A kinesin medley: biochemical and functional heterogeneity

Structure of the flotillin complex in a native membrane environment

In this study, we used cryoelectron microscopy to determine the structures of the Flotillin protein complex, part of the Stomatin, Prohibitin, Flotillin, and HflK/C (SPFH) superfamily, from cell-derived vesicles without detergents. It forms a right-handed helical barrel consisting of 22 pairs of Flotillin1 and Flotillin2 subunits, with a diameter of 32 nm at its wider end and 19 nm at its narrower end. Oligomerization is stabilized by the C terminus, which forms two helical layers linked by a β-strand, and coiled-coil domains that enable strong charge-charge intersubunit interactions. Flotillin interacts with membranes at both ends; through its SPFH1 domains at the wide end and the C terminus at the narrow end, facilitated by hydrophobic interactions and lipidation. The inward tilting of the SPFH domain, likely triggered by phosphorylation, suggests its role in membrane curvature induction, which could be connected to its proposed role in clathrin-independent endocytosis. The structure suggests a shared architecture across the family of SPFH proteins and will promote further research into Flotillin's roles in cell biology.

Microtubules: Evolving roles and critical cellular interactions

Microtubules are cytoskeletal elements known as drivers of directed cell migration, vesicle and organelle trafficking, and mitosis. In this review, we discuss new research in the lens that has shed light into further roles for stable microtubules in the process of development and morphogenesis. In the lens, as well as other systems, distinct roles for characteristically dynamic microtubules and stabilized populations are coming to light. Understanding the mechanisms of microtubule stabilization and the associated microtubule post-translational modifications is an evolving field of study. Appropriate cellular homeostasis relies on not only one cytoskeletal element, but also rather an interaction between cytoskeletal proteins as well as other cellular regulators. Microtubules are key integrators with actin and intermediate filaments, as well as cell–cell junctional proteins and other cellular regulators including myosin and RhoGTPases to maintain this balance.

Impact statement

The role of microtubules in cellular functioning is constantly expanding. In this review, we examine new and exciting fields of discovery for microtubule’s involvement in morphogenesis, highlight our evolving understanding of differential roles for stabilized versus dynamic subpopulations, and further understanding of microtubules as a cellular integrator.

Regulation of motor proteins, axonal transport deficits and adult-onset neurodegenerative diseases

Neurons affected in a wide variety of unrelated adult-onset neurodegenerative diseases (AONDs) typically exhibit a "dying back" pattern of degeneration, which is characterized by early deficits in synaptic function and neuritic pathology long before neuronal cell death. Consistent with this observation, multiple unrelated AONDs including Alzheimer's disease, Parkinson's disease, Huntington's disease, and several motor neuron diseases feature early alterations in kinase-based signaling pathways associated with deficits in axonal transport (AT), a complex cellular process involving multiple intracellular trafficking events powered by microtubule-based motor proteins. These pathogenic events have important therapeutic implications, suggesting that a focus on preservation of neuronal connections may be more effective to treat AONDs than addressing neuronal cell death. While the molecular mechanisms underlying AT abnormalities in AONDs are still being analyzed, evidence has accumulated linking those to a well-established pathological hallmark of multiple AONDs: altered patterns of neuronal protein phosphorylation. Here, we present a short overview on the biochemical heterogeneity of major motor proteins for AT, their regulation by protein kinases, and evidence revealing cell type-specific AT specializations. When considered together, these findings may help explain how independent pathogenic pathways can affect AT differentially in the context of each AOND.

Conventional kinesin: Biochemical heterogeneity and functional implications in health and disease

Intracellular trafficking events powered by microtubule-based molecular motors facilitate the targeted delivery of selected molecular components to specific neuronal subdomains. Within this context, we provide a brief review of mechanisms underlying the execution of axonal transport (AT) by conventional kinesin, the most abundant kinesin-related motor protein in the mature nervous system. We emphasize the biochemical heterogeneity of this multi-subunit motor protein, further discussing its significance in light of recent discoveries revealing its regulation by various protein kinases. In addition, we raise issues relevant to the mode of conventional kinesin attachment to cargoes and examine recent evidence linking alterations in conventional kinesin phosphorylation to the pathogenesis of adult-onset neurodegenerative diseases.

Role of kinesin-1 in the pathogenesis of SPG10, a rare form of hereditary spastic paraplegia

Molecular protein motors play key roles in processes such as intracellular cargo transport and brain wiring, and failure of function can give rise to serious diseases. Kinesin-1, a member of the kinesin superfamily (also known as KIFs) is a two-headed motor protein that uses energy derived from ATP hydrolysis to transport diverse types of intracellular cargo toward the plus-ends of microtubules within axons. Recent studies at the level of a single molecule have provided extensive knowledge on how kinesin-1 moves along microtubules. Further elucidation of kinesin-1 movement may shed light on its influence on axon generation, thereby leading to therapies for diseases such as spastic paraplegia type 10 (SPG10), the subject of this review. SPG10 is an autosomal dominant form of hereditary spastic paraplegia caused by mutations in KIF5A, which encodes one of the isoforms of kinesin-1 (KIF5A, KIF5B, and KIF5C). Although little is known about the cargo of KIF5A, a recent study revealed an axonal transport defect of mitochondria in a KIF5A (-/-) mouse model. This review discusses the consensus moving model of kinesin-1 and the pathogenicity of SPG10 caused by defective KIF5A function.

Rapid modulation of TRH and TRH-like peptide release in rat brain, pancreas, and testis by a GSK-3beta inhibitor

Antidepressants have been shown to be neuroprotective and able to reverse damage to glia and neurons. Thyrotropin-releasing hormone (TRH) is an endogenous antidepressant-like neuropeptide that reduces the expression of glycogen synthase kinase-3beta (GSK-3beta), an enzyme that hyperphosphorylates tau and is implicated in bipolar disorder, diabetes and Alzheimer's disease. In order to understand the potential role of GSK-3beta in the modulation of depression by TRH and TRH-like peptides and the therapeutic potential of GSK-3beta inhibitors for neuropsychiatric and metabolic diseases, young adult male Sprague-Dawley (SD) rats were (a) injected ip with 1.8mg/kg of GSK-3beta inhibitor VIII (GSKI) and sacrificed 0, 2, 4, 6, and 8h later or (b) injected with 0, 0.018, 0.18 or 1.8mg/kg GSKI and bled 4h later. Levels of TRH and TRH-like peptides were measured in various brain regions involved in mood regulation, pancreas and reproductive tissues. Large, 3-15-fold, increases of TRH and TRH-like peptide levels in cerebellum, for example, as well as other brain regions were noted at 2 and 4h. In contrast, a nearly complete loss of TRH and TRH-like peptides from testis within 2h and pancreas by 4h following GSKI injection was observed. We have previously reported similar acute effects of corticosterone in brain and peripheral tissues. Incubation of a decapsulated rat testis with either GSKI or corticosterone accelerated release of TRH, and TRH-like peptides. Glucocorticoids, via inhibition of GSK3-beta activity, may thus be involved in the inhibition of TRH and TRH-like peptide release in brain, thereby contributing to the depressogenic effect of this class of steroids. Corticosterone-induced acceleration of release of these peptides from testis may contribute to the decline in reproductive function and redirection of energy needed during life-threatening emergencies. These contrasting effects of glucocorticoid on peptide release appear to be mediated by GSK-3beta.

Conventional kinesin holoenzymes are composed of heavy and light chain homodimers

Conventional kinesin is a major microtubule-based motor protein responsible for anterograde transport of various membrane-bounded organelles (MBO) along axons. Structurally, this molecular motor protein is a tetrameric complex composed of two heavy (kinesin-1) chains and two light chain (KLC) subunits. The products of three kinesin-1 (kinesin-1A, -1B, and -1C, formerly KIF5A, -B, and -C) and two KLC (KLC1, KLC2) genes are expressed in mammalian nervous tissue, but the functional significance of this subunit heterogeneity remains unknown. In this work, we examine all possible combinations among conventional kinesin subunits in brain tissue. In sharp contrast with previous reports, immunoprecipitation experiments here demonstrate that conventional kinesin holoenzymes are formed of kinesin-1 homodimers. Similar experiments confirmed previous findings of KLC homodimerization. Additionally, no specificity was found in the interaction between kinesin-1s and KLCs, suggesting the existence of six variant forms of conventional kinesin, as defined by their gene product composition. Subcellular fractionation studies indicate that such variants associate with biochemically different MBOs and further suggest a role of kinesin-1s in the targeting of conventional kinesin holoenzymes to specific MBO cargoes. Taken together, our data address the combination of subunits that characterize endogenous conventional kinesin. Findings on the composition and subunit organization of conventional kinesin as described here provide a molecular basis for the regulation of axonal transport and delivery of selected MBOs to discrete subcellular locations.

Scaffold proteins of MAP-kinase modules

Mitogen-activated protein kinases (MAPKs) regulate critical signaling pathways involved in cell proliferation, differentiation and apoptosis. Recent studies have shown that a novel class of scaffold proteins mediates the structural and functional organization of the three-tier MAPK module. By linking the MAP3K, MAP2K and MAPK into a multienzyme complex, these MAPK-specific scaffold proteins provide an insulated physical conduit through which signals from the respective MAPK can be transmitted to the appropriate spatiotemporal cellular loci. Scaffold proteins play a determinant role in modulating the signaling strength of their cognate MAPK module by regulating the signal amplitude and duration. The scaffold proteins themselves are finely regulated resulting in dynamic intra- and inter-molecular interactions that can modulate the signaling outputs of MAPK modules. This review focuses on defining the diverse mechanisms by which these scaffold proteins interact with their respective MAPK modules and the role of such interactions in the spatiotemporal organization as well as context-specific signaling of the different MAPK modules.

Myosin-V, Kinesin-1, and Kinesin-3 cooperate in hyphal growth of the fungus Ustilago maydis

Long-distance transport is crucial for polar-growing cells, such as neurons and fungal hyphae. Kinesins and myosins participate in this process, but their functional interplay is poorly understood. Here, we investigate the role of kinesin motors in hyphal growth of the plant pathogen Ustilago maydis. Although the microtubule plus-ends are directed to the hyphal tip, of all 10 kinesins analyzed, only conventional kinesin (Kinesin-1) and Unc104/Kif1A-like kinesin (Kinesin-3) were up-regulated in hyphae and they are essential for extended hyphal growth. deltakin1 and deltakin3 mutant hyphae grew irregular and remained short, but they were still able to grow polarized. No additional phenotype was detected in deltakin1rkin3 double mutants, but polarity was lost in deltamyo5rkin1 and deltamyo5rkin3 mutant cells, suggesting that kinesins and class V myosin cooperate in hyphal growth. Consistent with such a role in secretion, fusion proteins of green fluorescent protein and Kinesin-1, Myosin-V, and Kinesin-3 accumulate in the apex of hyphae, a region where secretory vesicles cluster to form the fungal Spitzenkörper. Quantitative assays revealed a role of Kin3 in secretion of acid phosphatase, whereas Kin1 was not involved. Our data demonstrate that just two kinesins and at least one myosin support hyphal growth.

Organelle transport along microtubules - the role of KIFs

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

A novel CDK5-dependent pathway for regulating GSK3 activity and kinesin-driven motility in neurons

Neuronal transmission of information requires polarized distribution of membrane proteins within axonal compartments. Membrane proteins are synthesized and packaged in membrane-bounded organelles (MBOs) in neuronal cell bodies and later transported to axons by microtubule-dependent motor proteins. Molecular mechanisms underlying targeted delivery of MBOs to discrete axonal subdomains (i.e. nodes of Ranvier or presynaptic terminals) are poorly understood, but regulatory pathways for microtubule motors may be an essential step. In this work, pharmacological, biochemical and in vivo experiments define a novel regulatory pathway for kinesin-driven motility in axons. This pathway involves enzymatic activities of cyclin-dependent kinase 5 (CDK5), protein phosphatase 1 (PP1) and glycogen synthase kinase-3 (GSK3). Inhibition of CDK5 activity in axons leads to activation of GSK3 by PP1, phosphorylation of kinesin light chains by GSK3 and detachment of kinesin from transported cargoes. We propose that regulating the activity and localization of components in this pathway allows nerve cells to target organelle delivery to specific subcellular compartments. Implications of these findings for pathogenesis of neurodegenerative diseases such as Alzheimer's disease are discussed.

Ca2+-dependent dephosphorylation of kinesin heavy chain on beta-granules in pancreatic beta-cells. Implications for regulated beta-granule transport and insulin exocytosis

The specific biochemical steps required for glucose-regulated insulin exocytosis from beta-cells are not well defined. Elevation of glucose leads to increases in cytosolic [Ca2+]i and biphasic release of insulin from both a readily releasable and a storage pool of beta-granules. The effect of elevated [Ca2+]i on phosphorylation of isolated beta-granule membrane proteins was evaluated, and the phosphorylation of four proteins was found to be altered by [Ca2+]i. One (a 18/20-kDa doublet) was a Ca2+-dependent increase in phosphorylation, and, surprisingly, three others (138, 42, and 36 kDa) were Ca2+-dependent dephosphorylations. The 138-kDa beta-granule phosphoprotein was found to be kinesin heavy chain (KHC). At low levels of [Ca2+]i KHC was phosphorylated by casein kinase 2, but KHC was rapidly dephosphorylated by protein phosphatase 2B beta (PP2Bbeta) as [Ca2+]i increased. Inhibitors of PP2B specifically reduced the second, microtubule-dependent, phase of insulin secretion, suggesting that dephosphorylation of KHC was required for transport of beta-granules from the storage pool to replenish the readily releasable pool of beta-granules. This is distinct from synaptic vesicle exocytosis, because neurotransmitter release from synaptosomes did not require a Ca2+-dependent KHC dephosphorylation. These results suggest a novel mechanism for regulating KHC function and beta-granule transport in beta-cells that is mediated by casein kinase 2 and PP2B. They also implicate a novel regulatory role for PP2B/calcineurin in the control of insulin secretion downstream of a rise in [Ca2+]i.

Fast axonal transport: a site of acrylamide neurotoxicity?

The cellular and molecular site and mode of action of acrylamide (ACR) leading to neurotoxicity has been investigated for four decades, without resolution. Although fast axonal transport compromise has been the central theme for several hypotheses, the results of many studies appear contradictory. Our analysis of the literature suggests that differing experimental designs and parameters of measurement are responsible for these discrepancies. Further investigation has demonstrated consistent inhibition of the quantity of bi-directional fast transport following single ACR exposures. Repeated compromise in fast anterograde transport occurs with each exposure. Modification of neurofilaments, microtubules, energy-generating metabolic enzymes and motor proteins are evaluated as potential sites of action causing the changes in fast transport. Supportive and contradictory data to the hypothesis that deficient delivery of fast-transported proteins to the axon causes, or contributes to, neurotoxicity are critically summarized. A hypothesis of ACR action is presented as a framework for future investigations.

Mitosis and motor proteins in the filamentous ascomycete, Nectria haematococca, and some related fungi

Among filamentous fungi, mitosis has been studied in-depth in just a few species. The mitotic apparatuses in the ascomycetous Fusarium spp. are the most clearly and readily visualized in vivo within this group; fluorescent labeling is unnecessary. This superior cytological tractability has enabled detailed studies and revealing experiments that have led the way toward a more complete understanding of fungal mitosis. Some of the most important discoveries include the role of half-spindles in development of the bipolar spindle, the existence of true kinetochores in fungi, the unorthodox chromosome configurations and movements comprising metaphase and anaphase A, the attachment of astral microtubules to the plasmalemma, the role of the astral pulling force in elongating the spindle, an inwardly directed force within the spindle, and microtubule cross-bridging in both spindle and asters. Recent research has focused on the roles of microtubuleassociated motor proteins in Fusarium solani f. sp. pisi (anamorph of Nectria haematococca). Cytoplasmic dynein was shown to be involved in the development and/or maintenance of mitotic asters and necessary for motility and functionality of the interphase spindle pole body. The inwardly directed force within the anaphase spindle was shown to be produced by a kinesin-related protein, NhKRP1. Because of its superior cytological tractability, the considerable and unique knowledge we have of many aspects of its mitosis, and its genetic tractability, Fusarium solani f. sp. pisi is a good choice for further investigations of mitosis in filamentous fungi.

Cytoskeletal microdifferentiation: a mechanism for organizing morphological plasticity in dendrites

Experimental evidence suggests that microfilaments and microtubules play contrasting roles in regulating the balance between motility and stability in neuronal structures. Actin-containing microfilaments are associated with structural plasticity, both during development when their dynamic activity drives the exploratory activity of growth cones and after circuit formation when the actin-rich dendritic spines of excitatory synapses retain a capacity for rapid changes in morphology. By contrast, microtubules predominate in axonal and dendritic processes, which appear to be morphologically relatively more stable. To compare the cytoplasmic distributions and dynamics of microfilaments and microtubules we made time-lapse recordings of actin or the microtubule-associated protein 2 tagged with green fluorescent protein in neurons growing in dispersed culture or in tissue slices from transgenic mice. The results complement existing evidence indicating that the high concentrations of actin present in dendritic spines is a specialization for morphological plasticity. By contrast, microtubule-associated protein 2 is limited to the shafts of dendrites where time-lapse recordings show little evidence for dynamic activity. A parallel exists between the partitioning of microfilaments and microtubules in motile and stable domains of growing processes during development and between dendrite shafts and spines at excitatory synapses in established neuronal circuits. These data thus suggest a mechanism, conserved through development and adulthood, in which the differential dynamics of actin and microtubules determine the plasticity of neuronal structures.

Molecular motors and their functions in plants

Molecular motors that hydrolyze ATP and use the derived energy to generate force are involved in a variety of diverse cellular functions. Genetic, biochemical, and cellular localization data have implicated motors in a variety of functions such as vesicle and organelle transport, cytoskeleton dynamics, morphogenesis, polarized growth, cell movements, spindle formation, chromosome movement, nuclear fusion, and signal transduction. In non-plant systems three families of molecular motors (kinesins, dyneins, and myosins) have been well characterized. These motors use microtubules (in the case of kinesines and dyneins) or actin filaments (in the case of myosins) as tracks to transport cargo materials intracellularly. During the last decade tremendous progress has been made in understanding the structure and function of various motors in animals. These studies are yielding interesting insights into the functions of molecular motors and the origin of different families of motors. Furthermore, the paradigm that motors bind cargo and move along cytoskeletal tracks does not explain the functions of some of the motors. Relatively little is known about the molecular motors and their roles in plants. In recent years, by using biochemical, cell biological, molecular, and genetic approaches a few molecular motors have been isolated and characterized from plants. These studies indicate that some of the motors in plants have novel features and regulatory mechanisms. The role of molecular motors in plant cell division, cell expansion, cytoplasmic streaming, cell-to-cell communication, membrane trafficking, and morphogenesis is beginning to be understood. Analyses of the Arabidopsis genome sequence database (51% of genome) with conserved motor domains of kinesin and myosin families indicates the presence of a large number (about 40) of molecular motors and the functions of many of these motors remain to be discovered. It is likely that many more motors with novel regulatory mechanisms that perform plant-specific functions are yet to be discovered. Although the identification of motors in plants, especially in Arabidopsis, is progressing at a rapid pace because of the ongoing plant genome sequencing projects, only a few plant motors have been characterized in any detail. Elucidation of function and regulation of this multitude of motors in a given species is going to be a challenging and exciting area of research in plant cell biology. Structural features of some plant motors suggest calcium, through calmodulin, is likely to play a key role in regulating the function of both microtubule- and actin-based motors in plants.

Microtubules Are required for motility and positioning of vesicles and mitochondria in hyphal tip cells of Allomyces macrogynus

We have used video-enhanced light microscopy and digital image processing to characterize the intracellular motility and positioning of vesicles ( approximately 1-microm diameter) and mitochondria in growing hyphal tip cells of Allomyces macrogynus. These observations were coupled with cytoskeletal inhibitory experiments to define the roles of the microtubule and actin cytoskeletons in organelle translocation and positioning. Vesicles and mitochondria were abundant in apical and subapical hypha regions. Vesicles traveled along paths that were parallel to the longitudinal axis of the cell. Anterograde (i.e., toward the hyphal apex) and retrograde (i.e., away from the hyphal apex) movements of vesicles occurred at average rates of 4.0 and 2.2 microm/s, respectively. Bidirectional travel of vesicles along common paths was noted in the cortical cytoplasm. Mitochondria were aligned mostly parallel to the long axis of the hypha, except those extending into the hyphal apex, which were oriented toward the Spitzenkörper. In regions of the subapical hypha mitochondria were often restricted to the cortical cytoplasm and nuclei occupied the central cytoplasmic region. Mitochondria displayed rapid anterograde movements reaching speeds of 3.0 microm/s, but primarily maintained a constant position relative to either the advancing cytoplasm or the lateral cell wall. Cytoskeletal disruption experiments showed that the positioning of mitochondria and motility of vesicles and mitochondria were microtubule-based and suggested that the actin cytoskeleton played uncertain roles.

Kinesin is involved in regulation of rat pancreatic amylase secretion

BACKGROUND & AIMS

Kinesin has recently been localized to zymogen granules of pancreatic acini and is suggested to participate in exocytosis of exocrine pancreas. We examined the function of kinesin in regulated exocytosis of pancreatic acini in this study.

METHODS

Kinesin function in exocytosis was examined by introducing hexahistidine-tagged recombinant kinesin protein and antikinesin monoclonal antibody into streptolysin-O-permeabilized acini. Intracellular localization of introduced recombinant kinesin was investigated by immunohistochemistry. Interaction between recombinant kinesin and the microtubule network was confirmed by nocodazole pretreatment of acini. Kinesin regulation by secretagogues was investigated by examining their effect on adenosine triphosphatase (ATPase) activity of endogenous kinesin.

RESULTS

Recombinant kinesin enhanced calcium-stimulated amylase release from streptolysin-O-permeabilized acini. Introduced recombinant kinesin was localized to both the microtubule network and zymogen granule. Nocodazole pretreatment of acini abolished the enhancing effect of recombinant kinesin on calcium-stimulated amylase release. Antikinesin antibody inhibited amylase release stimulated by the combination of calcium and cyclic adenosine monophosphate (cAMP) but not that stimulated by calcium alone. Secretin and 8-bromo-cAMP increased ATPase activity of endogenous kinesin.

CONCLUSIONS

Kinesin plays a stimulatory role in regulated exocytosis of pancreatic acini and is involved in stimulus-secretion coupling through a cAMP-dependent pathway.

Release of kinesin from vesicles by hsc70 and regulation of fast axonal transport

The nature of kinesin interactions with membrane-bound organelles and mechanisms for regulation of kinesin-based motility have both been surprisingly difficult to define. Most kinesin is recovered in supernatants with standard protocols for purification of motor proteins, but kinesin recovered on membrane-bound organelles is tightly bound. Partitioning of kinesin between vesicle and cytosolic fractions is highly sensitive to buffer composition. Addition of either N-ethylmaleimide or EDTA to homogenization buffers significantly increased the fraction of kinesin bound to organelles. Given that an antibody against kinesin light chain tandem repeats also releases kinesin from vesicles, these observations indicated that specific cytoplasmic factors may regulate kinesin release from membranes. Kinesin light tandem repeats contain DnaJ-like motifs, so the effects of hsp70 chaperones were evaluated. Hsc70 released kinesin from vesicles in an MgATP-dependent and N-ethylmaleimide-sensitive manner. Recombinant kinesin light chains inhibited kinesin release by hsc70 and stimulated the hsc70 ATPase. Hsc70 actions may provide a mechanism to regulate kinesin function by releasing kinesin from cargo in specific subcellular domains, thereby effecting delivery of axonally transported materials.

Localization of the mixed-lineage kinase DLK/MUK/ZPK to the Golgi apparatus in NIH 3T3 cells

DLK/MUK/ZPK is a serine/threonine kinase that belongs to the mixed-lineage (MLK) subfamily of protein kinases. As is the case for most members of this family, relatively little is known about the physiological role of DLK/MUK/ZPK in mammalian cells. Because analysis of subcellular distribution may provide important clues concerning the potential in vivo function of a protein, an antiserum was generated against the amino terminal region of murine DLK/MUK/ZPK and used for localization studies in wild-type NIH 3T3 cells. Light microscopic immunocytochemistry experiments performed with the antiserum revealed that DLK/MUK/ZPK was specifically localized in a juxtanuclear structure characteristic of the Golgi complex. In support of this, treatment of cells with brefeldin A, a drug known to disintegrate the Golgi apparatus, caused disruption of DLK/MUK/ZPK perinuclear staining. Ultrastructural observation of NIH 3T3 cells also confirmed this localization, showing that most of the immunoreactivity was detected on membranes of the stacked Golgi cisternae. Consistent with localization studies, biochemical analyses revealed that DLK/MUK/ZPK was predominantly associated with Golgi membranes on fractionation of cellular extracts and was entirely partitioned into the aqueous phase when membranes were subjected to Triton X-114 extraction. On the basis of these findings, we suggest that DLK/MUK/ZPK is a peripheral membrane protein tightly associated with the cytoplasmic face of the Golgi apparatus. (J Histochem Cytochem 47:1287-1296, 1999)

Characterization of katD, a kinesin-like protein gene specifically expressed in floral tissues of Arabidopsis thaliana

Kinesin and kinesin-like proteins (KLPs) are microtubule-based motor proteins that play important roles in organelle transport. Based on the homology to these proteins, a katD cDNA has now been isolated from a library prepared from flowers of Arabidopsis thaliana ecotype Columbia. Sequence analysis of the katD cDNA revealed an open reading frame of 2691bp [corrected], encoding a protein of 987 amino acids. Comparison of the nucleotide sequences of katD genomic and cDNA clones revealed the presence of 18 introns, 17 of which conform to the GU-AG rule. The central region of the KatD polypeptide exhibits substantial amino acid sequence homology to the motor domain of kinesin heavy chains, although the motor domain of KatD appears to be phylogenetically distant from those of other KLPs in plants. The amino-terminal region of KatD shares marked sequence similarity with the calponin homology domain, whereas the approximately 240-residue carboxyl-terminal region shows no significant homology to other known proteins. The predicted secondary structure of KatD revealed the lack of an alpha-helical coiled coil structure typical of kinesin heavy chains, suggesting that KatD may function as a monomeric motor. A recombinant truncated KatD protein containing the putative motor domain was shown both to bind to mammalian microtubules in a manner dependent on a non-hydrolyzable ATP analog, and to possess microtubule-dependent ATPase activity. Immunoblot and Northern blot analyses showed that both KatD protein and mRNA are expressed specifically in floral tissues. These results suggest that the structurally distinct KatD protein functions as a floral tissue-specific motor protein.

Structures of kinesin and kinesin-microtubule interactions

Several X-ray crystal structures of kinesin motor domains have recently been solved at high resolution ( approximately 0.2-0.3 nm), in both their monomeric and dimeric states. They show the folding of the polypeptide chain and different arrangements of subunits in the dimer. In addition, cryo-electron microscopy and image reconstruction have revealed microtubules decorated with kinesin at intermediate resolution ( approximately 2 nm), showing the distribution and orientation of kinesin heads on the microtubule surface. The comparison of the X-ray and electron microscopy results yields a model of how monomeric motor domains bind to the microtubule but the binding of dimeric motors, their stoichiometry, or the influence of nucleotides remains a matter of debate.

Direct interaction of microtubule- and actin-based transport motors

The microtubule network is thought to be used for long-range transport of cellular components in animal cells whereas the actin network is proposed to be used for short-range transport, although the mechanism(s) by which this transport is coordinated is poorly understood. For example, in sea urchins long-range Ca2+-regulated transport of exocytotic vesicles requires a microtubule-based motor, whereas an actin-based motor is used for short-range transport. In neurons, microtubule-based kinesin motor proteins are used for long-range vesicular transport but microtubules do not extend into the neuronal termini, where actin filaments form the cytoskeletal framework, and kinesins are rapidly degraded upon their arrival in neuronal termini, indicating that vesicles may have to be transferred from microtubules to actin tracks to reach their final destination. Here we show that an actin-based vesicle-transport motor, MyoVA, can interact directly with a microtubule-based transport motor, KhcU. As would be expected if these complexes were functional, they also contain kinesin light chains and the localization of MyoVA and KhcU overlaps in the cell. These results indicate that cellular transport is, in part, coordinated through the direct interaction of different motor molecules.

A role for cyclin-dependent kinase(s) in the modulation of fast anterograde axonal transport: effects defined by olomoucine and the APC tumor suppressor protein

Proteins that interact with both cytoskeletal and membrane components are candidates to modulate membrane trafficking. The tumor suppressor proteins neurofibromin (NF1) and adenomatous polyposis coli (APC) both bind to microtubules and interact with membrane-associated proteins. The effects of recombinant NF1 and APC fragments on vesicle motility were evaluated by measuring fast axonal transport along microtubules in axoplasm from squid giant axons. APC4 (amino acids 1034-2844) reduced only anterograde movements, whereas APC2 (aa 1034-2130) or APC3 (aa 2130-2844) reduced both anterograde and retrograde transport. NF1 had no effect on organelle movement in either direction. Because APC contains multiple cyclin-dependent kinase (CDK) consensus phosphorylation motifs, the kinase inhibitor olomoucine was examined. At concentrations in which olomoucine is specific for cyclin-dependent kinases (5 microM), it reduced only anterograde transport, whereas anterograde and retrograde movement were both affected at concentrations at which other kinases are inhibited as well (50 microM). Both anterograde and retrograde transport also were inhibited by histone H1 and KSPXK peptides, substrates for proline-directed kinases, including CDKs. Our data suggest that CDK-like axonal kinases modulate fast anterograde transport and that other axonal kinases may be involved in modulating retrograde transport. The specific effect of APC4 on anterograde transport suggests a model in which the binding of APC to microtubules may limit the activity of axonal CDK kinase or kinases in restricted domains, thereby affecting organelle transport.

Image reconstructions of microtubules decorated with monomeric and dimeric kinesins: comparison with x-ray structure and implications for motility

We have decorated microtubules with monomeric and dimeric kinesin constructs, studied their structure by cryoelectron microscopy and three-dimensional image reconstruction, and compared the results with the x-ray crystal structure of monomeric and dimeric kinesin. A monomeric kinesin construct (rK354, containing only a short neck helix insufficient for coiled-coil formation) decorates microtubules with a stoichiometry of one kinesin head per tubulin subunit (alpha-beta-heterodimer). The orientation of the kinesin head (an anterograde motor) on the microtubule surface is similar to that of ncd (a retrograde motor). A longer kinesin construct (rK379) forms a dimer because of the longer neck helix forming a coiled-coil. Unexpectedly, this construct also decorates the microtubule with a stoichiometry of one head per tubulin subunit, and the orientation is similar to that of the monomeric construct. This means that the interaction with microtubules causes the two heads of a kinesin dimer to separate sufficiently so that they can bind to two different tubulin subunits. This result is in contrast to recent models and can be explained by assuming that the tubulin-kinesin interaction is antagonistic to the coiled-coil interaction within a kinesin dimer.

Molecular diversity in neurosecretion: reflections on the hypothalamo-neurohypophysial system

1. The diversity of molecules involved in various aspects of neurosecretion, such as proprotein processing, axonal transport of large dense core vesicles (LDCVs), and regulated secretion, is discussed in the context of the hypothalamo-neurohypophysial system (HNS). 2. Recent studies have uncovered a family of at least seven processing enzymes known as proprotein convertases (PCs) which are involved in proteolytically cleaving protein precursors at paired basic amino acid motifs to yield biologically active peptides. Three of these, PC1(3), 2, and 5, are found in neurons and are involved in producing regulated secretory peptide products. 3. The axonal transport of LDCVs occurs on microtubule tracks by still unknown mechanisms. There are over 11 distinct kinesin-related molecules that have now been identified as possible microtubule motor candidates. 4. Calcium channels in the nervous system are known to be derived from at least five alpha-subunit and four beta-subunit genes with multiple alternatively spliced isoforms in each case. These could account, in part, for the varied calcium currents found in the HNS. 5. The large number of proteins and isoforms now demonstrated to be involved in regulated secretion are discussed, with a focus on LDCV compositions and the synaptotagmin gene family.

Interaction of monomeric and dimeric kinesin with microtubules

The binding stoichiometry of kinesin to microtubules was determined using several biochemical and biophysical approaches (chemical crosslinking, binding assays, scanning transmission electron microscopy (STEM), image reconstruction, and X-ray scattering). The results show that each tubulin dimer associates with one kinesin head, irrespective of whether kinesin occurs in a monomeric or dimeric form in solution. Moreover, these heads appear to align along the protofilament axis generating a 16 nm periodicity of successive kinesin dimers. This is consistent with a "tightrope" model of movement where the first head of the dimer provides a guiding signal for the following one.

The MAP kinase kinase kinase MLK2 co-localizes with activated JNK along microtubules and associates with kinesin superfamily motor KIF3

The MLK (mixed lineage) ser/thr kinases are most closely related to the MAP kinase kinase kinase family. In addition to a kinase domain, MLK1, MLK2 and MLK3 each contain an SH3 domain, a leucine zipper domain and a potential Rac/Cdc42 GTPase-binding (CRIB) motif. The C-terminal regions of the proteins are essentially unrelated. Using yeast two-hybrid analysis and in vitro dot-blots, we show that MLK2 and MLK3 interact with the activated (GTP-bound) forms of Rac and Cdc42, with a slight preference for Rac. Transfection of MLK2 into COS cells leads to strong and constitutive activation of the JNK (c-Jun N-terminal kinase) MAP kinase cascade, but also to activation of ERK (extracellular signal-regulated kinase) and p38. When expressed in fibroblasts, MLK2 co-localizes with active, dually phosphorylated JNK1/2 to punctate structures along microtubules. In an attempt to identify proteins that affect the activity and localization of MLK2, we have screened a yeast two-hybrid cDNA library. MLK2 and MLK3 interact with members of the KIF3 family of kinesin superfamily motor proteins and with KAP3A, the putative targeting component of KIF3 motor complexes, suggesting a potential link between stress activation and motor protein function.

A fungal kinesin required for organelle motility, hyphal growth, and morphogenesis

A gene (NhKIN1) encoding a kinesin was cloned from Nectria haematococca genomic DNA by polymerase chain reaction amplification, using primers corresponding to conserved regions of known kinesin-encoding genes. Sequence analysis showed that NhKIN1 belongs to the subfamily of conventional kinesins and is distinct from any of the currently designated kinesin-related protein subfamilies. Deletion of NhKIN1 by transformation-mediated homologous recombination caused several dramatic phenotypes: a 50% reduction in colony growth rate, helical or wavy hyphae with reduced diameter, and subcellular abnormalities including withdrawal of mitochondria from the growing hyphal apex and reduction in the size of the Spitzenkörper, an apical aggregate of secretory vesicles. The effects on mitochondria and Spitzenkörper were not due to altered microtubule distribution, as microtubules were abundant throughout the length of hyphal tip cells of the mutant. The rate of spindle elongation during anaphase B of mitosis was reduced 11%, but the rate was not significantly different from that of wild type. This lack of a substantial mitotic phenotype is consistent with the primary role of the conventional kinesins in organelle motility rather than mitosis. Our results provide further evidence that the microtubule-based motility mechanism has a direct role in apical transport of secretory vesicles and the first evidence for its role in apical transport of mitochondria in a filamentous fungus. They also include a unique demonstration that a microtubule-based motor protein is essential for normal positioning of the Spitzenkörper, thus providing a new insight into the cellular basis for the aberrant hyphal morphology.

Cloning and expression of human mitotic centromere-associated kinesin gene

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

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.

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.

Characterization of KIFC2, a neuronal kinesin superfamily member in mouse

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

Active transport of photoactivated tubulin molecules in growing axons revealed by a new electron microscopic analysis

To determine whether tubulin molecules transported in axons are polymers or oligomers, we carried out electron microscopic analysis of the movement of the tubulin molecules after photoactivation. Although previous optical microscopic analyses after photobleaching or photoactivation had suggested that most of the axonal microtubules were stationary, they were not sufficiently sensitive to allow detection of actively transported tubulin molecules which were expected to be only a small fraction of total tubulin molecules in axons. In addition, some recent studies using indirect approaches suggested active polymer transport as a mechanism for tubulin transport (Baas, P.W., F.J. Ahmad. 1993. J. Cell Biol. 120:1427-1437; Yu, W., V.E. Centonze, F.J. Ahmad, and P.W. Bass, 1993, J. Cell Biol. 122:349-359; Ahmad, F.J., and P.W. Bass. 1995. J. Cell Sci. 108:2761-2769). So, whether transported tubulin molecules are polymers or not remain to be determined. To clear up this issue, we made fluorescent marks on the tubulin molecules in the axons using a photoactivation technique and performed electron microscopic immunocytochemistry using anti-fluorescein antibody. Using this new method we achieved high resolution and high sensitivity for detecting the transported tubulin molecules. In cells fixed after permeabilization, we found no translocated microtubules. In those fixed without permeabilization, in which oligomers and heterodimers in addition to polymers were preserved, we found much more label in the regions distal to the photoactivated regions than in the proximal regions. These data indicated that tubulin molecules are transported not as polymers but as heterodimers or oligomers by an active mechanism rather than by diffusion.

Differential expression of ubiquitous and neuronal kinesin heavy chains during differentiation of human neuroblastoma and PC12 cells

Kinesin is a microtubule-based motor protein involved in intracellular organelle transport. Neurons are characterized by the presence of at least two isoforms of conventional kinesin: ubiquitous kinesin, expressed in all cells and tissues, and neuronal kinesin, whose pattern of expression is confined to neuronal cells. In order to investigate whether the two kinesin motors, which are encoded by different genes, may play distinct biological roles in neurons, we studied their expression during neuronal differentiation. Human neuroblastoma SH-SY5Y and IMR32 cells and rat phaeochromocytoma PC12 cells were used as an in vitro system for neuronal differentiation and were induced to differentiate in the presence of retinoic acid, a combination of dibutyryl cAMP and 5-bromodeoxyuridine, and nerve growth factor respectively. The expression level of each kinesin isoform was evaluated by quantitative immunoblot before and after pharmacological treatment. We found that in all cell types the expression level of neuronal kinesin, but not of ubiquitous kinesin, is stimulated during differentiation. In particular, SH-SY5Y cells show a 4.5-fold, IMR32 cells a 3-fold and PC12 cells a 7-fold increase in the level of expression of neuronal kinesin. By Northern blot analysis we found that the selective increase in the expression of neuronal kinesin is paralleled by an increase in its mRNA, indicating that there is a transcriptional control of the expression of this kinesin isoform during differentiation of neuroblastoma and PC12 cells. Our results suggest that these cells represent an adequate model to study the function of conventional kinesin and its isoforms.

Biochemical and functional diversity of microtubule motors in the nervous system

The fact that multiple microtubule-based motors exist in brain inevitably raises questions about their function. Transcripts for at least seven kinesin superfamily genes and even more dynein heavy chain genes have been detected in brain cDNA libraries. The challenge now is to match their gene products to specific functions in cells of the nervous system. Recent studies have attempted to establish a function for each microtubule motor by using recombinant protein and immunochemical approaches.