Heterotrimeric kinesin II is the microtubule motor protein responsible for pigment dispersion in Xenopus melanophores

Drosophila KAP Interacts with the Kinesin II Motor Subunit KLP64D to Assemble Chordotonal Sensory Cilia, but Not Sperm Tails

BACKGROUND

Kinesin II-mediated anterograde intraflagellar transport (IFT) is essential for the assembly and maintenance of flagella and cilia in various cell types. Kinesin associated protein (KAP) is identified as the non-motor accessory subunit of Kinesin II, but its role in the corresponding motor function is not understood.

RESULTS

We show that mutations in the Drosophila KAP (DmKap) gene could eliminate the sensory cilia as well as the sound-evoked potentials of Johnston's organ (JO) neurons. Ultrastructure analysis of these mutants revealed that the ciliary axonemes are absent. Mutations in Klp64D, which codes for a Kinesin II motor subunit in Drosophila, show similar ciliary defects. All these defects are rescued by exclusive expression of DmKAP and KLP64D/KIF3A in the JO neurons of respective mutants. Furthermore, reduced copy number of the DmKap gene was found to enhance the defects of hypomorphic Klp64D alleles. Unexpectedly, however, both the DmKap and the Klp64D mutant adults produce vigorously motile sperm with normal axonemes.

CONCLUSIONS

KAP plays an essential role in Kinesin II function, which is required for the axoneme growth and maintenance of the cilia in Drosophila type I sensory neurons. However, the flagellar assembly in Drosophila spermatids does not require Kinesin II and is independent of IFT.

Dimerization properties of a Xenopus laevis kinesin-II carboxy-terminal stalk fragment

We have analysed the structural and physical properties of the carboxy-terminal stalk region of a kinesin-II, Xenopus kinesin-like protein 3A/B (Xklp3A/B), which we showed to be essential for heterodimerization in a previous work (De Marco et al., 2001). We expressed the corresponding A-stalk and B-stalk fragments and investigated their modes of interaction by analytical ultracentrifugation (AUC), circular dichroism spectroscopy, denaturation assays and electron microscopy. Co-expression of the A-stalk and B-stalk produced the properly folded, hetero-dimeric coiled coil at high yields. The dimeric nature of the complex was confirmed by AUC. We also found that the isolated A-stalk fragment forms a stable helix by itself and shows a significant tendency towards homodimer and higher-order complex formation. In the absence of the corresponding A-stalk fragment, the isolated B-stalk fragment remains partially unfolded, which suggests that the A-stalk provides a template structure for the B-stalk in order to recompose the complete heterodimeric coiled coil.

Kinesin II and regulated intraflagellar transport of Chlamydomonas aurora protein kinase

The assembly and functioning of cilia and flagella depend on a complex system of traffic between the organelles and the cell body. Two types of transport into these organelles have been identified. The best characterized is constitutive: in a process termed intraflagellar transport (IFT), flagellar structural components are continuously carried into cilia and flagella on transport complexes termed IFT particles via the microtubule motor protein kinesin II. Previous studies have shown that the flagella of the unicellular green alga Chlamydomonas exhibit a second type of protein import that is regulated. During fertilization, the Chlamydomonas aurora protein kinase CALK undergoes regulated translocation from the cell body into the flagella. The motor that powers this second, regulated type of movement is unknown. Here, we have examined the cellular properties of the CALK in Chlamydomonas and used a kinesin II mutant to test the idea that the motor protein is essential for regulated translocation of proteins into flagella. We found that the CALK that is transported into flagella of wild-type gametes becomes part of a membrane-associated complex, that kinesin II is essential for the normal localization of this Chlamydomonas aurora protein kinase in unactivated gametes and that the cAMP-induced translocation of the protein kinase into flagella is disrupted in the fla10 mutants. Our results indicate that, in addition to its role in the constitutive transport of IFT particles and their cargo, kinesin II is essential for regulated translocation of proteins into flagella.

The molecular motor toolbox for intracellular transport

Eukaryotic cells create internal order by using protein motors to transport molecules and organelles along cytoskeletal tracks. Recent genomic and functional studies suggest that five cargo-carrying motors emerged in primitive eukaryotes and have been widely used throughout evolution. The complexity of these "Toolbox" motors expanded in higher eukaryotes through gene duplication, alternative splicing, and the addition of associated subunits, which enabled new cargoes to be transported. Remarkably, fungi, parasites, plants, and animals have distinct subsets of Toolbox motors in their genomes, suggesting an underlying diversity of strategies for intracellular transport.

Dynactin is required for bidirectional organelle transport

Kinesin II is a heterotrimeric plus end-directed microtubule motor responsible for the anterograde movement of organelles in various cell types. Despite substantial literature concerning the types of organelles that kinesin II transports, the question of how this motor associates with cargo organelles remains unanswered. To address this question, we have used Xenopus laevis melanophores as a model system. Through analysis of kinesin II-mediated melanosome motility, we have determined that the dynactin complex, known as an anchor for cytoplasmic dynein, also links kinesin II to organelles. Biochemical data demonstrates that the putative cargo-binding subunit of Xenopus kinesin II, Xenopus kinesin II-associated protein (XKAP), binds directly to the p150Glued subunit of dynactin. This interaction occurs through aa 530-793 of XKAP and aa 600-811 of p150Glued. These results reveal that dynactin is required for transport activity of microtubule motors of opposite polarity, cytoplasmic dynein and kinesin II, and may provide a new mechanism to coordinate their activities.

Dynactin polices two-way organelle traffic

How is the bidirectional motion of organelles controlled? In this issue, Deacon et al. (2003) reveal the unexpected finding that dynactin (previously known to control dynein-based motility) binds to kinesin II and regulates anterograde movement of Xenopus melanosomes. This result suggests that dynactin may be a key player in coordinating vesicle traffic in this system.

The cytoskeleton in fish melanophore melanosome positioning

Melanophore melanosomes organelles can be regulated to move and locate correspondingly to many other different organelle types. Comparing lessons from analysis of a specific melanosome distribution can, therefore, contribute to the understanding of distribution of other organelles, and vice versa. From such data, it is now generally accepted that microtubules provide directed long-distance movement, while cell peripheral movements include microfilaments. In fish melanophores, both actin and dynein exhibit counter-forces to the kinesin-like protein in maintaining the evenly dispersed state, while actin and kinesin exhibit counter-forces to dynein in many other systems. Lessons from elevating cAMP levels indicate the presence of a peripheral feedback regulatory system involved in maintaining the evenly dispersed state. Studies from dynein inhibition suggest that the kinesin-like protein involved in fish melanosome dispersal is regulated in contrast to many other systems. One would further expect melanosome transport to be regulated also on actin/myosin, in order to prevent actin-dependent capture of melanosomes during the microtubule-dependent aggregation and dispersion. General findings will be discussed in comparison with positioning and movement of other organelle types in cells. Finally, recent data on melanosome-dependent organising of microtubules show that dynein is involved in nucleating microtubules extending from melanosome aggregates in melanophore fragments.

Kinesin-II is not essential for mitosis and cell growth in Chlamydomonas

The FLA10 gene product (Fla10p) in Chlamydomonas, a heterotrimeric kinesin-II, plays a crucial role in flagellar assembly as a motor protein driving intraflagellar transport. This protein has also been suggested to play a role in mitosis based on its localization to mitotic spindle. A role for Fla10p in mitosis has been difficult to test because to date only conditional (temperature-sensitive) mutant alleles were available, and it is not known whether these retain residual function for mitosis at the non-permissive temperature. In this report, we describe a null allele of fla10 produced by insertional mutagenesis. This mutant does not assemble flagella, but proliferates at a rate identical to that of wild type cells. Observation of microtubule organization in the cell body revealed that normal mitotic spindles are formed in dividing mutant cells. Thus, we conclude that FLA10 kinesin plays no significant roles in mitosis.

Interactions and regulation of molecular motors in Xenopus melanophores

Many cellular components are transported using a combination of the actin- and microtubule-based transport systems. However, how these two systems work together to allow well-regulated transport is not clearly understood. We investigate this question in the Xenopus melanophore model system, where three motors, kinesin II, cytoplasmic dynein, and myosin V, drive aggregation or dispersion of pigment organelles called melanosomes. During dispersion, myosin V functions as a "molecular ratchet" to increase outward transport by selectively terminating dynein-driven minus end runs. We show that there is a continual tug-of-war between the actin and microtubule transport systems, but the microtubule motors kinesin II and dynein are likely coordinated. Finally, we find that the transition from dispersion to aggregation increases dynein-mediated motion, decreases myosin V--mediated motion, and does not change kinesin II--dependent motion. Down-regulation of myosin V contributes to aggregation by impairing its ability to effectively compete with movement along microtubules.

Modulation of substrate adhesion dynamics via microtubule targeting requires kinesin-1

Recent studies have shown that the targeting of substrate adhesions by microtubules promotes adhesion site disassembly (Kaverina, I., O. Krylyshkina, and J.V. Small. 1999. J. Cell Biol. 146:1033-1043). It was accordingly suggested that microtubules serve to convey a signal to adhesion sites to modulate their turnover. Because microtubule motors would be the most likely candidates for effecting signal transmission, we have investigated the consequence of blocking microtubule motor activity on adhesion site dynamics. Using a function-blocking antibody as well as dynamitin overexpression, we found that a block in dynein-cargo interaction induced no change in adhesion site dynamics in Xenopus fibroblasts. In comparison, a block of kinesin-1 activity, either via microinjection of the SUK-4 antibody or of a kinesin-1 heavy chain construct mutated in the motor domain, induced a dramatic increase in the size and reduction in number of substrate adhesions, mimicking the effect observed after microtubule disruption by nocodazole. Blockage of kinesin activity had no influence on either the ability of microtubules to target substrate adhesions or on microtubule polymerisation dynamics. We conclude that conventional kinesin is not required for the guidance of microtubules into substrate adhesions, but is required for the focal delivery of a component(s) that retards their growth or promotes their disassembly.

Kinectin is a key effector of RhoG microtubule-dependent cellular activity

RhoG is a member of the Rho family of GTPases that activates Rac1 and Cdc42 through a microtubule-dependent pathway. To gain understanding of RhoG downstream signaling, we performed a yeast two-hybrid screen from which we identified kinectin, a 156-kDa protein that binds in vitro to conventional kinesin and enhances microtubule-dependent kinesin ATPase activity. We show that RhoG(GTP) specifically interacts with the central domain of kinectin, which also contains a RhoA binding domain in its C terminus. Interaction was confirmed by coprecipitation of kinectin with active RhoG(G12V) in COS-7 cells. RhoG, kinectin, and kinesin colocalize in REF-52 and COS-7 cells, mainly in the endoplasmic reticulum but also in lysosomes. Kinectin distribution in REF-52 cells is modulated according to endogenous RhoG activity. In addition, by using injection of anti-kinectin antibodies that challenge RhoG-kinectin interaction or by blocking anti-kinesin antibodies, we show that RhoG morphogenic activity relies on kinectin interaction and kinesin activity. Finally, kinectin overexpression elicits Rac1- and Cdc42-dependent cytoskeletal effects and switches cells to a RhoA phenotype when RhoG activity is inhibited or microtubules are disrupted. The functional links among RhoG, kinectin, and kinesin are further supported by time-lapse videomicroscopy of COS-7 cells, which showed that the microtubule-dependent lysosomal transport is facilitated by RhoG activation or kinectin overexpression and is severely stemmed upon RhoG inhibition. These data establish that kinectin is a key mediator of microtubule-dependent RhoG activity and suggest that kinectin also mediates RhoG- and RhoA-dependent antagonistic pathways.

Self-organization of a radial microtubule array by dynein-dependent nucleation of microtubules

Polarized radial arrays of cytoplasmic microtubules (MTs) with minus ends clustered at the cell center define the organization of the cytoplasm through interaction with microtubule motors bound to membrane organelles or chromosomes. It is generally assumed that the radial organization results from nucleation of MTs at the centrosome. However, radial MT array can also be attained through self-organization that requires the activity of a minus-end-directed MT motor, cytoplasmic dynein. In this study we examine the role of cytoplasmic dynein in the self-organization of a radial MT array in cytoplasmic fragments of fish melanophores lacking the centrosome. After activation of dynein motors bound to membrane-bound organelles, pigment granules, the fragments rapidly form polarized radial arrays of MTs and position pigment aggregates at their centers. We show that rearrangement of MTs in the cytoplasm is achieved through dynein-dependent MT nucleation. The radial pattern is generated by continuous disassembly and reassembly of MTs and concurrent minus-end-directed transport of pigment granules bearing the nucleation sites.

Cell cycle regulation of myosin-V by calcium/calmodulin-dependent protein kinase II

Organelle transport by myosin-V is down-regulated during mitosis, presumably by myosin-V phosphorylation. We used mass spectrometry phosphopeptide mapping to show that the tail of myosin-V was phosphorylated in mitotic Xenopus egg extract on a single serine residue localized in the carboxyl-terminal organelle-binding domain. Phosphorylation resulted in the release of the motor from the organelle. The phosphorylation site matched the consensus sequence of calcium/calmodulin-dependent protein kinase II (CaMKII), and inhibitors of CaMKII prevented myosin-V release. The modulation of cargo binding by phosphorylation is likely to represent a general mechanism regulating organelle transport by myosin-V.

Analysis of heterodimer formation by Xklp3A/B, a newly cloned kinesin-II from Xenopus laevis

kinesin-II motor proteins are composed of two different kinesin-like motor proteins and one cargo binding subunit. Here we report the cloning of a new member of the kinesin-II superfamily, Xklp3A from Xenopus laevis, which forms a heterodimeric complex with Xklp3B. The heterodimer formation properties between Xklp3A and B have been tested in vitro using reticulocyte lysate expression and immunoprecipitation. To this end we produced a series of Xklp3A and B constructs of varying length and tested their propensity for heterodimer formation. We could demonstrate that, in contrast to conventional kinesin, the critical domains for heterodimer formation in Xklp3A/B are located at the C-terminal end of the stalk. Neither the neck nor the highly charged stretches after the neck region, which are typical of kinesins-II, are required for heterodimer formation, nor do they prevent homodimer formation. Dimerization is controlled by a cooperative mechanism between the C-terminal coiled-coil segments. Classical trigger sites were not identified. The critical regions for dimerization exhibit a very high degree of sequence conservation among equivalent members of the kinesin-II family.

Expression of KIF3C kinesin during neural development and in vitro neuronal differentiation

KIF3A, KIF3B and KIF3C are kinesin-related motor subunits of the KIF3 family that associate to form the kinesin-II motor complex in which KIF3C and KIF3B are alternative partners of KIF3A. We have analysed the expression of Kif3 mRNAs during prenatal murine development. Kif3c transcripts are detectable from embryonic day 12.5 and persist throughout development both in the CNS and in some peripheral ganglia. Comparison of the expression patterns of the Kif3 genes revealed that Kif3c and Kif3a mRNAs colocalize in the CNS, while only Kif3a is also present outside the CNS. In contrast, Kif3b is detectable in several non-neural tissues. We have also performed immunocytochemical analyses of the developing rat brain and have found the presence of the KIF3C protein in selected brain regions and in several fibre systems. Using neuroblastoma cells as an in vitro model for neuronal differentiation, we found that retinoic acid stimulated the expression of the three Kif3 and the kinesin-associated protein genes, although with different time courses. The selective expression of Kif3c in the nervous system during embryonic development and its up-regulation during neuroblastoma differentiation suggest a role for this motor during maturation of neuronal cells.

Molecular motors and membrane traffic in Dictyostelium

Phagocytosis and membrane traffic in general are largely dependent on the cytoskeleton and their associated molecular motors. The myosin family of motors, especially the unconventional myosins, interact with the actin cortex to facilitate the internalization of external materials during the early steps of phagocytosis. Members of the kinesin and dynein motor families, which mediate transport along microtubules (MTs), facilitate the intracellular processing of the internalized materials and the movement of membrane. Recent studies indicate that some unconventional myosins are also involved in membrane transport, and that the MT- and actin-dependent transport systems might interact with each other. Studies in Dictyostelium have led to the discovery of many motors involved in critical steps of phagocytosis and membrane transport. With the ease of genetic and biochemical approaches, the established functional analysis to test phagocytosis and vesicle transport, and the effort of the Dictyostelium cDNA and Genome Projects, Dictyostelium will continue to be a superb model system to study phagocytosis in particular and cytoskeleton and motors in general.

Analysis of the roles of kinesin and dynein motors in microtubule-based transport in the Caenorhabditis elegans nervous system

The heteromeric kinesins constitute a subfamily of kinesin-related motor complexes that function in several distinct intracellular transport events. The founding member of this subfamily, heterotrimeric kinesin II, has been purified and characterized from early sea urchin embryos, where it was shown using antibody perturbation to be required for the synthesis of motile cilia, presumably by driving the anterograde transport of raft complexes. To further characterize heteromeric kinesin transport pathways, and to attempt to identify cargo molecules, we are using the model organism Caenorhabditis elegans to exploit its well-characterized nervous system and simple genetics. Here we describe methods for large-scale nematode growth and partial purification of kinesin-related holoenzymes from C. elegans, and an in vivo transport assay that allows the direct labeling and visualization of motor complexes and putative cargo molecules moving in living C. elegans neurons. This transport assay is being used to characterize the in vivo transport properties of motor enzymes in living cells, and to exploit a number of existing mutations in C. elegans that may represent constituents of heteromeric kinesin-driven transport pathways, for example, the retrograde intraflagellar transport motor CHE-3 dynein, as well as cargo molecules and/or regulatory molecules.

Expression and partial characterization of kinesin-related proteins in differentiating and adult skeletal muscle

Using pan-kinesin antibodies to screen a differentiating C2C12 cell library, we identified the kinesin proteins KIF3A, KIF3B, and conventional kinesin heavy chain to be present in differentiating skeletal muscle. We compared the expression and subcellular localization characteristics of these kinesins in myogenic cells to others previously identified in muscle, neuronal, and mitotic systems (KIF1C, KIF3C, and mitotic-centromere-associated kinesin). Because members of the KIF3 subfamily of kinesin-related proteins showed altered subcellular fractionation characteristics in differentiating cells, we focused our study of kinesins in muscle on the function of kinesin-II. Kinesin-II is a motor complex comprised of dimerized KIF3A and KIF3B proteins and a tail-associated protein, KAP. The Xenopus homologue of KIF3B, Xklp3, is predominantly localized to the region of the Golgi apparatus, and overexpression of motorless-Xklp3 in Xenopus A6 cells causes mislocalization of Golgi components (). In C2C12 myoblasts and myotubes, KIF3B is diffuse and punctate, and not primarily associated with the Golgi. Overexpression of motorless-KIF3B does not perturb localization of Golgi components in myogenic cells, and myofibrillogenesis is normal. In adult skeletal muscle, KIF3B colocalizes with the excitation-contraction-coupling membranes. We propose that these membranes, consisting of the transverse-tubules and sarcoplasmic reticulum, are dynamic structures in which kinesin-II may function to actively assemble and maintain in myogenic cells.

Nitric oxide modulates intracellular translocation of pigment organelles in Xenopus laevis melanophores

Pigment organelles in Xenopus laevis melanophores are used by the animal to change skin color, and they provide a good model for studying intracellular organelle transport. Movement of organelles and vesicles along the cytoskeleton is essential for many processes, such as axonal transport, endocytosis, and intercompartmental trafficking. Nitric oxide (NO) is a signaling molecule that plays a role in, among other things, relaxation of blood vessels, sperm motility, and polymerization of actin. Our study focused on the effect NO exerts on cytoskeleton-mediated transport, which has previously received little attention. We found that an inhibitor of NO synthesis, N-nitro-L-arginine methyl ester (L-NAME), reduced the melatonin-induced aggregation of the pigment organelles, melanosomes. Preaggregated melanosomes dispersed after treatment with L-NAME but not after exposure to the inactive stereoisomer (D-NAME) or the substrate for NO synthesis (L-arginine). Signal transduction by NO can be mediated through the activation of soluble guanylate cyclase (sGC), which leads to increased production of cGMP and activation of cGMP-dependent kinases (PKG). We found that both the sGC inhibitor 1H-(1,2,4) oxadiazolo(4,3-a)quinoxalin-1-one (ODQ) and the cGMP analogue 8-bromoguanosine 3':5'-cyclic monophosphate (8-Br-cGMP) reduced melanosome aggregation, whereas the PKG inhibitor KT582 did not. Our results demonstrate that melanosome aggregation depends on synthesis of NO, and NO deprivation causes dispersion. It seems, thus, as if NO and cGMP are essential and can regulate melanosome translocation.

Dynein, dynactin, and kinesin II's interaction with microtubules is regulated during bidirectional organelle transport

The microtubule motors, cytoplasmic dynein and kinesin II, drive pigmented organelles in opposite directions in Xenopus melanophores, but the mechanism by which these or other motors are regulated to control the direction of organelle transport has not been previously elucidated. We find that cytoplasmic dynein, dynactin, and kinesin II remain on pigment granules during aggregation and dispersion in melanophores, indicating that control of direction is not mediated by a cyclic association of motors with these organelles. However, the ability of dynein, dynactin, and kinesin II to bind to microtubules varies as a function of the state of aggregation or dispersion of the pigment in the cells from which these molecules are isolated. Dynein and dynactin bind to microtubules when obtained from cells with aggregated pigment, whereas kinesin II binds to microtubules when obtained from cells with dispersed pigment. Moreover, the microtubule binding activity of these motors/dynactin can be reversed in vitro by the kinases and phosphatase that regulate the direction of pigment granule transport in vivo. These findings suggest that phosphorylation controls the direction of pigment granule transport by altering the ability of dynein, dynactin, and kinesin II to interact with microtubules.

Microtubule-based transport systems in neurons: the roles of kinesins and dyneins

The large size and extreme polarization of neurons is crucial to their ability to communicate at long distances and to form the complex cellular networks of the nervous system. The size, shape, and compartmentalization of these specialized cells must be generated and supported by the cytoskeletal systems of intracellular transport. One of the major systems is the microtubule-based transport system along which kinesin and dynein motor proteins generate force and drive the traffic of many cellular components. This review describes our current understanding of the functions of kinesins and dyneins and how these motor proteins may be harnessed to generate some of the unique properties of neuronal cells.

Local inhibition of cortical rotation in Xenopus eggs by an anti-KRP antibody

The dorsal-ventral axis of amphibian embryos is specified by the "cortical rotation," a translocation of the egg cortex relative to the vegetal yolk mass. The mechanism of cortical rotation is not understood but is thought to involve an array of aligned, commonly oriented microtubules. We have demonstrated an essential requirement for kinesin-related proteins (KRPs) in the cortical rotation by microinjection beneath the vegetal cortex of an antipeptide antibody recognising multiple Xenopus egg KRPs. Time-lapse videomicroscopy revealed a striking local inhibition of the cortical rotation around the injection site, indicating that KRP-mediated translocation of the cortex is generated by forces acting across the vegetal subcortical region. Anti-tubulin immunofluorescence showed that the antibody disrupted both formation and maintenance of the aligned microtubule array. Direct examination of rhodamine-labelled microtubules by confocal microscopy showed that the anti-KRP antibody provoked striking three-dimensional flailing movement of the subcortical microtubules. In contrast, microtubules in antibody-free regions undulated only within the plane of the cortex, a significant population exhibiting little or no net movement. These findings suggest that KRPs have a critical role during cortical rotation in tethering microtubules to the cortex and that they may not contribute significantly to the translocation force as previously thought.

Melanosome and erythrosome positioning regulates cAMP-induced movement in chromatophores from spotted triplefin, Grahamina capito

This study investigated regulation of uniform positioning of melanosomes and erythrosomes in chromatophores from spotted triplefin Grahamina capito from New Zealand, by modulating levels of intracellular cAMP. Elevated cAMP levels, caused by forskolin treatment, inhibited aggregation and induced rapid dispersion of melanosomes and erythrosomes. The dispersing organelles moved to and accumulated at the cell periphery, leading to an abnormal hyperdispersed state with a melanosome- or erythrosome-depleted cell center. Minutes after hyperdispersion, these organelles reversed direction and moved towards the center again to finally distribute throughout the cells. When chromatophores with initially dispersed melanosomes or erythrosomes were treated with forskolin, no hyperdispersion was seen, but the erythrosomes aggregated slowly. Disassembly of actin by latrunculin resulted in a similar but constant hyperdispersed melanosome and erythrosome distribution. The results show that cAMP not only disperses but also aggregates melanosomes and erythrosomes, and that it is the intracellular position of these organelles that determine the directionality of the cAMP-induced movement. To ascertain the even distribution in the dispersed state, regulatory components associated with the actin cytoskeleton in the cell periphery might modify activity of cytoplasmic dynein or kinesin upon contact with dispersing melanosomes or erythrosomes.

TPX2, A novel xenopus MAP involved in spindle pole organization

TPX2, the targeting protein for Xenopus kinesin-like protein 2 (Xklp2), was identified as a microtubule-associated protein that mediates the binding of the COOH-terminal domain of Xklp2 to microtubules (Wittmann, T., H. Boleti, C. Antony, E. Karsenti, and I. Vernos. 1998. J. Cell Biol. 143:673-685). Here, we report the cloning and functional characterization of Xenopus TPX2. TPX2 is a novel, basic 82.4-kD protein that is phosphorylated during mitosis in a microtubule-dependent way. TPX2 is nuclear during interphase and becomes localized to spindle poles in mitosis. Spindle pole localization of TPX2 requires the activity of the dynein-dynactin complex. In late anaphase TPX2 becomes relocalized from the spindle poles to the midbody. TPX2 is highly homologous to a human protein of unknown function and thus defines a new family of vertebrate spindle pole components. We investigated the function of TPX2 using spindle assembly in Xenopus egg extracts. Immunodepletion of TPX2 from mitotic egg extracts resulted in bipolar structures with disintegrating poles and a decreased microtubule density. Addition of an excess of TPX2 to spindle assembly reactions gave rise to monopolar structures with abnormally enlarged poles. We conclude that, in addition to its function in targeting Xklp2 to microtubule minus ends during mitosis, TPX2 also participates in the organization of spindle poles.

Kinesin superfamily protein 3 (KIF3) motor transports fodrin-associating vesicles important for neurite building

Kinesin superfamily proteins (KIFs) comprise several dozen molecular motor proteins. The KIF3 heterotrimer complex is one of the most abundantly and ubiquitously expressed KIFs in mammalian cells. To unveil the functions of KIF3, microinjection of function-blocking monovalent antibodies against KIF3 into cultured superior cervical ganglion (SCG) neurons was carried out. They significantly blocked fast axonal transport and brought about inhibition of neurite extension. A yeast two-hybrid binding assay revealed the association of fodrin with the KIF3 motor through KAP3. This was further confirmed by using vesicles collected from large bundles of axons (cauda equina), from which membranous vesicles could be prepared in pure preparations. Both immunoprecipitation and immunoelectron microscopy indicated the colocalization of fodrin and KIF3 on the same vesicles, the results reinforcing the evidence that the cargo of the KIF3 motor consists of fodrin-associating vesicles. In addition, pulse-labeling study implied partial comigration of both molecules as fast flow components. Taken together, the KIF3 motor is engaged in fast axonal transport that conveys membranous components important for neurite extension.

Understanding the functions of kinesin-II

Species ranging from Chlamydomonas to humans possess the heterotrimeric kinesin-II holoenzyme composed of two different motor subunits and one non-motor accessory subunit. An important function of kinesin-II is that it transports the components needed for the construction and maintenance of cilia and flagella from the site of synthesis in the cell body to the site of growth at the distal tip. Recent work suggests that kinesin-II does not directly interact with these components, but rather via a large protein complex, which has been termed a raft (intraflagellar transport (IFT)). While ciliary transport is the best-established function for kinesin-II, evidence has been reported for possible roles in neuronal transport, melanosome transport, the secretory pathway and during mitosis.

Brefeldin A-dependent membrane tubule formation reconstituted in vitro is driven by a cell cycle-regulated microtubule motor

Treatment of cultured cells with brefeldin A (BFA) induces the formation of extensive membrane tubules from the Golgi apparatus, trans-Golgi network, and early endosomes in a microtubule-dependent manner. We have reconstituted this transport process in vitro using Xenopus egg cytosol and a rat liver Golgi-enriched membrane fraction. The presence of BFA results in the formation of an intricate, interconnected tubular membrane network, a process that, as in vivo, is inhibited by nocodazole, the H1 anti-kinesin monoclonal antibody, and by membrane pretreatment with guanosine 5'-O-(3-thiotriphosphate). Surprisingly, membrane tubule formation is not due to the action of conventional kinesin or any of the other motors implicated in Golgi membrane dynamics. Two candidate motors of approximately 100 and approximately 130 kDa have been identified using the H1 antibody, both of which exhibit motor properties in a biochemical assay. Finally, BFA-induced membrane tubule formation does not occur in metaphase cytosol, and because membrane binding of both candidate motors is not altered after incubation in metaphase compared with interphase cytosol, these results suggest that either the ATPase or microtubule-binding activity of the relevant motor is cell cycle regulated.

Membrane trafficking, organelle transport, and the cytoskeleton

Cytoskeleton-associated motor proteins typically drive organelle movements in eukaryotic cells in a manner that is tightly regulated, both spatially and temporally. In the past year, a novel organelle transport mechanism utilizing actin polymerization was described. Important advances were also made in the assignment of functions to several new motors and in our understanding of how motor proteins are regulated during organelle transport. In addition, insights were gained into how and why organelles are transported cooperatively along the microtubule and actin cytoskeletons, and into the importance of motor-mediated transport in the organization of the cytoskeleton itself.

Motors, clutches and brakes for membrane traffic: a commemorative review in honor of Thomas Kreis

Membranous organelles interact with a wide variety of cytoskeletal proteins that allow them to be organized into dynamic, yet stable, structures with distinct subcellular addresses. This review provides an up-to-date summary of the motor enzymes and membrane-microtubule crosslinking proteins that have been implicated in this process, and discusses the potential impact membrane anchoring may have on cellular architecture.

Stirring up development with the heterotrimeric kinesin KIF3

KIF3 is a heterotrimeric member of the kinesin superfamily of microtubule associated motors. This functionally diverse family of motor is involved in anterograde transport of membrane bound organelles in neurons and melanosomes, mediates transport between the endoplasmic reticulum and the Golgi, and transports protein complexes within cilia and flagella required for their morphogenesis. Interestingly, a mutation of KIF3, which impairs ciliogenesis in nodal cells, prevents the unidirectional leftward flow (nodal flow) of putative morphogens during embryogenesis, thereby altering the development of left-right asymmetry in mammals.

The road less traveled: emerging principles of kinesin motor utilization

Proteins of the kinesin superfamily utilize a conserved catalytic motor domain to generate movements in a wide variety of cellular processes. In this review, we discuss the rapid expansion in our understanding of how eukaryotic cells take advantage of these proteins to generate force and movement in diverse functional contexts. We summarize several recent examples revealing that the simplest view of a kinesin motor protein binding to and translocating a cargo along a microtubule track is inadequate. In fact, this paradigm captures only a small subset of the many ways in which cells harness force production of the generation of intracellular movements and functions. We also highlight several situations where the catalytic kinesin motor domain may not be used to generate movement, but instead may be used in other biochemical and functional contexts. Finally, we review some recent ideas about kinesin motor regulation, redundancy, and cargo attachment strategies.

Cooperation between microtubule- and actin-based motor proteins

Organelle transport has been proposed to proceed in two steps: long-range transport along microtubules and local delivery via actin filaments. This model is supported by recent studies of pigment transport in several cell types and transport in neurons, and in several cases, class V myosin has been implicated as the actin-based motor. Mutations in mice (dilute) and yeast (myo2) have also implicated this class of myosin in organelle transport, and genetic interactions in yeast have indicated that a kinesin-related protein (Smy1p) plays a supporting role. This link between members of two different motor superfamilies has now taken a surprising turn: There is evidence for a physical interaction between class V myosins and kinesin or Smy1p in both mice and yeast.

Polyglutamylation of atlantic cod tubulin: immunochemical localization and possible role in pigment granule transport

In higher organisms, there is a large variety of tubulin isoforms, due to multiple tubulin genes and extensive post-translational modification. The properties of microtubules may be modulated by their tubulin isoform composition. Polyglutamylation is a post-translational modification that is thought to influence binding of both structural microtubule associated proteins (MAPs) and mechano-chemical motors to tubulin. The present study investigates the role of tubulin polyglutamylation in a vesicle transporting system, cod (Gadus morhua) melanophores. We did this by microinjecting an antibody against polyglutamylated tubulin into these cells. To put our results into perspective, and to be able to judge their universal application, we characterized cod tubulin polyglutamylation by Western blotting technique, and compared it to what is known from mammals. We found high levels of polyglutamylation in tissues and cell types whose functions are highly dependent on interactions between microtubules and motor proteins. Microinjection of the anti-polyglutamylation antibody GT335 into cultured melanophores interfered with pigment granule dispersion, while dynein-dependent aggregation was unaffected. Additional experiments showed that GT335-injected cells were able to aggregate pigment even when actin filaments were depolymerized, indicating that the maintained ability of pigment aggregation in these cells was indeed microtubule-based and did not depend upon actin filaments. The results indicate that dynein and the kinesin-like dispersing motor protein in cod melanophores bind to tubulin on slightly different sites, and perhaps depend differentially on polyglutamylation for their interaction with microtubules. The binding site of the dispersing motor may bind directly to the polyglutamate chain, or more closely than dynein.

Kinesin-II is required for axonal transport of choline acetyltransferase in Drosophila

KLP64D and KLP68D are members of the kinesin-II family of proteins in Drosophila. Immunostaining for KLP68D and ribonucleic acid in situ hybridization for KLP64D demonstrated their preferential expression in cholinergic neurons. KLP68D was also found to accumulate in cholinergic neurons in axonal obstructions caused by the loss of kinesin light chain. Mutations in the KLP64D gene cause uncoordinated sluggish movement and death, and reduce transport of choline acetyltransferase from cell bodies to the synapse. The inviability of KLP64D mutations can be rescued by expression of mammalian KIF3A. Together, these data suggest that kinesin-II is required for the axonal transport of a soluble enzyme, choline acetyltransferase, in a specific subset of neurons in Drosophila. Furthermore, the data lead to the conclusion that the cargo transport requirements of different classes of neurons may lead to upregulation of specific pathways of axonal transport.

Reconstitution of membrane transport powered by a novel dimeric kinesin motor of the Unc104/KIF1A family purified from Dictyostelium

Motor-powered movement along microtubule tracks is important for membrane organization and trafficking. However, the molecular basis for membrane transport is poorly understood, in part because of the difficulty in reconstituting this process from purified components. Using video microscopic observation of organelle transport in vitro as an assay, we have purified two polypeptides (245 and 170 kD) from Dictyostelium extracts that independently reconstitute plus-end-directed membrane movement at in vivo velocities. Both polypeptides were found to be kinesin motors, and the 245-kD protein (DdUnc104) is a close relative of Caenorhabditis elegans Unc104 and mouse KIF1A, neuron-specific motors that deliver synaptic vesicle precursors to nerve terminals. A knockout of the DdUnc104 gene produces a pronounced defect in organelle transport in vivo and in the reconstituted assay. Interestingly, DdUnc104 functions as a dimeric motor, in contrast to other members of this kinesin subfamily, which are monomeric.

Molecular mechanisms of pigment transport in melanophores

We present an overview of the research on intracellular transport in pigment cells, with emphasis on the most recent discoveries. Pigment cells of lower vertebrates have been traditionally used as a model for studies of intracellular transport mechanisms, because these cells transport pigment organelles to the center or to the periphery of the cell in a highly co-ordinated fashion. It is now well established that both aggregation and dispersion of pigment in melanophores require two elements of the cytoskeleton: microtubules and actin filaments. Melanosomes are moved along these cytoskeletal tracks by motor proteins. Recent studies have identified the motors responsible for pigment dispersion and aggregation in melanophores. We propose a model for the possible roles of the two cytoskeletal transport systems and how they might interact. We also discuss the putative mechanisms of regulation of pigment transport, especially phosphorylation. Last, we suggest areas of research that will receive attention in the future in order to elucidate the mechanisms of organelle transport.

Regulation of melanosome movement in the cell cycle by reversible association with myosin V

Previously, we have shown that melanosomes of Xenopus laevis melanophores are transported along both microtubules and actin filaments in a coordinated manner, and that myosin V is bound to purified melanosomes (Rogers, S., and V.I. Gelfand. 1998. Curr. Biol. 8:161-164). In the present study, we have demonstrated that myosin V is the actin-based motor responsible for melanosome transport. To examine whether myosin V was regulated in a cell cycle-dependent manner, purified melanosomes were treated with interphase- or metaphase-arrested Xenopus egg extracts and assayed for in vitro motility along Nitella actin filaments. Motility of organelles treated with mitotic extract was found to decrease dramatically, as compared with untreated or interphase extract-treated melanosomes. This mitotic inhibition of motility correlated with the dissociation of myosin V from melanosomes, but the activity of soluble motor remained unaffected. Furthermore, we find that myosin V heavy chain is highly phosphorylated in metaphase extracts versus interphase extracts. We conclude that organelle transport by myosin V is controlled by a cell cycle-regulated association of this motor to organelles, and that this binding is likely regulated by phosphorylation of myosin V during mitosis.

Membrane motors

Research over the past 18 months has revealed that many membranous organelles move along both actin filaments and microtubules. It is highly likely that the activity of the microtubule motors, myosins and static linker proteins present on any organelle are co-ordinately regulated and that this control is linked to the processes of membrane traffic itself.

Microtubule-based endoplasmic reticulum motility in Xenopus laevis: activation of membrane-associated kinesin during development

The endoplasmic reticulum (ER) in animal cells uses microtubule motor proteins to adopt and maintain its extended, reticular organization. Although the orientation of microtubules in many somatic cell types predicts that the ER should move toward microtubule plus ends, motor-dependent ER motility reconstituted in extracts of Xenopus laevis eggs is exclusively a minus end-directed, cytoplasmic dynein-driven process. We have used Xenopus egg, embryo, and somatic Xenopus tissue culture cell (XTC) extracts to study ER motility during embryonic development in Xenopus by video-enhanced differential interference contrast microscopy. Our results demonstrate that cytoplasmic dynein is the sole motor for microtubule-based ER motility throughout the early stages of development (up to at least the fifth embryonic interphase). When egg-derived ER membranes were incubated in somatic XTC cytosol, however, ER tubules moved in both directions along microtubules. Data from directionality assays suggest that plus end-directed ER tubule extensions contribute approximately 19% of the total microtubule-based ER motility under these conditions. In XTC extracts, the rate of ER tubule extensions toward microtubule plus ends is lower ( approximately 0.4 microm/s) than minus end-directed motility ( approximately 1.3 microm/s), and plus end-directed motility is eliminated by a function-blocking anti-conventional kinesin heavy chain antibody (SUK4). In addition, we provide evidence that the initiation of plus end-directed ER motility in somatic cytosol is likely to occur via activation of membrane-associated kinesin.

Two heteromeric kinesin complexes in chemosensory neurons and sensory cilia of Caenorhabditis elegans

Chemosensation in the nervous system of the nematode Caenorhabditis elegans depends on sensory cilia, whose assembly and maintenance requires the transport of components such as axonemal proteins and signal transduction machinery to their site of incorporation into ciliary structures. Members of the heteromeric kinesin family of microtubule motors are prime candidates for playing key roles in these transport events. Here we describe the molecular characterization and partial purification of two heteromeric kinesin complexes from C. elegans, heterotrimeric CeKinesin-II and dimeric CeOsm-3. Transgenic worms expressing green fluorescent protein driven by endogenous heteromeric kinesin promoters reveal that both CeKinesin-II and CeOsm-3 are expressed in amphid, inner labial, and phasmid chemosensory neurons. Additionally, immunolocalization experiments on fixed worms show an intense concentration of CeKinesin-II and CeOsm-3 polypeptides in the ciliated endings of these chemosensory neurons and a punctate localization pattern in the corresponding cell bodies and dendrites. These results, together with the phenotypes of known mutants in the pathway of sensory ciliary assembly, suggest that CeKinesin-II and CeOsm-3 drive the transport of ciliary components required for sequential steps in the assembly of chemosensory cilia.

Role of xklp3, a subunit of the Xenopus kinesin II heterotrimeric complex, in membrane transport between the endoplasmic reticulum and the Golgi apparatus

The function of the Golgi apparatus is to modify proteins and lipids synthesized in the ER and sort them to their final destination. The steady-state size and function of the Golgi apparatus is maintained through the recycling of some components back to the ER. Several lines of evidence indicate that the spatial segregation between the ER and the Golgi apparatus as well as trafficking between these two compartments require both microtubules and motors. We have cloned and characterized a new Xenopus kinesin like protein, Xklp3, a subunit of the heterotrimeric Kinesin II. By immunofluorescence it is found in the Golgi region. A more detailed analysis by EM shows that it is associated with a subset of membranes that contain the KDEL receptor and are localized between the ER and Golgi apparatus. An association of Xklp3 with the recycling compartment is further supported by a biochemical analysis and the behavior of Xklp3 in BFA-treated cells. The function of Xklp3 was analyzed by transfecting cells with a dominant-negative form lacking the motor domain. In these cells, the normal delivery of newly synthesized proteins to the Golgi apparatus is blocked. Taken together, these results indicate that Xklp3 is involved in the transport of tubular-vesicular elements between the ER and the Golgi apparatus.