant alpha adducin exhibited increased levels of cytoplasmic spectrin, which was colocalized with the mutant alpha adducin in a punctate pattern. Immunofluorescence with the phosphoadducin-specific antibody revealed the RTPS-serine phosphorylation of adducin in postsynaptic areas in the developing rat hippocampus. High levels of the phosphoadducin were detected in the dendritic spines of cultured hippocampal neurons. Spectrin also was a component of dendritic spines, although at distinct sites from the ones containing phosphoadducin. These data demonstrate that adducin is a significant in vivo substrate for PKC or other PMA-activated kinases in a variety of cells, and that phosphorylation of adducin occurs in dendritic spines that are believed to respond to external signals by changes in morphology and reorganization of cytoskeletal structures.

Adducin preferentially recruits spectrin to the fast growing ends of actin filaments in a complex requiring the MARCKS-related domain and a newly defined oligomerization domain

Adducin is a protein associated with spectrin and actin in membrane skeletons of erythrocytes and possibly other cells. Adducin has activities in in vitro assays of association with the sides of actin filaments, capping the fast growing ends of actin filaments, and recruiting spectrin to actin filaments. This study presents evidence that adducin exhibits a preference for the fast growing ends of actin filaments for recruiting spectrin to actin and for direct association with actin. beta-Adducin-(335-726) promoted recruitment of spectrin to gelsolin-sensitive sites at fast growing ends of actin filaments with half-maximal activity at 15 nM and to gelsolin-insensitive sites with half-maximal activity at 75 nM. beta-Adducin-(335-726) also exhibited a preference for actin filament ends in direct binding assays; the half-maximal concentration for binding of adducin to gelsolin-sensitive sites at filament ends was 60 nM, and the Kd for binding to lateral sites was 1.5 microM. The concentration of beta-adducin-(335-726) of 60 nM required for half-maximal binding to filament ends is in the same range as the concentration of 150 nM required for half-maximal actin capping activity. All interactions of adducin with actin require the myristoylated alanine-rich protein kinase C substrate-related domain as well as a newly defined oligomerization site localized in the neck domain of adducin. Surprisingly, the head domain of adducin is not required for spectrin-actin interactions, although it could play a role in forming tetramers. The relative activities of adducin imply that an important role of adducin in cells is to form a complex with the fast growing ends of actin filaments that recruits spectrin and prevents addition or loss of actin subunits.

ADP ribosylation factor regulates spectrin binding to the Golgi complex

Homologues of two major components of the well-characterized erythrocyte plasma-membrane-skeleton, spectrin (a not-yet-cloned isoform, betaI Sigma* spectrin) and ankyrin (AnkG119 and an approximately 195-kDa ankyrin), associate with the Golgi complex. ADP ribosylation factor (ARF) is a small G protein that controls the architecture and dynamics of the Golgi by mechanisms that remain incompletely understood. We find that activated ARF stimulates the in vitro association of betaI Sigma* spectrin with a Golgi fraction, that the Golgi-associated betaI Sigma* spectrin contains epitopes characteristic of the betaI Sigma2 spectrin pleckstrin homology (PH) domain known to bind phosphatidylinositol 4,5-bisphosphate (PtdInsP2), and that ARF recruits betaI Sigma* spectrin by inducing increased PtdInsP2 levels in the Golgi. The stimulation of spectrin binding by ARF is independent of its ability to stimulate phospholipase D or to recruit coat proteins (COP)-I and can be blocked by agents that sequester PtdInsP2. We postulate that a PH domain within betaI Sigma* Golgi spectrin binds PtdInsP2 and acts as a regulated docking site for spectrin on the Golgi. Agents that block the binding of spectrin to the Golgi, either by blocking the PH domain interaction or a constitutive Golgi binding site within spectrin's membrane association domain I, inhibit the transport of vesicular stomatitis virus G protein from endoplasmic reticulum to the medial compartment of the Golgi complex. Collectively, these results suggest that the Golgi-spectrin skeleton plays a central role in regulating the structure and function of this organelle.

Identification of a candidate human spectrin Src homology 3 domain-binding protein suggests a general mechanism of association of tyrosine kinases with the spectrin-based membrane skeleton

Spectrin is a widely expressed protein with specific isoforms found in erythroid and nonerythroid cells. Spectrin contains an Src homology 3 (SH3) domain of unknown function. A cDNA encoding a candidate spectrin SH3 domain-binding protein was identified by interaction screening of a human brain expression library using the human erythroid spectrin (alphaI) SH3 domain as a bait. Five isoforms of the alphaI SH3 domain-binding protein mRNA were identified in human brain. Mapping of SH3 binding regions revealed the presence of two alphaI SH3 domain binding regions and one Abl-SH3 domain binding region. The gene encoding the candidate spectrin SH3 domain-binding protein has been located to human chromosome 10p11.2 --> p12. The gene belongs to a recently identified family of tyrosine kinase-binding proteins, and one of its isoforms is identical to e3B1, an eps8-binding protein (Biesova, Z., Piccoli, C., and Wong, W. T. (1997)Oncogene 14, 233-241). Overexpression of the green fluorescent protein fusion of the SH3 domain-binding protein in NIH3T3 cells resulted in cytoplasmic punctate fluorescence characteristic of the reticulovesicular system. This fluorescence pattern was similar to that obtained with the anti-human erythroid spectrin alphaI SigmaI/betaI SigmaI antibody in untransfected NIH3T3 cells; in addition, the anti-alphaI SigmaI/betaI SigmaI antibody also stained Golgi apparatus. Immunofluorescence obtained using antibodies against alphaI SigmaI/++betaI SigmaI spectrin and Abl tyrosine kinase but not against alphaII/betaII spectrin colocalized with the overexpressed green fluorescent protein-SH3-binding protein. Based on the conservation of the spectrin SH3 binding site within members of this protein family and published interactions, a general mechanism of interactions of tyrosine kinases with the spectrin-based membrane skeleton is proposed.

Actin microfilaments are essential for the cytological positioning and morphology of the Golgi complex

The organization and function of the Golgi complex was studied in normal rat kidney cells following disruption of the actin cytoskeleton induced by cytochalasin D. In cells treated with these reagents, the reticular and perinuclear Golgi morphology acquired a cluster shape restricted to the centrosome region. Golgi complex alteration affected all Golgi subcompartments as revealed by double fluorescence staining with antibodies to the cis/middle Mannosidase II and the trans-Golgi network TGN38 proteins or vital staining with the lipid derivate C6-NBD-ceramide. The ultrastructural and stereological analysis showed that the Golgi cisternae remained attached in a stacked conformation, but they were swollen and contained electron-dense intra-cisternal bodies. The Golgi complex cluster remained linked to microtubules since it was fragmented and dispersed after treatment with nocodazole. Moreover, the reassembly of Golgi fragments after the disruption of the microtubuli with nocodazole does not utilize the actin microfilaments. The actin microfilament requirement for the disassembly and reassembly of the Golgi complex and for the ER-Golgi vesicular transport were also studied. The results show that actin microfilaments are not needed for either the retrograde fusion of the Golgi complex with the endoplasmic reticulum promoted by brefeldin A or the anterograde reassembly after the removal of the drug, or the ER-Golgi transport of VSV-G glycoprotein. However, actin microfilaments are directly involved in the subcellular localization and the morphology of the Golgi complex.

Cell cycle regulation of organelle transport

Microtubule- and actin-based motors play a wide range of vital roles in the organisation and function of cells during both interphase and mitosis, all of which are likely to be under strict control. Here, we describe how one of these roles--the movement of membranes--is regulated through the cell cycle. Organelle movement in many species is greatly reduced in mitosis as compared to interphase, and this change occurs concomitantly with an inhibition of most membrane traffic functions. Data from in vitro studies is shedding light on how microtubule motor regulation may be achieved.

The role of the dynactin complex in intracellular motility

Dynactin is a multisubunit complex that binds to the minus-end-directed microtubule motor cytoplasmic dynein and may provide a link between the motor and its cargo. Results from genetic studies in Saccharomyces cerevisiae, Neurospora crassa, Aspergillus nidulans, and Drosophila have suggested that cytoplasmic dynein and dynactin function in the same cellular pathways. p150Glued, a vertebrate homologue of the Drosophila gene Glued, is the largest polypeptide in the dynactin complex with multiple protein interactions. Centractin, the most abundant dynactin subunit polypeptide, forms an actin-like filament at the base of the complex. Studies on dynamitin, the 50-kDa dynactin subunit, predict a role for dynactin in mitotic spindle assembly. Other subunits of dynactin have also been cloned and characterized; these studies have provided insight into the role of the complex in essential cellular processes.

Epithelial and fibroblastoid cells contain numerous cell-type specific putative microtubule-regulating proteins, among which are ezrin and fodrin

Upon cell junction formation, the microtubules of polarizing epithelial cells become reorganized by unknown signaling mechanisms and regulating proteins. Microtubule-associated (MAPs) and other types of proteins are likely to be involved in this process, but most of these are unknown. Such proteins are called here collectively microtubule-regulating proteins (MRPs). As a first step towards their characterization, we used co-sedimentation of cytosolic proteins of MDCK cells and A72, a dog fibroblastoid line, with an excess of taxol-stabilized MTs, to obtain a cell fraction enriched in putative MRPs ("MRPs"). Additional tests have led to the inventory of around 40 "MRPs" among the 80 proteins present in the microtubule pellet. We also found that "MRPs" are recovered in higher amounts from MDCK cytosol, and that half of these are cell-type specific. These results corroborate data from yeast cells and insect eggs, and show that in mammalian somatic cells too, a large number of proteins seems to be involved in microtubule regulation, and that different cell types use a specific set of MRPs. "MRPs" found in both cell types are the intermediate chain of cytoplasmic dynein, Arp1, the major subunit of the dynactin complex, and CLIP-170. Two MDCK-specific "MRPs" were identified as the actin-binding proteins ezrin and alpha-fodrin. These results are discussed with regard to a possible involvement of ezrin and fodrin in morphogenetic interactions of microtubules with the membrane cytoskeleton in polarizing epithelia upon junction formation.

Arp2/3 complex from Acanthamoeba binds profilin and cross-links actin filaments

The Arp2/3 complex was first purified from Acanthamoeba castellanii by profilin affinity chromatography. The mechanism of interaction with profilin was unknown but was hypothesized to be mediated by either Arp2 or Arp3. Here we show that the Arp2 subunit of the complex can be chemically cross-linked to the actin-binding site of profilin. By analytical ultracentrifugation, rhodamine-labeled profilin binds Arp2/3 complex with a Kd of 7 microM, an affinity intermediate between the low affinity of profilin for barbed ends of actin filaments and its high affinity for actin monomers. These data suggest the barbed end of Arp2 is exposed, but Arp2 and Arp3 are not packed together in the complex exactly like two actin monomers in a filament. Arp2/3 complex also cross-links actin filaments into small bundles and isotropic networks, which are mechanically stiffer than solutions of actin filaments alone. Arp2/3 complex is concentrated at the leading edge of motile Acanthamoeba, and its localization is distinct from that of alpha-actinin, another filament cross-linking protein. Based on localization and actin filament nucleation and cross-linking activities, we propose a role for Arp2/3 in determining the structure of the actin filament network at the leading edge of motile cells.

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.

Cytoskeletal proteins and Golgi dynamics

Association of the Golgi complex with cytoskeletal elements, in particular microtubules, is required for maintenance of the Golgi's characteristic spatial location within cells and for efficient delivery of proteins and lipids to diverse cellular sites. Recent work has suggested the mechanisms underlying this association involve components, such as ankyrin and spectrin, that facilitate Golgi membrane association with motor proteins, including cytoplasmic dynein, kinesin and myosin. Understanding how these associations are regulated and what roles they play in Golgi trafficking and dynamics is fundamental for insight into the spatial and functional integration of secretory membrane traffic.

Germline cyst formation in Drosophila

In a wide variety of organisms, gametes develop within clusters of interconnected germline cells called cysts. Four major principles guide the construction of most cysts: synchronous division, a maximally branched pattern of interconnection between cells, specific changes in cyst geometry, and cyst polarization. The fusome is a germline-specific organelle that is associated with cyst formation in many insects and is likely to play an essential role in these processes. This review examines the cellular and molecular processes that underlie fusome formation and cyst initiation, construction, and polarization in Drosophila melanogaster. The studies described here highlight the importance of cyst formation to the subsequent development of functional gametes.

Cytoplasmic dynein and dynactin are required for the transport of microtubules into the axon

Previous work from our laboratory suggested that microtubules are released from the neuronal centrosome and then transported into the axon (Ahmad, F.J., and P.W. Baas. 1995. J. Cell Sci. 108: 2761-2769). In these studies, cultured sympathetic neurons were treated with nocodazole to depolymerize most of their microtubule polymer, rinsed free of the drug for a few minutes to permit a burst of microtubule assembly from the centrosome, and then exposed to nanomolar levels of vinblastine to suppress further microtubule assembly from occurring. Over time, the microtubules appeared first near the centrosome, then dispersed throughout the cytoplasm, and finally concentrated beneath the periphery of the cell body and within developing axons. In the present study, we microinjected fluorescent tubulin into the neurons at the time of the vinblastine treatment. Fluorescent tubulin was not detected in the microtubules over the time frame of the experiment, confirming that the redistribution of microtubules observed with the experimental regime reflects microtubule transport rather than microtubule assembly. To determine whether cytoplasmic dynein is the motor protein that drives this transport, we experimentally increased the levels of the dynamitin subunit of dynactin within the neurons. Dynactin, a complex of proteins that mediates the interaction of cytoplasmic dynein and its cargo, dissociates under these conditions, resulting in a cessation of all functions of the motor tested to date (Echeverri, C.J., B.M. Paschal, K.T. Vaughan, and R.B. Vallee. 1996. J. Cell Biol. 132: 617-633). In the presence of excess dynamitin, the microtubules did not show the outward progression but instead remained near the centrosome or dispersed throughout the cytoplasm. On the basis of these results, we conclude that cytoplasmic dynein and dynactin are essential for the transport of microtubules from the centrosome into the axon.

Kinesin and dynein superfamily proteins and the mechanism of organelle transport

Cells transport and sort proteins and lipids, after their synthesis, to various destinations at appropriate velocities in membranous organelles and protein complexes. Intracellular transport is thus fundamental to cellular morphogenesis and functioning. Microtubules serve as a rail on which motor proteins, such as kinesin and dynein superfamily proteins, convey their cargoes. This review focuses on the molecular mechanism of organelle transport in cells and describes kinesin and dynein superfamily proteins.

Speculating about spectrin: new insights into the Golgi-associated cytoskeleton

Amid the continual flux of membranes and proteins through the Golgi, the distinctive structure and compartmentalization of the cisternal stacks are preserved. Microtubules and associated motors are required to maintain Golgi structure and for transport to and from the organelle. There is also evidence for Golgi-associated myosins. Recent research has identified a novel Golgi-associated spectrin-based network. In this review, we discuss evidence for this network and the possible roles for spectrin in maintaining Golgi structure and in vesicular transport to and from the Golgi. Overall the link between the cytoskeleton and the Golgi appears to be dynamic in nature, in keeping with the continuous flux of proteins and lipids through this organelle.

Localization of proteins to the Golgi apparatus

For the Golgi apparatus to perform its various unique roles it must maintain a population of resident proteins. These residents include the enzymes that modify the proteins and lipids passing through the Golgi, as well as the proteins involved in vesicle formation and protein sorting. For several of these residents, it has been possible to identify regions that are crucial for specifying a Golgi localization. Consideration of how these targeting domains could function has provided insights into the organization of the Golgi and its protein and lipid content.

Molecular motors and a spectrin matrix associate with Golgi membranes in vitro

Cytoplasmic dynein is a microtubule minus-end-directed motor that is thought to power the transport of vesicles from the TGN to the apical cortex in polarized epithelial cells. Trans-Golgi enriched membranes, which were isolated from primary polarized intestinal epithelial cells, contain both the actin-based motor myosin-I and dynein, whereas isolated Golgi stacks lack dynein but contain myosin-I (Fath, K.R., G.M. Trimbur, and D.R. Burgess. 1994. J. Cell Biol. 126:661-675). We show now that Golgi stacks in vitro bind dynein supplied from cytosol in the absence of ATP, and bud small membranes when incubated with cytosol and ATP. Cytosolic dynein binds to regions of stacks that are destined to bud because dynein is present in budded membranes, but absent from stacks after budding. Budded membranes move exclusively towards microtubule minus-ends in in vitro motility assays. Extraction studies suggest that dynein binds to a Golgi peripheral membrane protein(s) that resists extraction by ice-cold Triton X-100. In the presence of cytosol, these membrane ghosts can move towards the minus-ends of microtubules. Detergent-extracted Golgi stacks and TGN-containing membranes are closely associated with an amorphous matrix composed in part of spectrin and ankyrin. Although spectrin has been proposed to help link dynein to organellar membranes, we found that functional dynein may bind to extracted membranes independently of spectrin and ankyrin.

The interaction between cytoplasmic dynein and dynactin is required for fast axonal transport

Fast axonal transport is characterized by the bidirectional, microtubule-based movement of membranous organelles. Cytoplasmic dynein is necessary but not sufficient for retrograde transport directed from the synapse to the cell body. Dynactin is a heteromultimeric protein complex, enriched in neurons, that binds to both microtubules and cytoplasmic dynein. To determine whether dynactin is required for retrograde axonal transport, we examined the effects of anti-dynactin antibodies on organelle transport in extruded axoplasm. Treatment of axoplasm with antibodies to the p150(Glued) subunit of dynactin resulted in a significant decrease in the velocity of microtubule-based organelle transport, with many organelles bound along microtubules. We examined the molecular mechanism of the observed inhibition of motility, and we demonstrated that antibodies to p150(Glued) disrupted the binding of cytoplasmic dynein to dynactin and also inhibited the association of cytoplasmic dynein with organelles. In contrast, the anti-p150(Glued) antibodies had no effect on the binding of dynactin to microtubules nor on cytoplasmic dynein-driven microtubule gliding. These results indicate that the interaction between cytoplasmic dynein and the dynactin complex is required for the axonal transport of membrane-bound vesicles and support the hypothesis that dynactin may function as a link between the organelle, the microtubule, and cytoplasmic dynein during vesicle transport.

Overexpression of the dynamitin (p50) subunit of the dynactin complex disrupts dynein-dependent maintenance of membrane organelle distribution

Dynactin is a multisubunit complex that plays an accessory role in cytoplasmic dynein function. Overexpression in mammalian cells of one dynactin subunit, dynamitin, disrupts the complex, resulting in dissociation of cytoplasmic dynein from prometaphase kinetochores, with consequent perturbation of mitosis (Echeverri, C.J., B.M. Paschal, K.T. Vaughan, and R.B. Vallee. 1996. J. Cell Biol. 132:617-634). Based on these results, dynactin was proposed to play a role in linking cytoplasmic dynein to kinetochores and, potentially, to membrane organelles. The current study reports on the dynamitin interphase phenotype. In dynamitin-overexpressing cells, early endosomes (labeled with antitransferrin receptor), as well as late endosomes and lysosomes (labeled with anti-lysosome-associated membrane protein-1 [LAMP-1]), were redistributed to the cell periphery. This redistribution was disrupted by nocodazole, implicating an underlying plus end-directed microtubule motor activity. The Golgi stack, monitored using sialyltransferase, galactosyltransferase, and N-acetylglucosaminyltransferase I, was dramatically disrupted into scattered structures that colocalized with components of the intermediate compartment (ERGIC-53 and ERD-2). The disrupted Golgi elements were revealed by EM to represent short stacks similar to those formed by microtubule-depolymerizing agents. Golgi-to-ER traffic of stack markers induced by brefeldin A was not inhibited by dynamitin overexpression. Time-lapse observations of dynamitin-overexpressing cells recovering from brefeldin A treatment revealed that the scattered Golgi elements do not undergo microtubule-based transport as seen in control cells, but rather, remain stationary at or near their ER exit sites. These results indicate that dynactin is specifically required for ongoing centripetal movement of endocytic organelles and components of the intermediate compartment. Results similar to those of dynamitin overexpression were obtained by microinjection with antidynein intermediate chain antibody, consistent with a role for dynactin in mediating interactions of cytoplasmic dynein with specific membrane organelles. These results suggest that dynamitin plays a pivotal role in regulating organelle movement at the level of motor-cargo binding.

The involvement of the intermediate chain of cytoplasmic dynein in binding the motor complex to membranous organelles of Xenopus oocytes

Cytoplasmic dynein is one of the major motor proteins involved in intracellular transport. It is a protein complex consisting of four subunit classes: heavy chains, intermediate chains (ICs), light intermediate chains, and light chains. In a previous study, we had generated new monoclonal antibodies to the ICs and mapped the ICs to the base of the motor. Because the ICs have been implicated in targeting the motor to cargo, we tested whether these new antibodies to the intermediate chain could block the function of cytoplasmic dynein. When cytoplasmic extracts of Xenopus oocytes were incubated with either one of the monoclonal antibodies (m74-1, m74-2), neither organelle movement nor network formation was observed. Network formation and membrane transport was blocked at an antibody concentration as low as 15 micrograms/ml. In contrast to these observations, no effect was observed on organelle movement and tubular network formation in the presence of a control antibody at concentrations as high as 0.5 mg/ml. After incubating cytoplasmic extracts or isolated membranes with the monoclonal antibodies m74-1 and m74-2, the dynein IC polypeptide was no longer detectable in the membrane fraction by SDS-PAGE immunoblot, indicating a loss of cytoplasmic dynein from the membrane. We used a panel of dynein IC truncation mutants and mapped the epitopes of both antibodies to the N-terminal coiled-coil domain, in close proximity to the p150Glued binding domain. In an IC affinity column binding assay, both antibodies inhibited the IC-p150Glued interaction. Thus these findings demonstrate that direct IC-p150Glued interaction is required for the proper attachment of cytoplasmic dynein to membranes.

Na,K-ATPase transport from endoplasmic reticulum to Golgi requires the Golgi spectrin-ankyrin G119 skeleton in Madin Darby canine kidney cells

Spectrin (betaISigma*) and ankyrin (AnkG119) associate with Golgi membranes and the dynactin complex, but their role in vesicle trafficking remains uncertain. We find that the actin-binding domain and membrane-association domain 1 (MAD1) of betaI spectrin together form a constitutive Golgi targeting signal in transfected MDCK cells. Expression of this signal in transfected cells disrupts the endogenous Golgi spectrin skeleton and blocks transport of alpha- and beta-Na,K-ATPase and vesicular stomatitis virus-G protein from the endoplasmic reticulum (ER) but does not disrupt the formation of Golgi stacks, the distribution of beta-COP, or the transport and surface display of E-cadherin. The Golgi spectrin skeleton is thus required for the transport of a subset of membrane proteins from the ER to the Golgi. We postulate that together with polyfunctional adapter proteins such as AnkG119, Golgi spectrin forms a docking complex that acts prior to the cis-Golgi, presumably with vesicular-tubular clusters (VTCs or ERGIC), to sequester specific membrane proteins into vesicles transiting between the ER and Golgi, and subsequently (probably involving other isoforms of spectrin and ankyrin) to mediate cargo transport within the Golgi and to other membrane compartments. We hypothesize that this vesicular spectrin-ankyrin adapter-protein trafficking (or tethering) system (SAATS) mediates the capture and transport of many membrane proteins and acts in conjunction with vesicle-targeting molecules to effect the efficient transport of cargo proteins.

Spectrosomes and fusomes anchor mitotic spindles during asymmetric germ cell divisions and facilitate the formation of a polarized microtubule array for oocyte specification in Drosophila

In the Drosophila ovary, membrane skeletal proteins such as the adducin-like Hts protein(s), spectrin, and ankyrin are found in the spectrosome, an organelle in germline stem cells (GSC) and their differentiated daughter cells (cystoblasts). These proteins are also components of the fusome, a cytoplasmic structure that spans the cystoblast's progeny that develop to form a germline cyst consisting of 15 nurse cells and an oocyte. Spectrosomes and fusomes are associated with one pole of spindles during mitosis and are implicated in cyst formation and oocyte differentiation. Here we show that the asymmetric behavior of the spectrosome persists throughout the cell cycle of GSC. Eliminating the spectrosome by the htsl mutation leads to randomized spindle orientation, suggesting that the spectrosome anchors the spindle to ensure the asymmetry of GSC division; eliminating the fusome in developing cysts results in defective spindles and randomized spindle orientation as well as asynchronous and reduced cystocyte divisions. These observations suggest that fusomes are required for the proper formation and asymmetric orientation of mitotic spindles. Moreover, they reinforce the notion that fusomes are required for the four synchronous divisions of the cystoblast leading to cyst formation. In htsl cysts which lack fusomes and fail to incorporate a hts gene product(s) into ring canals following cyst formation, polarized microtubule networks do not form, the dynamics of cytoplasmic dynein is disrupted, and oskar and orb RNAs fail to be transported to the future oocyte. These observations support the proposed role of fusomes and ring canals in organizing a polarized microtubule-based transport system for RNA localization that leads to oocyte differentiation.

Deletion of nudC, a nuclear migration gene of Aspergillus nidulans, causes morphological and cell wall abnormalities and is lethal

Nuclear migration is required for normal development in both higher and lower eukaryotes. In fungi this process is mediated by cytoplasmic dynein. It is believed that this motor protein is anchored to the cell membrane and moves nuclei by capturing and pulling on spindle pole body microtubules. To date, four genes have been identified and shown to be required for this process in Aspergillus nidulans. The nudA and nudG genes, respectively, encode the heavy and light chains of cytoplasmic dynein, and the nudF and nudC gene products encode proteins of 49 and 22 kDa. The precise biochemical functions of the nudF and nudC genes have not yet been identified. In this report we further investigate NUDC protein function by deleting the nudC gene. Surprisingly, although deletion of nudA and nudF affect nuclear migration, deletion of nudC profoundly affected the morphology and composition of the cell wall. Spores of the strain deleted for nudC grew spherically and lysed. The thickness of the cell wall was increased in the deletion mutant and wall polymer composition was abnormal. This phenotype could be repressed by growth on osmotically buffered medium at low temperature. Similar, but less severe, effects were also noted in a strain depleted for NUDC by down-regulation. These results suggest a possible relationship between fungal cell wall biosynthesis and nuclear migration.

Myosin II is involved in the production of constitutive transport vesicles from the TGN

The participation of nonmuscle myosins in the transport of organelles and vesicular carriers along actin filaments has been documented. In contrast, there is no evidence for the involvement of myosins in the production of vesicles involved in membrane traffic. Here we show that the putative TGN coat protein p200 (Narula, N., I. McMorrow, G. Plopper, J. Doherty, K.S. Matlin, B. Burke, and J.L. Stow. 1992. J. Cell Biol. 114: 1113-1124) is myosin II. The recruitment of myosin II to Golgi membranes is dependent on actin and is regulated by G proteins. Using an assay that studies the release of transport vesicles from the TGN in vitro, we provide functional evidence that p200/myosin is involved in the assembly of basolateral transport vesicles carrying vesicular stomatitis virus G protein (VSVG) from the TGN of polarized MDCK cells. The 50% reduced efficiency in VSVG vesicle release from the TGN in vitro after depletion of p200/myosin II could be reestablished to control levels by the addition of purified nonmuscle myosin II. Several inhibitors of the actin-stimulated ATPase activity of myosin specifically inhibited the release of VSVG-containing vesicles from the TGN.

The microtubule motor cytoplasmic dynein is required for spindle orientation during germline cell divisions and oocyte differentiation in Drosophila

During animal development cellular differentiation is often preceded by an asymmetric cell division whose polarity is determined by the orientation of the mitotic spindle. In the fruit fly, Drosophila melanogaster, the oocyte differentiates in a 16-cell syncytium that arises from a cystoblast which undergoes 4 synchronous divisions with incomplete cytokinesis. During these divisions, spindle orientation is highly ordered and is thought to impart a polarity to the cyst that is necessary for the subsequent differentiation of the oocyte. Using mutations in the Drosophila cytoplasmic dynein heavy chain gene, Dhc64C, we show that cytoplasmic dynein is required at two stages of oogenesis. Early in oogenesis, dynein mutations disrupt spindle orientation in dividing cysts and block oocyte determination. The localization of dynein in mitotic cysts suggests spindle orientation is mediated by the microtubule motor cytoplasmic dynein. Later in oogenesis, dynein function is necessary for proper differentiation, but does not appear to participate in morphogen localization within the oocyte. These results provide evidence for a novel developmental role for the cytoplasmic dynein motor in cellular determination and differentiation.

Golgi membrane skeleton: identification, localization and oligomerization of a 195 kDa ankyrin isoform associated with the Golgi complex

To extend our finding of a Golgi-localized form of the membrane skeleton protein spectrin, we have identified an isoform of ankyrin that associates at steady state with the Golgi complex. Immuno-light and -electron microscopy show that this ankyrin isoform localizes to the perinuclear cytoplasm on tubular vesicular structures that co-stain with Golgi marker proteins. An antiserum raised against erythrocyte ankyrin, which was used to identify the Golgi ankyrin, recognized three prominent polypeptides of 220, 213 and 195 kDa in MDCK cells. Affinity purification of this antiserum against each of these MDCK cell ankyrins revealed that only an antibody specific for the 195 kDa form retained the ability to stain the Golgi complex; affinity purified antibody preparations specific for both the 220 and 213 kDa forms stained punctate and reticular cytoplasmic structures distinct from the Golgi complex. Antibody specific for the 195 kDa ankyrin did not recognize a recently identified 119 kDa ankyrin that is also localized to the Golgi. The 195 kDa Golgi ankyrin binds purified erythrocyte spectrin, and rapidly co-sediments with Golgi beta-spectrin during brief, low speed centrifugation of Triton X-100 extracts of MDCK cells. Golgi ankyrin and beta-spectrin are retained on tubular vesicular ‘Golgi ghosts’ following extraction of cultured cells with Triton X-100. Significantly, Golgi ghost tubules containing ankyrin/spectrin are co-linear with individual microtubules, suggesting a role for both Golgi membrane skeleton and microtubules in spatial localization of the Golgi. Golgi ankyrin dissociates from Golgi membranes during mitosis and in cells treated with brefeldin A, indicating that Golgi ankyrin has a dynamic assembly state similar to that of Golgi spectrin and other Golgi membrane coat proteins.

Dynactin phosphorylation is modulated in response to cellular effectors

Reversible protein phosphorylation has been implicated in the regulation of organelle transport by cytoplasmic dynein. Motor function may be modulated directly by the phosphorylation of dynein or through the phosphorylation of an accessory factor. Dynactin binds to cytoplasmic dynein and is a required activator for dynein-driven vesicular motility. In metabolic labeling studies we have determined that the p150Glued subunit of dynactin is a phosphoprotein. Treatment of Rat2 cells with okadaic acid or with activators of protein kinase A or protein kinase C caused a marked increase in the incorporation of 32P into p150Glued; the increased phosphorylation correlated with activated vesicular transport. Phosphoamino-acid analysis of p150Glued isolated from cells treated with okadaic acid or with activators of either protein kinase A or protein kinase C indicated exclusive labeling of phosphoserine. These results suggest that the phosphorylation of dynactin may serve to regulate intracellular transport catalyzed by cytoplasmic dynein.

Casein kinase II binds to and phosphorylates cytoplasmic dynein

We have isolated a 27-kDa protein that binds to cytoplasmic dynein. Microsequencing of a 17-amino acid peptide of this polypeptide yielded a sequence which completely matched the predicted sequence of the beta subunit of casein kinase II, a highly conserved serine/threonine kinase. Affinity chromatography using a dynein column indicates that both the alpha and beta subunits of casein kinase II are retained by the column from rat brain cytosol. Although dynactin is also bound to the column, casein kinase II is not a dynactin subunit. Casein kinase II does not co-immunoprecipitate with dynactin, and it binds to a dynein intermediate chain column which has been preblocked with excess p150(Glued), a treatment that inhibits the binding of dynactin from cytosol. Bacterially expressed and purified rat dynein intermediate chain can be phosphorylated by casein kinase II in vitro. Further, native cytoplasmic dynein purified from rat brain can also be phosphorylated by casein kinase II in vitro. We propose that CKII may be involved in the regulation of dynein function possibly by altering its cargo specificity or its ability to interact with dynactin.