Kinesin-mediated organelle translocation revealed by specific cellular manipulations

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.

TER94, a Drosophila homolog of the membrane fusion protein CDC48/p97, is accumulated in nonproliferating cells: in the reproductive organs and in the brain of the imago

We have cloned a Drosophila homolog of the membrane fusion protein CDC48/p97. The open reading frame of the Drosophila homolog encodes an 801 amino acid long protein (TER94), which shows high similarity to the known CDC48/p97 sequences. The chromosomal position of TER94 is 46 C/D. TER94 is expressed in embryo, in pupae and in imago, but is suppressed in larva. In the imago, the immunoreactivity was exclusively present in the head and in the gonads of both sexes. In the head the most striking staining was observed in the entire neuropil of the mushroom body and in the antennal glomeruli. Besides TER94, sex-specific forms were also detected in the gonads of the imago: p47 in the ovaries and p98 in the testis. TER94/p47 staining was observed in the nurse cells and often in the oöcytes, while TER94/p98 staining was present in the sperm bundles. On the basis of its distribution we suggest that TER94 functions in the protein transport utilizing endoplasmic reticulum and Golgi derived vesicles.

A specific light chain of kinesin associates with mitochondria in cultured cells

The motor protein kinesin is implicated in the intracellular transport of organelles along microtubules. Kinesin light chains (KLCs) have been suggested to mediate the selective binding of kinesin to its cargo. To test this hypothesis, we isolated KLC cDNA clones from a CHO-K1 expression library. Using sequence analysis, they were found to encode five distinct isoforms of KLCs. The primary region of variability lies at the carboxyl termini, which were identical or highly homologous to carboxyl-terminal regions of rat KLC B and C, human KLCs, sea urchin KLC isoforms 1-3, and squid KLCs. To examine whether the KLC isoforms associate with different cytoplasmic organelles, we made an antibody specific for a 10-amino acid sequence unique to B and C isoforms. In an indirect immunofluorescence assay, this antibody specifically labeled mitochondria in cultured CV-1 cells and human skin fibroblasts. On Western blots of total cell homogenates, it recognized a single KLC isoform, which copurified with mitochondria. Taken together, these data indicate a specific association of a particular KLC (B type) with mitochondria, revealing that different KLC isoforms can target kinesin to different cargoes.

Interaction of a Golgi-associated kinesin-like protein with Rab6

Rab guanosine triphosphatases regulate vesicular transport and membrane traffic within eukaryotic cells. Here, a kinesin-like protein that interacts with guanosine triphosphate (GTP)-bound forms of Rab6 was identified. This protein, termed Rabkinesin-6, was localized to the Golgi apparatus and shown to play a role in the dynamics of this organelle. The carboxyl-terminal domain of Rabkinesin-6, which contains the Rab6-interacting domain, inhibited the effects of Rab6-GTP on intracellular transport. Thus, a molecular motor is a potential effector of a Rab protein, and coordinated action between members of these two families of proteins could control membrane dynamics and directional vesicular traffic.

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.

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.

DiOC6(3): a useful dye for staining the endoplasmic reticulum

The present review discusses the fluorescent organelle probe, DiOC6(3), with reference to its structure, chemistry, availability, spectral properties, labeling procedures, vital staining characteristics, and major applications in cellular and molecular biology. The specificity of dye for endoplasmic reticulum is summarized. We examine the simplicity and advantages of the fluorescent dye system for evaluating structure and function of endoplasmic reticulum. Other significant uses of the dye are also discussed.

Suppression of KIF2 in PC12 cells alters the distribution of a growth cone nonsynaptic membrane receptor and inhibits neurite extension

In the present study, we present evidence about the cellular functions of KIF2, a kinesin-like superfamily member having a unique structure in that its motor domain is localized at the center of the molecule (Noda Y., Y. Sato-Yoshitake, S. Kondo, M. Nangaku, and N. Hirokawa. 1995. J. Cell Biol. 129:157-167.). Using subcellular fractionation techniques, isopicnic sucrose density centrifugation of microsomal fractions from developing rat cerebral cortex, and immunoisolation with KIF2 antibodies, we have now identified a type of nonsynaptic vesicle that associates with KIF2. This type of organelle lacks synaptic vesicle markers (synapsin, synaptophysin), amyloid precursor protein, GAP-43, or N-cadherin. On the other hand, it contains betagc, which is a novel variant of the beta subunit of the IGF-1 receptor, which is highly enriched in growth cone membranes. Both betagc and KIF2 are upregulated by NGF in PC12 cells and highly concentrated in growth cones of developing neurons. We have also analyzed the consequences of KIF2 suppression by antisense oligonucleotide treatment on nerve cell morphogenesis and the distribution of synaptic and nonsynaptic vesicle markers. KIF2 suppression results in a dramatic accumulation of betagc within the cell body and in its complete disappearance from growth cones; no alterations in the distribution of synapsin, synaptophysin, GAP-43, or amyloid percursor protein are detected in KIF2-suppressed neurons. Instead, all of them remained highly enriched at nerve terminals. KIF2 suppression also produces a dramatic inhibition of neurite outgrowth; this phenomenon occurs after betagc has disappeared from growth cones. Taken collectively, our results suggest an important role for KIF2 in neurite extension, a phenomenon that may be related with the anterograde transport of a type of nonsynaptic vesicle that contains as one of its components a growth cone membrane receptor for IGF-1, a growth factor implicated in nerve cell development.

Kinesin is essential for cell morphogenesis and polarized secretion in Neurospora crassa

Kinesin is a force-generating molecule that is thought to translocate organelles along microtubules, but its precise cellular function is still unclear. To determine the role of kinesin in vivo, we have generated a kinesin-deficient strain in the simple cell system Neurospora crassa. Null cells exhibit severe alterations in cell morphogenesis, notably hyphal extension, morphology and branching. Surprisingly, the movement of organelles visualized by video microscopy is hardly affected, but apical hyphae fail to establish a Spitzenkörper, an assemblage of secretory vesicles intimately linked to cell elongation and morphogenesis in Neurospora and other filamentous fungi. As cell morphogenesis depends on polarized secretion, our findings demonstrate that a step in the secretory pathway leading to cell shape determination and cell elongation cannot tolerate a loss of kinesin function. The defect is suggested to affect the transport of small, secretory vesicles to the site involved in protrusive activity, resulting in the uncoordinated insertion of new cell wall material over much of the cell surface. These observations have implications for the presumptive function of kinesin in more complex cell systems.

Suppression of the expression of a pancreatic beta-cell form of the kinesin heavy chain by antisense oligonucleotides inhibits insulin secretion from primary cultures of mouse beta-cells

Granular/vesicular transport is thought to be supported by microtubule-based force-generating adenosine triphosphatases such as kinesin. Kinesin is a motor molecule that has been well studied in brain and other neuronal tissues. Although vesicular transport is important for pancreatic beta-cell secretory activities, the role of kinesin in beta-cell function has not been investigated. It is hypothesized that kinesin functions as a translocator that associates with both microtubules and insulin-containing granules in beta-cells and transports the secretory granules from deep within the cytoplasm, where insulin is synthesized and processed, to the surface of beta-cells upon secretory stimulation. To test this hypothesis, a mouse beta-cell kinesin heavy chain complementary DNA was cloned and sequenced. Kinesin expression in primary cultures of mouse beta-cells then was selectively suppressed by antimouse beta-cell kinesin heavy chain antisense oligonucleotide treatment. Analysis of insulin secretion determined that the basal level of insulin secretion from the treated cells was decreased by 50%. Furthermore, glucose-stimulated insulin release from treated beta-cells was reduced by almost 70% after suppression of kinesin expression by antisense treatment. The findings from this study provide the first direct evidence that kinesin, a microtubule-based motor protein, plays an important role in insulin secretion.

Molecular requirements for bi-directional movement of phagosomes along microtubules

Microtubules facilitate the maturation of phagosomes by favoring their interactions with endocytic compartments. Here, we show that phagosomes move within cells along tracks of several microns centrifugally and centripetally in a pH- and microtubule-dependent manner. Phagosome movement was reconstituted in vitro and required energy, cytosol and membrane proteins of this organelle. The activity or presence of these phagosome proteins was regulated as the organelle matured, with "late" phagosomes moving threefold more frequently than "early" ones. The majority of moving phagosomes were minus-end directed; the remainder moved towards microtubule plus-ends and a small subset moved bi-directionally. Minus-end movement showed pharmacological characteristics expected for dyneins, was inhibited by immunodepletion of cytoplasmic dynein and could be restored by addition of cytoplasmic dynein. Plus-end movement displayed pharmacological properties of kinesin, was inhibited partially by immunodepletion of kinesin and fully by addition of an anti-kinesin IgG. Immunodepletion of dynactin, a dynein-activating complex, inhibited only minus-end directed motility. Evidence is provided for a dynactin-associated kinase required for dynein-mediated vesicle transport. Movement in both directions was inhibited by peptide fragments from kinectin (a putative kinesin membrane receptor), derived from the region to which a motility-blocking antibody binds. Polypeptide subunits from these microtubule-based motility factors were detected on phagosomes by immunoblotting or immunoelectron microscopy. This is the first study using a single in vitro system that describes the roles played by kinesin, kinectin, cytoplasmic dynein, and dynactin in the microtubule-mediated movement of a purified membrane organelle.

Immunochemical analysis of kinesin light chain function

The kinesin heterotetramer consists of two heavy and two light chains. Kinesin light chains have been proposed to act in binding motor protein to cargo, but evidence for this has been indirect. A library of monoclonal antibodies directed against conserved epitopes throughout the kinesin light chain sequence were used to map light chain functional architecture and to assess physiological functions of these domains. Immunocytochemistry with all antibodies showed a punctate pattern that was detergent soluble. A monoclonal antibody (KLC-All) made against a highly conserved epitope in the tandem repeat domain of light chains inhibited fast axonal transport in isolated axoplasm by decreasing both the number and velocity of vesicles moving, whereas an antibody against a conserved amino terminus epitope had no effect. KLC-All was equally effective at inhibiting both anterograde and retrograde transport. Neither antibody inhibited microtubule-binding or ATPase activity in vitro. KLC-All was unique among antibodies tested in releasing kinesin from purified membrane vesicles, suggesting a mechanism of action for inhibition of axonal transport. These results provide further evidence that conventional kinesin is a motor for fast axonal transport and demonstrate that kinesin light chains play an important role in kinesin interaction with membranes.

Host cell invasion by trypanosomes requires lysosomes and microtubule/kinesin-mediated transport

Invasion of mammalian cells by the protozoan parasite Trypanosoma cruzi occurs by an actin-independent mechanism distinct from phagocytosis. Clusters of host lysosomes are observed at the site of parasite attachment, and lysosomal markers are detected in the vacuolar membrane at early stages of the entry process. These observations led to the hypothesis that the trypanosomes recruit host lysosomes to their attachment site, and that lysosomal fusion serves as a source of membrane to form the parasitophorous vacuole. Here we directly demonstrate directional migration of lysosomes to the parasite entry site, using time-lapse video-enhanced microscopy of L6E9 myoblasts exposed to T. cruzi trypomastigotes. BSA-gold-loaded lysosomes moved towards the cell periphery, in the direction of the parasite attachment site, but only when their original position was less than 11-12 microns from the invasion site. Lysosomes more distant from the invasion area exhibited only the short multi-directional saltatory movements previously described for lysosomes, regardless of their proximity to the cell margins. Specific depletion of peripheral lysosomes was obtained by microinjection of NRK cells with antibodies against the cytoplasmic domain of lgp 120, a treatment that aggregated lysosomes in the perinuclear area and inhibited T. cruzi entry. The microtubule-binding drugs nocodazole, colchicine, vinblastine, and taxol also inhibited invasion, in both NRK and L6E9 cells. Furthermore, microinjection of antibodies to the heavy chain of kinesin blocked the acidification-induced, microtubule-dependent redistribution of lysosomes to the host cell periphery, and reduced trypomastigote entry. Our results therefore demonstrate that during T. cruzi invasion of host cells lysosomes are mobilized from the immediately surrounding area, and that availability of lysosomes at the cell periphery and microtubule/kinesin-mediated transport are requirements for parasite entry.

Microtubule-associated protein-dependent binding of phagosomes to microtubules

In macrophages, phagosome movement is microtubule-dependent. Microtubules are a prerequisite for phagosome maturation because they facilitate interactions between phagosomes and organelles of the endocytic pathway. We have established an in vitro assay that measures the binding of purified phagosomes to microtubules. This binding depends on the presence of membrane proteins, most likely integral to the surface of phagosomes, and on macrophage cytosol. The cytosolic binding factor can interact with microtubules prior to the addition of phagosomes to the assay, suggesting that it is a microtubule-associated protein (MAP). Consistent with this, depletion of MAPs from the cytosol by microtubule affinity removes all binding activity. Microtubule motor proteins show no binding activity, whereas a crude MAP preparation is sufficient to support binding and to restore full binding activity to MAP-depleted cytosol. We show that the activating MAP factor is a heat-sensitive protein(s) that migrates at around 150 kDa by gel filtration.

Comparison of the intracellular distribution of cytoplasmic dynein and kinesin in cultured cells: motor protein location does not reliably predict function

While immunolocalization methods have been used as a reasonable means to judge where a given molecule may be active in the cellular milieu, the correlation between distribution and function for proteins involved in intracellular transport may not be clear cut. To address the question of specificity and reproducibility of immunolocalization of microtubule-based motor proteins, we have co-localized cytoplasmic dynein and kinesin by immunofluorescence microscopy using two specific antibodies for each motor molecule. The results indicate that cytoplasmic dynein and kinesin appear to co-localize on a small subset of vesicles, but largely reside or accumulate on morphologically distinct organelles. In addition, anti-kinesin antibodies differing in their epitope specificity label different cellular compartments. To address the question of whether the distribution of motor molecules is representative of organelles that are undergoing active transport, we have altered the activity of vesicle trafficking pathways in fibroblasts using several different methods, including cytoplasmic acidification and disruption of cellular compartments with brefeldin A, nocodazole and okadaic acid. Analysis of the distribution of cytoplasmic dynein and kinesin under these conditions indicates that immunolocalization data alone are not reliable indicators of sites of likely function for these microtubule-based motors.

Microtubules and microtubule motors: mechanisms of regulation

Microtubule-based motility is precisely regulated, and the targets of regulation may be the motor proteins, the microtubules, or both components of this intricately controlled system. Regulation of microtubule behavior can be mediated by cell cycle-dependent changes in centrosomal microtubule nucleating ability and by cell-specific, microtubule-associated proteins (MAPs). Changes in microtubule organization and dynamics have been correlated with changes in phosphorylation. Regulation of motor proteins may be required both to initiate movement and to dictate its direction. Axonemal and cytoplasmic dyneins as well as kinesin can be phosphorylated and this modification may affect the motor activities of these enzymes or their ability to interact with organelles. A more complete understanding of how motors can be modulated by phosphorylation, either of the motor proteins or of other associated substrates, will be necessary in order to understand how bidirectional transport is regulated.

Point mutation of adenosine triphosphate-binding motif generated rigor kinesin that selectively blocks anterograde lysosome membrane transport

In the study of motor proteins, the molecular mechanism of mechanochemical coupling, as well as the cellular role of these proteins, is an important issue. To assess these questions we introduced cDNA of wild-type and site-directed mutant kinesin heavy chains into fibroblasts, and analyzed the behavior of the recombinant proteins and the mechanisms involved in organelle transports. Overexpression of wild-type kinesin significantly promoted elongation of cellular processes. Wild-type kinesin accumulated at the tips of the long processes, whereas the kinesin mutants, which contained either a T93N- or T93I mutation in the ATP-binding motif, tightly bound to microtubules in the center of the cells. These mutant kinesins could bind to microtubules in vitro, but could not dissociate from them even in the presence of ATP, and did not support microtubule motility in vitro, thereby indicating rigor-type mutations. Retrograde transport from the Golgi apparatus to the endoplasmic reticulum, as well as lysosome dispersion, was shown to be a microtubule-dependent, plus-end-directed movement. The latter was selectively blocked in the rigor-mutant cells, although the microtubule minus-end-directed motion of lysosomes was not affected. We found the point mutations that make kinesin motor in strong binding state with microtubules in vitro and showed that this mutant causes a dominant effect that selectively blocks anterograde lysosome membrane transports in vivo.

Protein trafficking in kinetoplastid protozoa

The kinetoplastid protozoa infect hosts ranging from invertebrates to plants and mammals, causing diseases of medical and economic importance. They are the earliest-branching organisms in eucaryotic evolution to have either mitochondria or peroxisome-like microbodies. Investigation of their protein trafficking enables us to identify characteristics that have been conserved throughout eucaryotic evolution and also reveals how far variations, or alternative mechanisms, are possible. Protein trafficking in kinetoplastids is in many respects similar to that in higher eucaryotes, including mammals and yeasts. Differences in signal sequence specificities exist, however, for all subcellular locations so far examined in detail--microbodies, mitochondria, and endoplasmic reticulum--with signals being more degenerate, or shorter, than those of their higher eucaryotic counterparts. Some components of the normal array of trafficking mechanisms may be missing in most (if not all) kinetoplastids: examples are clathrin-coated vesicles, recycling receptors, and mannose 6-phosphate-mediated lysosomal targeting. Other aspects and structures are unique to the kinetoplastids or are as yet unexplained. Some of these peculiarities may eventually prove to be weak points that can be used as targets for chemotherapy; others may turn out to be much more widespread than currently suspected.

Membrane traffic motors

There is a wealth of data suggesting that microtubules and associated motor proteins play important roles in orchestrating membrane traffic within higher eukaryotes, with myosins and actin filaments fulfilling similar functions in organisms such as fungi, algae and plants. In addition, evidence is accumulating that both cytoskeletal systems can co-operate within one cell. Recent studies have highlighted how individual motor proteins can act at multiple steps in the membrane-traffic pathways, and in contrast, how more than one motor type may be involved in each transport step and in generating organelle morphology.

Organization of organelles and membrane traffic by microtubules

Organelles of the central membrane system of higher eukaryotes have been shown to utilize microtubules both for maintenance of their characteristic spatial distributions and for efficient transport of their protein and lipid to diverse sites within the cell. Recent work addressing the mechanisms that underlie this organization provides new insights regarding the roles of microtubules and microtubule motors in influencing organelle dynamics and specific membrane traffic routes through the cytoplasm.