Cargo binding and regulatory sites in the tail of fungal conventional kinesin

Intracellular Transport and Kinesin Superfamily Proteins, KIFs: Structure, Function, and Dynamics

Various molecular cell biology and molecular genetic approaches have indicated significant roles for kinesin superfamily proteins (KIFs) in intracellular transport and have shown that they are critical for cellular morphogenesis, functioning, and survival. KIFs not only transport various membrane organelles, protein complexes, and mRNAs for the maintenance of basic cellular activity, but also play significant roles for various mechanisms fundamental for life, such as brain wiring, higher brain functions such as memory and learning and activity-dependent neuronal survival during brain development, and for the determination of important developmental processes such as left-right asymmetry formation and suppression of tumorigenesis. Accumulating data have revealed a molecular mechanism of cargo recognition involving scaffolding or adaptor protein complexes. Intramolecular folding and phosphorylation also regulate the binding activity of motor proteins. New techniques using molecular biophysics, cryoelectron microscopy, and X-ray crystallography have detected structural changes in motor proteins, synchronized with ATP hydrolysis cycles, leading to the development of independent models of monomer and dimer motors for processive movement along microtubules.

The kinesin-1 motor protein is regulated by a direct interaction of its head and tail

Kinesin-1 is a molecular motor protein that transports cargo along microtubules. Inside cells, the vast majority of kinesin-1 is regulated to conserve ATP and to ensure its proper intracellular distribution and coordination with other molecular motors. Regulated kinesin-1 folds in half at a hinge in its coiled-coil stalk. Interactions between coiled-coil regions near the enzymatically active heads at the N terminus and the regulatory tails at the C terminus bring these globular elements in proximity and stabilize the folded conformation. However, it has remained a mystery how kinesin-1's microtubule-stimulated ATPase activity is regulated in this folded conformation. Here, we present evidence for a direct interaction between the kinesin-1 head and tail. We photochemically cross-linked heads and tails and produced an 8-A cryoEM reconstruction of the cross-linked head-tail complex on microtubules. These data demonstrate that a conserved essential regulatory element in the kinesin-1 tail interacts directly and specifically with the enzymatically critical Switch I region of the head. This interaction suggests a mechanism for tail-mediated regulation of the ATPase activity of kinesin-1. In our structure, the tail makes simultaneous contacts with the kinesin-1 head and the microtubule, suggesting the tail may both regulate kinesin-1 in solution and hold it in a paused state with high ADP affinity on microtubules. The interaction of the Switch I region of the kinesin-1 head with the tail is strikingly similar to the interactions of small GTPases with their regulators, indicating that other kinesin motors may share similar regulatory mechanisms.

Phosphorylation relieves autoinhibition of the kinetochore motor Cenp-E

During mitosis, chromosome alignment depends on the regulated dynamics of microtubules and on motor protein activities. At the kinetochore, the interplay between microtubule-binding proteins, motors, and kinases is poorly understood. Cenp-E is a kinetochore-associated kinesin involved in chromosome congression, but the mechanism by which this is achieved is unclear. Here, we present a study of the regulation of Cenp-E motility by using purified full-length (FL) Xenopus Cenp-E protein, which demonstrates that FL Cenp-E is a genuine plus-end-directed motor. Furthermore, we find that the Cenp-E tail completely blocks the motility of Cenp-E in vitro. This is achieved through direct interaction between its motor and tail domains. Finally, we show that Cenp-E autoinhibition is reversed by MPS1- or CDK1-cyclin B-mediated phosphorylation of the Cenp-E tail. This suggests a model of dynamic control of Cenp-E motility, and hence chromosome congression, dependent upon phosphorylation at the kinetochore.

Association of the Trp719Arg polymorphism in kinesin-like protein 6 with myocardial infarction and coronary heart disease in 2 prospective trials: the CARE and WOSCOPS trials

OBJECTIVES

We asked whether 35 genetic polymorphisms, previously found to be associated with cardiovascular disease, were associated with myocardial infarction (MI) in the CARE (Cholesterol and Recurrent Events) trial and with coronary heart disease (CHD) in the WOSCOPS (West of Scotland Coronary Prevention Study) trial and whether the risk associated with these polymorphisms could be reduced by pravastatin treatment.

BACKGROUND

Identification of genetic polymorphisms associated with CHD may improve assessment of CHD risk and understanding of disease pathophysiology.

METHODS

We tested the association between genotype and recurrent MI in the CARE study and between genotype and primary CHD in the WOSCOPS trial using regression models that adjusted for conventional risk factors: Cox proportional hazards models for the CARE study and conditional logistic regression models for a nested case-control study of the WOSCOPS trial.

RESULTS

We found that Trp719Arg (rs20455) in KIF6 was associated with coronary events. KIF6 encodes kinesin-like protein 6, a member of the molecular motor superfamily. In placebo-treated patients, carriers of the KIF6 719Arg allele (59.4% of the CARE trial cohort) had a hazard ratio of 1.50 (95% confidence interval [CI] 1.05 to 2.15) in the CARE trial and an odds ratio of 1.55 (95% CI 1.14 to 2.09) in the WOSCOPS trial. Among carriers, the absolute risk reduction by pravastatin was 4.89% (95% CI 1.81% to 7.97%) in the CARE trial and 5.49% (95% CI 3.52% to 7.46%) in the WOSCOPS trial.

CONCLUSIONS

In both the CARE and the WOSCOPS trials, carriers of the KIF6 719Arg allele had an increased risk of coronary events, and pravastatin treatment substantially reduced that risk.

Polymorphism in KIF6 gene and benefit from statins after acute coronary syndromes: results from the PROVE IT-TIMI 22 study

OBJECTIVES

We explored whether the benefit of intensive versus moderate statin therapy would be greater in carriers of KIF6 719Arg than in noncarriers.

BACKGROUND

The 719Arg variant of Trp719Arg (rs20455), a polymorphism in kinesin-like protein 6, is associated with greater risk of coronary events and greater benefit from pravastatin versus placebo.

METHODS

We genotyped 1,778 acute coronary syndrome patients within the PROVE IT-TIMI 22 (Pravastatin or Atorvastatin Evaluation and Infection Therapy: Thrombolysis in Myocardial Infarction 22) trial and investigated different intensities of statin therapy in carriers of 719Arg and in noncarriers using Cox proportional hazards models that adjusted for traditional risk factors.

RESULTS

Benefit from intensive, compared with moderate, statin therapy was significantly greater in the 59% of the cohort who were carriers (hazard ratio [HR] 0.59, 95% confidence interval [CI] 0.45 to 0.77) than in those who were noncarriers (HR 0.94, 95% CI 0.70 to 1.27; p = 0.018 for interaction between 719Arg carrier status and treatment). Absolute risk reduction was 10.0% in carriers versus 0.8% in noncarriers. The benefit of intensive therapy in carriers was significant as early as day 30 of therapy. Carriers and noncarriers did not differ in on-treatment low-density lipoprotein cholesterol, triglyceride, or C-reactive protein (CRP) levels.

CONCLUSIONS

Carriers of 719Arg receive significantly greater benefit from intensive statin therapy than do noncarriers, a superior benefit that appears to be due to a mechanism distinct from lipid or CRP lowering. Functional studies of the KIF6 kinesin are warranted, given the consistent association of Trp719Arg with risk of coronary events and statin benefit.

Properties of the Kinesin-1 motor DdKif3 from Dictyostelium discoideum

The amoeba Dictyostelium discoideum possesses genes for 13 different kinesins. Here we characterize DdKif3, a member of the Kinesin-1 family. Kinesin-1 motors form homodimers that can move micrometer-long distances on microtubules using the energy derived from ATP hydrolysis. We expressed recombinant motors in Escherichia coli and tested them in different in vitro assays. Full-length and truncated Kif3 motors were active in gliding and ATPase assays. They showed a strong dependence on ionic strength. Like the full-length motor, the truncated DdKif3-592 motor (aa 1-592; comprising motor domain, neck, and partial stalk) reached its maximum speed of around 2.0micrcom s(-1) at a potassium acetate concentration of 200mM. The shortened DdKif3-342 motor (aa 1-342; comprising motor domain, partial neck) showed a high ATP turnover, comparable to that of the fungal Kinesin-1, Nkin. Results from the duty cycle calculations and gliding assays indicate that DdKif3 is a processive motor. A GFP-fusion protein revealed a mainly cytoplasmic localization of DdKif3. Immunofluorescence staining makes an association with the endoplasmic reticulum or mitochondria unlikely. Despite a similar phylogenetic distance to both metazoa and fungi, in terms of its biochemical properties DdKif3 revealed a closer similarity to fungal than animal kinesins.

Molecular dissection and expression of the LdK39 kinesin in the human pathogen, Leishmania donovani

In this study we show for the first time the intracellular distribution of a K39 kinesin homologue in Leishmania donovani, a medically important parasite of humans. Further, we demonstrated that this motor protein is expressed in both the insect and mammalian developmental forms (i.e. promastigote and amastigotes) of this organism. Moreover, in both of these parasite developmental stages, immunofluorescence indicated that the LdK39 kinesin accumulated at anterior and posterior cell poles and that it displayed a peripheral localization consistent with the cortical cytoskeleton. Using a molecular approach, we identified, cloned and characterized the first complete open reading frame for the gene (LdK39) encoding this large (> 358 kDa) motor protein in L. donovani. Based on these observations, we subsequently used a homologous episomal expression system to dissect and express the functional domains that constitute the native molecule. Cell fractionation experiments demonstrated that LdK39 was soluble and that it bound to detergent-extracted cytoskeletons of these parasites in an ATP-dependent manner. The cumulative results of these experiments are consistent with LdK39 functioning as an ATP-dependent kinesin which binds to and travels along the cortical cytoskeleton of this important human pathogen.

Two kinesins from Arabidopsis, KatB and KatC, have a second microtubule-binding site in the tail domain

Kinesins, as a kind of microtubule-based motor proteins, have a conserved microtubule-binding site in their motor domain. Here we report that two homologous kinesins in Arabidopsis thaliana, KatB and KatC, contain a second microtubule-binding site in their tail domains. The prokaryotic-expressed N-terminal tail domain of the KatC heavy chain can bind to microtubules in an ATP-insensitive manner. To identify the precise region responsible for the binding, a serious of truncated KatC cDNAs encoding KatC N-terminal regions in different lengths, KatC1-128, KatC1-86, KatC1-73 and KatC1-63, fused to Histidine-tags, were expressed in E. coli and affinity-purified. Microtubule cosedimentation assays show that the site at amino acid residues 74-86 in KatC is important for microtubulebinding. By similarity, we obtained three different lengths of KatB N-terminal regions, KatB1-384, KatB1-77, and KatB1-63, and analyzed their microtubule-binding ability. Cosedimentation assays indicate that the KatB tail domain can also bind to microtubules at the same site as and in a similar manner to KatC. Fluorescence microscopic observations show that the microtubule-binding site at the tail domain of KatB or KatC can induce microtubules bundling only when the stalk domain is present. Through pull-down assays, we show that KatB1-385 and KatC1-394 are able to interact specifically with themselves and with each other in vitro. These findings are significant for identifying a previously uncharacterized microtubule-binding site in the two kinesin proteins, KatB and KatC, and the functional relations between them.

Two binding partners cooperate to activate the molecular motor Kinesin-1

The regulation of molecular motors is an important cellular problem, as motility in the absence of cargo results in futile adenosine triphosphate hydrolysis. When not transporting cargo, the microtubule (MT)-based motor Kinesin-1 is kept inactive as a result of a folded conformation that allows autoinhibition of the N-terminal motor by the C-terminal tail. The simplest model of Kinesin-1 activation posits that cargo binding to nonmotor regions relieves autoinhibition. In this study, we show that binding of the c-Jun N-terminal kinase-interacting protein 1 (JIP1) cargo protein is not sufficient to activate Kinesin-1. Because two regions of the Kinesin-1 tail are required for autoinhibition, we searched for a second molecule that contributes to activation of the motor. We identified fasciculation and elongation protein zeta1 (FEZ1) as a binding partner of kinesin heavy chain. We show that binding of JIP1 and FEZ1 to Kinesin-1 is sufficient to activate the motor for MT binding and motility. These results provide the first demonstration of the activation of a MT-based motor by cellular binding partners.

Jump-starting kinesin

When it is not actively transporting cargo, conventional Kinesin-1 is present in the cytoplasm in a folded conformation that cannot interact effectively with microtubules (MTs). Two important and largely unexplored aspects of kinesin regulation are how it is converted to an active species when bound to cargo and the related issue of how kinesin discriminates among its many potential cargo molecules. Blasius et al. (see p. 11 of this issue) report that either binding of the cargo linker c-Jun N-terminal kinase–interacting protein 1 (JIP1) to the light chains (LCs) or binding of fasciculation and elongation protein ζ1 (FEZ1) to the heavy chains (HCs) is insufficient for activation but that activation occurs when both are present simultaneously. A related paper by Cai et al. (see p. 51 of this issue) provides structural insight into the conformation of the folded state in the cell obtained by fluorescence resonance energy transfer analysis.

Insight into the mechanism of fast movement of myosin from Chara corallina

Chara myosin, two-headed plant myosin belonging to class XI, slides F-actin at maximally 60 microm s(-1). To elucidate the mechanism of this fast sliding, we extensively investigated its mechanochemical properties. The maximum actin activated ATPase activity, Vmax, was 21.3 s(-1) head(-1) in a solution, but when myosin was immobilized on the surface, its activity was 57.6 s(-1) head(-1) at 2 mg ml(-1) of F-actin. The sliding velocity and the actin activated ATPase activity were greatly inhibited by ADP, suggesting that ADP dissociation was the rate limiting step. With the extensive assay of motility by varying the surface density, the duty ratio of Chara myosin was found to be 0.49-0.44 from velocity measurements and 0.34 from the landing rate analysis. At the surface density of 10 molecules microm(-2), Chara myosin exhibited pivot movement under physiological conditions. Based on the results obtained, we will discuss the sliding mechanism of Chara myosin according to the working stroke model in terms of its physiological aspects. aspects.

Regulating cytoskeleton-based vesicle motility

During vesicular transport, the assembly of the coat complexes and the selection of cargo proteins must be coordinated with the subsequent translocation of vesicles from the donor to an acceptor compartment. Here, we review recent progress toward uncovering the molecular mechanisms that connect transport vesicles to the protein machinery responsible for cytoskeleton-mediated motility. An emerging theme is that vesicle cargo proteins, either directly or through binding interactions with coat proteins, are able to influence cytoskeletal dynamics and motor protein function. Hence, a vesicle's cargo composition may help direct its intracellular motility and targeting.

Cargo selection by specific kinesin light chain 1 isoforms

Kinesin-1 drives the movement of diverse cargoes, and it has been proposed that specific kinesin light chain (KLC) isoforms target kinesin-1 to these different structures. Here, we test this hypothesis using two in vitro motility assays, which reconstitute the movement of rough endoplasmic reticulum (RER) and vesicles present in a Golgi membrane fraction. We generated GST-tagged fusion proteins of KLC1B and KLC1D that included the tetratricopeptide repeat domain and the variable C-terminus. We find that preincubation of RER with KLC1B inhibits RER motility, whereas KLC1D does not. In contrast, Golgi fraction vesicle movement is inhibited by KLC1D but not KLC1B reagents. Both RER and vesicle movement is inhibited by preincubation with the GST-tagged C-terminal domain of ubiquitous kinesin heavy chain (uKHC), which binds to the N-terminal domain of uKHC and alters its interaction with microtubules. We propose that although the TRR domains are required for cargo binding, it is the variable C-terminal region of KLCs that are vital for targeting kinesin-1 to different cellular structures.

A peptide zipcode sufficient for anterograde transport within amyloid precursor protein

Fast anterograde transport of membrane-bound organelles delivers molecules synthesized in the neuronal cell body outward to distant synapses. Identification of the molecular "zipcodes" on organelles that mediate attachment and activation of microtubule-based motors for this directed transport is a major area of inquiry. Here we identify a short peptide sequence (15 aa) from the cytoplasmic C terminus of amyloid precursor protein (APP-C) sufficient to mediate the anterograde transport of peptide-conjugated beads in the squid giant axon. APP-C beads travel at fast axonal transport rates (0.53 mum/s average velocity, 0.9 mum/s maximal velocity) whereas beads coupled to other peptides coinjected into the same axon remain stationary at the injection site. This transport appears physiologic, because it mimics behavior of endogenous squid organelles and of beads conjugated to C99, a polypeptide containing the full-length cytoplasmic domain of amyloid precursor protein (APP). Beads conjugated to APP lacking the APP-C domain are not transported. Coinjection of APP-C peptide reduces C99 bead motility by 75% and abolishes APP-C bead motility, suggesting that the soluble peptide competes with protein-conjugated beads for axoplasmic motor(s). The APP-C domain is conserved (13/15 aa) from squid to human, and peptides from either squid or human APP behave similarly. Thus, we have identified a conserved peptide zipcode sufficient to direct anterograde transport of exogenous cargo and suggest that one of APP's roles may be to recruit and activate axonal machinery for endogenous cargo transport.

Mechanism of tail-mediated inhibition of kinesin activities studied using synthetic peptides

We used a truncated form of human conventional kinesin (K560) and a set of synthetic tail-derived peptides to investigate the mechanism by which the kinesin tail domain inhibits the protein's ATPase and motor activities. A peptide that spans residues 904-933 (C3) exhibited the strongest inhibitory effect on steady-state motility and ATPase activity. This inhibition reflected diminished binding of the ADP-bound kinesin head to the microtubule. Although peptide C3 bound to both K560 and microtubules, gliding assays using subtilisin-treated microtubules suggested that the binding to the microtubule contributes only little to the inhibition if there is sufficient affinity between the peptide and kinesin. We suggest that tail-mediated inhibition of kinesin activity is mainly the product of allosteric inhibition induced by the intramolecular binding of the kinesin tail domain to the motor domain, but simultaneous binding of the tail to the microtubule also may exert a minor effect.

Back on track – On the role of the microtubule for kinesin motility and cellular function

The evolution of cytoskeletal filaments (actin- and intermediate-filaments, and the microtubules) and their associated motor- and non-motor-proteins has enabled the eukaryotic cell to achieve complex organizational and structural tasks. This ability to control cellular transport processes and structures allowed for the development of such complex cellular organelles like cilia or flagella in single-cell organisms and made possible the development and differentiation of multi-cellular organisms with highly specialized, polarized cells. Also, the faithful segregation of large amounts of genetic information during cell division relies crucially on the reorganization and control of the cytoskeleton, making the cytoskeleton a key prerequisite for the development of highly complex genomes. Therefore, it is not surprising that the eukaryotic cell continuously invests considerable resources in the establishment, maintenance, modification and rearrangement of the cytoskeletal filaments and the regulation of its interaction with accessory proteins. Here we review the literature on the interaction between microtubules and motor-proteins of the kinesin-family. Our particular interest is the role of the microtubule in the regulation of kinesin motility and cellular function. After an introduction of the kinesin-microtubule interaction we focus on two interrelated aspects: (1) the active allosteric participation of the microtubule during the interaction with kinesins in general and (2) the possible regulatory role of post-translational modifications of the microtubule in the kinesin-microtubule interaction.

Review: regulation mechanisms of Kinesin-1

Kinesin-1 microtubule motors are common kinesin motors from protozoa, fungi and animals. They transport vesicular or particle cargo in a strictly regulated manner. The relatively well-studied tail inhibition mechanism is based on a conformational change that leads to an interaction of Kinesin-1's tail with the junction of neck and hinge regions. This folding causes a decrease in microtubule binding and motor activity. In fungal Kinesin-1 motors several lines of evidence suggest that a conserved tyrosine in the neck coiled-coil mediates this inhibition. In the active state, a region surrounding a conserved tryptophan in the hinge stabilises the neck coiled-coil, and prevents the tyrosine from inhibiting. Although animal and fungal Kinesin-1 motors are clearly homologous and function according to the same chemo-mechanical mechanism, they differ in their regulation. Unlike fungal Kinesin-1s, animal kinesins associate with light chains that are important for regulation and cargo interaction. Several proteins interacting with animal Kinesin-1 heavy or light chains are known, among them typical scaffolding proteins that seem to link Kinesin-1 to signalling pathways.

Conventional kinesin mediates microtubule-microtubule interactions in vivo

Conventional kinesin is a ubiquitous organelle transporter that moves cargo toward the plus-ends of microtubules. In addition, several in vitro studies indicated a role of conventional kinesin in cross-bridging and sliding microtubules, but in vivo evidence for such a role is missing. In this study, we show that conventional kinesin mediates microtubule-microtubule interactions in the model fungus Ustilago maydis. Live cell imaging and ultrastructural analysis of various mutants in Kin1 revealed that this kinesin-1 motor is required for efficient microtubule bundling and participates in microtubule bending in vivo. High levels of Kin1 led to increased microtubule bending, whereas a rigor-mutation in the motor head suppressed all microtubule motility and promoted strong microtubule bundling, indicating that kinesin can form cross-bridges between microtubules in living cells. This effect required a conserved region in the C terminus of Kin1, which was shown to bind microtubules in vitro. In addition, a fusion protein of yellow fluorescent protein and the Kin1tail localized to microtubule bundles, further supporting the idea that a conserved microtubule binding activity in the tail of conventional kinesins mediates microtubule-microtubule interactions in vivo.

Rab Proteins, Connecting Transport and Vesicle Fusion

Small GTPases of the Rab family control timing of vesicle fusion. Fusion of two vesicles can only occur when they have been brought into close contact. Transport by microtubule- or actin-based motor proteins will facilitate this process in vivo. Ideally, transport and vesicle fusion are linked activities. Active, GTP-bound Rab proteins dock on specific compartments and are therefore perfect candidates to control transport of the different compartments. Recently, a number of Rab proteins were identified that control motor protein recruitment to their specific target membranes. By cycling through inactive and active states, Rab proteins are able to control motor protein-mediated transport and subsequent fusion of intracellular structures in both spatial and timed manners.

Self-organized density patterns of molecular motors in arrays of cytoskeletal filaments

The stationary states of systems with many molecular motors are studied theoretically for uniaxial and centered (asterlike) arrangements of cytoskeletal filaments using Monte Carlo simulations and a two-state model. Mutual exclusion of motors from binding sites of the filaments is taken into account. For small overall motor concentration, the density profiles are exponential and algebraic in uniaxial and centered filament systems, respectively. For uniaxial systems, exclusion leads to the coexistence of regions of high and low densities of bound motors corresponding to motor traffic jams, which grow upon increasing the overall motor concentration. These jams are insensitive to the motor behavior at the end of the filament. In centered systems, traffic jams remain small and an increase in the motor concentration leads to a flattening of the profile if the motors move inwards, and to the buildup of a concentration maximum in the center of the aster if motors move outwards. In addition to motor density patterns, we also determine the corresponding patterns of the motor current.

The E-hook of tubulin interacts with kinesin's head to increase processivity and speed

Kinesins are dimeric motor proteins that move processively along microtubules. It has been proposed that the processivity of conventional kinesins is increased by electrostatic interactions between the positively charged neck of the motor and the negatively charged C-terminus of tubulin (E-hook). In this report we challenge this anchoring hypothesis by studying the motility of a fast fungal kinesin from Neurospora crassa (NcKin). NcKin is highly processive despite lacking the positive charges in the neck. We present a detailed analysis of how proteolytic removal of the E-hook affects truncated monomeric and dimeric constructs of NcKin. Upon digestion we observe a strong reduction of the processivity and speed of dimeric motor constructs. Monomeric motors with truncated or no neck display the same reduction of microtubule gliding speed as dimeric constructs, suggesting that the E-hook interacts with the head only. The E-hook has no effect on the strongly bound states of NcKin as microtubule digestion does not alter the stall forces produced by single dimeric motors, suggesting that the E-hook affects the interaction site of the kinesin.ADP-head and the microtubule. In fact, kinetic and binding experiments indicate that removal of the E-hook shifts the binding equilibrium of the weakly attached kinesin.ADP-head toward a more strongly bound state, which may explain reduced processivity and speed on digested microtubules.

The KLP-6 kinesin is required for male mating behaviors and polycystin localization in Caenorhabditis elegans

BACKGROUND

Male mating behavior of the nematode Caenorhabditis elegans offers an intriguing model to study the genetics of sensory behavior, cilia function, and autosomal dominant polycystic kidney disease (ADPKD). The C. elegans polycystins LOV-1 and PKD-2 act in male-specific sensory cilia required for response and vulva-location mating behaviors.

RESULTS

Here, we identify and characterize a new mating mutant, sy511. sy511 behavioral phenotypes were mapped to a mutation in the klp-6 locus, a gene encoding a member of the kinesin-3 family (previously known as the UNC-104/Kif1A family). KLP-6 has a single homolog of unknown function in vertebrate genomes, including fish, chicken, mouse, rat, and human. We show that KLP-6 expresses exclusively in sensory neurons with exposed ciliated endings and colocalizes with the polycystins in cilia of male-specific neurons. Cilia of klp-6 mutants appear normal, suggesting a defect in sensory neuron function but not development. KLP-6 structure-function analysis reveals that the putative cargo binding domain directs the motor to cilia. Consistent with a motor-cargo association between KLP-6 and the polycystins, klp-6 is required for PKD-2 localization and function within cilia. Genetically, we find klp-6 regulates behavior through polycystin-dependent and -independent pathways.

CONCLUSION

Multiple ciliary transport pathways dependent on kinesin-II, OSM-3, and KLP-6 may act sequentially to build cilia and localize sensory ciliary membrane proteins such as the polycystins. We propose that KLP-6 and the polycystins function as an evolutionarily conserved ciliary unit. KLP-6 promises new routes to understanding cilia function, behavior, and ADPKD.

The complex interplay between the neck and hinge domains in kinesin-1 dimerization and motor activity

Kinesin-1 dimerizes via the coiled-coil neck domain. In contrast to animal kinesins, neck dimerization of the fungal kinesin-1 NcKin requires additional residues from the hinge. Using chimeric constructs containing or lacking fungal-specific elements, the proximal part of the hinge was shown to stabilize the neck coiled-coil conformation in a complex manner. The conserved fungal kinesin hinge residue W384 caused neck coiled-coil formation in a chimeric NcKin construct, including parts of the human kinesin-1 stalk. The stabilizing effect was retained in a NcKinW384F mutant, suggesting important pi-stacking interactions. Without the stalk, W384 was not sufficient to induce coiled-coil formation, indicating that W384 is part of a cluster of several residues required for neck coiled-coil folding. A W384-less chimera of NcKin and human kinesin possessed a non-coiled-coil neck conformation and showed inhibited activity that could be reactivated when artificial interstrand disulfide bonds were used to stabilize the neck coiled-coil conformation. On the basis of yeast two-hybrid data, we propose that the proximal hinge can bind kinesin's cargo-free tail domain and causes inactivation of kinesin by disrupting the neck coiled-coil conformation.

Molecular motors and mechanisms of directional transport in neurons

Intracellular transport is fundamental for neuronal morphogenesis, function and survival. Many proteins are selectively transported to either axons or dendrites. In addition, some specific mRNAs are transported to dendrites for local translation. Proteins of the kinesin superfamily participate in selective transport by using adaptor or scaffolding proteins to recognize and bind cargoes. The molecular components of RNA-transporting granules have been identified, and it is becoming clear how cargoes are directed to axons and dendrites by kinesin superfamily proteins. Here we discuss the molecular mechanisms of directional axonal and dendritic transport with specific emphasis on the role of motor proteins and their mechanisms of cargo recognition.

N-terminal kinesins: many and various

Molecular motors are a fascinating group of proteins that have vital roles in a huge variety of cellular processes. They all share the ability to produce force through the hydrolysis of adenosine triphosphate, and fall into classes groups: the kinesins, myosins and the dyneins. The kinesin superfamily itself can be split into three major groups depending on the position of the motor domain, which is localized N-terminally, C-terminally, or internally. This review focuses on the N-terminal kinesins, providing a brief overview of their roles within the cell, and illustrating recent key developments in our understanding of how these proteins function.

Transport of Drosophila fragile X mental retardation protein-containing ribonucleoprotein granules by kinesin-1 and cytoplasmic dynein

Transport and translation of mRNA are tightly coupled to ensure strict temporal and spatial expression of nascent proteins. Fragile X mental retardation protein (FMRP) has been shown to be involved in translational regulation and is found in ribonucleoprotein (RNP) granules that travel along dendrites of neurons. In this study, GFP-tagged Drosophila homologue of FMRP (dFMR) was used to visualize RNP granule movement in Drosophila S2 cells. GFP-dFMR form granules that contain both endogenous dFMR and mRNA. Live fluorescence microscopy revealed that dFMR-containing RNP granules move bidirectionally in thin processes formed by S2 cells in the presence of cytochalasin D. Knocking down the heavy chains of either kinesin-1 (kinesin heavy chain) or cytoplasmic dynein (dynein heavy chain) by RNA interference blocks the movement of the dFMR granules. In contrast, knockdown of kinesin light chain (KLC), which is typically necessary for movement of membrane organelles by kinesin-1, had no effect on the dFMR granule translocation. In immunoprecipitation assays, dFMR associates with both kinesin heavy chain and dynein heavy chain, but not KLC. Based on these findings, we conclude that dFMR-containing RNP granules are moved by both kinesin-1 and cytoplasmic dynein and that KLC is not essential and is likely missing from RNP-transporting kinesin-1.

In vitro selection of Jun-associated proteins using mRNA display

Although yeast two-hybrid assay and biochemical methods combined with mass spectrometry have been successfully employed for the analyses of protein-protein interactions in the field of proteomics, these methods encounter various difficulties arising from the usage of living cells, including inability to analyze toxic proteins and restriction of testable interaction conditions. Totally in vitro display technologies such as ribosome display and mRNA display are expected to circumvent these difficulties. In this study, we applied an mRNA display technique to screening for interactions of a basic leucine zipper domain of Jun protein in a mouse brain cDNA library. By performing iterative affinity selection and sequence analyses, we selected 16 novel Jun-associated protein candidates in addition to four known interactors. By means of real-time PCR and pull-down assay, 10 of the 16 newly discovered candidates were confirmed to be direct interactors with Jun in vitro. Furthermore, interaction of 6 of the 10 proteins with Jun was observed in cultured cells by means of co-immunoprecipitation and observation of subcellular localization. These results demonstrate that this in vitro display technology is effective for the discovery of novel protein-protein interactions and can contribute to the comprehensive mapping of protein-protein interactions.

The ribosome receptor, p180, interacts with kinesin heavy chain, KIF5B

The conventional microtubule-dependent motor protein kinesin consists of heavy and light chains both of which have been documented to bind a variety of potential linker or cargo proteins. In this study we employed a yeast two-hybrid assay to identify additional binding partners of the kinesin heavy chain isoform KIF5B. A human brain cDNA library was screened with a bait corresponding to amino acid residues 814-963 of human KIF5B. This screen identified the ribosome receptor, p180, as a KIF5B-binding protein. The sites of interaction are residues 1294-1413 of p180 and the C-terminal half of the cargo binding-domain of KIF5B (residues 867-907). The KIF5B-binding site in p180 is homologous to the previously determined KIF5B-binding site in kinectin. The interacting regions of p180 and KIF5B consist almost entirely of heptad repeats, suggesting the interaction is a coiled-coil. A role for the kinesin/p180 interaction may include mRNA localization and/or transport of endoplasmic reticulum-derived vesicles.

Nuclear migration and positioning in filamentous fungi

Genetic analyses of nuclear distribution mutants have indicated that functions of the microtubule motor, cytoplasmic dynein, and its regulators are important for nuclear positioning in filamentous fungi. Here we review these studies and also present the need to further dissect how dynein and its associated microtubule cytoskeleton are involved mechanistically in nuclear positioning in the multinucleated hyphae.

Lessons from the genome sequence of Neurospora crassa: tracing the path from genomic blueprint to multicellular organism

We present an analysis of over 1,100 of the approximately 10,000 predicted proteins encoded by the genome sequence of the filamentous fungus Neurospora crassa. Seven major areas of Neurospora genomics and biology are covered. First, the basic features of the genome, including the automated assembly, gene calls, and global gene analyses are summarized. The second section covers components of the centromere and kinetochore complexes, chromatin assembly and modification, and transcription and translation initiation factors. The third area discusses genome defense mechanisms, including repeat induced point mutation, quelling and meiotic silencing, and DNA repair and recombination. In the fourth section, topics relevant to metabolism and transport include extracellular digestion; membrane transporters; aspects of carbon, sulfur, nitrogen, and lipid metabolism; the mitochondrion and energy metabolism; the proteasome; and protein glycosylation, secretion, and endocytosis. Environmental sensing is the focus of the fifth section with a treatment of two-component systems; GTP-binding proteins; mitogen-activated protein, p21-activated, and germinal center kinases; calcium signaling; protein phosphatases; photobiology; circadian rhythms; and heat shock and stress responses. The sixth area of analysis is growth and development; it encompasses cell wall synthesis, proteins important for hyphal polarity, cytoskeletal components, the cyclin/cyclin-dependent kinase machinery, macroconidiation, meiosis, and the sexual cycle. The seventh section covers topics relevant to animal and plant pathogenesis and human disease. The results demonstrate that a large proportion of Neurospora genes do not have homologues in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. The group of unshared genes includes potential new targets for antifungals as well as loci implicated in human and plant physiology and disease.

The early onset dystonia protein torsinA interacts with kinesin light chain 1

Early onset dystonia is a movement disorder caused by loss of a glutamic acid residue (Glu(302/303)) in the carboxyl-terminal portion of the AAA+ protein, torsinA. We identified the light chain subunit (KLC1) of kinesin-I as an interacting partner for torsinA, with binding occurring between the tetratricopeptide repeat domain of KLC1 and the carboxyl-terminal region of torsinA. Coimmunoprecipitation analysis demonstrated that wild-type torsinA and kinesin-I form a complex in vivo. In cultured cortical neurons, both proteins co-localized along processes with enrichment at growth cones. Wild-type torsinA expressed in CAD cells co-localized with endogenous KLC1 at the distal end of processes, whereas mutant torsinA remained confined to the cell body. Subcellular fractionation of adult rat brain revealed torsinA and KLC associated with cofractionating membranes, and both proteins were co-immunoprecipitated after cross-linking cytoplasmically oriented proteins on isolated rat brain membranes. These studies suggest that wild-type torsinA undergoes anterograde transport along microtubules mediated by kinesin and may act as a molecular chaperone regulating kinesin activity and/or cargo binding.

Molecular motor proteins of the kinesin superfamily proteins (KIFs): structure, cargo and disease

Intracellular organelle transport is essential for morphogenesis and functioning of the cell. Kinesins and kinesin-related proteins make up a large superfamily of molecular motors that transport cargoes such as vesicles, organelles (e.g. mitochondria, peroxisomes, lysosomes), protein complexes (e.g. elements of the cytoskeleton, virus particles), and mRNAs in a microtubule- and ATP-dependent manner in neuronal and non-neuronal cells. Until now, more than 45 kinesin superfamily proteins (KIFs) have been identified in the mouse and human genomes. Elucidating the transport pathways mediated by kinesins, the identities of the cargoes moved, and the nature of the proteins that link kinesin motors to cargoes are areas of intense investigation. This review focuses on the structure, the binding partners of kinesins and kinesin-based human diseases.

β-Dystrobrevin interacts directly with kinesin heavy chain in brain

β-Dystrobrevin, a member of the dystrobrevin protein family, is a dystrophin-related and -associated protein restricted to non-muscle tissues and is highly expressed in kidney, liver and brain. Dystrobrevins are now thought to play an important role in intracellular signal transduction, in addition to providing a membrane scaffold in muscle, but the precise role of β-dystrobrevin has not yet been determined. To study β-dystrobrevin's function in brain, we used the yeast two-hybrid approach to look for interacting proteins. Four overlapping clones were identified that encoded Kif5A, a neuronal member of the Kif5 family of proteins that consists of the heavy chains of conventional kinesin. A direct interaction of β-dystrobrevin with Kif5A was confirmed by in vitro and in vivo association assays. Co-immunoprecipitation with a monoclonal kinesin heavy chain antibody precipitated both α- and β-dystrobrevin, indicating that this interaction is not restricted to the β-dystrobrevin isoform. The site for Kif5A binding to β-dystrobrevin was localized in a carboxyl-terminal region that seems to be important in heavy chain-mediated kinesin interactions and is highly homologous in all three Kif5 isoforms, Kif5A, Kif5B and Kif5C. Pull-down and immunofluorescence experiments also showed a direct interaction between β-dystrobrevin and Kif5B. Our findings suggest a novel function for dystrobrevin as a motor protein receptor that might play a major role in the transport of components of the dystrophin-associated protein complex to specific sites in the cell.

Microtubules provide directional cues for polarized axonal transport through interaction with kinesin motor head

Post-Golgi carriers of various newly synthesized axonal membrane proteins, which possess kinesin (KIF5)-driven highly processive motility, were transported from the TGN directly to axons. We found that KIF5 has a preference to the microtubules in the initial segment of axon. Low dose paclitaxel treatment caused missorting of KIF5, as well as axonal membrane proteins to the tips of dendrites. Microtubules in the initial segment of axons showed a remarkably high affinity to EB1-YFP, which was known to bind the tips of growing microtubules. These findings revealed unique features of the microtubule cytoskeletons in the initial segment, and suggested that they provide directional information for polarized axonal transport.

Processive and nonprocessive models of kinesin movement

Conventional kinesin is the prototypic member of a family of diverse proteins that use the chemical energy of ATP hydrolysis to generate force and move along microtubules. These proteins, which are involved in a wide range of cellular functions, have been identified in protozoa, fungi, plants, and animals and possess a high degree of sequence conservation among species in their motor domains. The biochemical properties of kinesin and its homologues, in conjunction with the recently solved three-dimensional structures of several kinesin motors, have contributed to our understanding of the mechanism of kinesin movement along microtubules. We discuss several models for movement, including the hand-over-hand, inchworm, and biased diffusion models of processive movement, as well as models of nonprocessive movement.

Trafficking of signaling modules by kinesin motors

The human genome has more than 40 kinesin genes whose protein products organize intracellular traffic along microtubules. Research during the past two years has begun to elucidate the cargoes carried by kinesins and the nature of the kinesin-cargo linkage. Modular protein-protein interactions connect kinesins to diverse cellular molecules, which, apart from their other functions, serve as kinesin-cargo linkers. Many of these newly identified linkers are scaffolds for signaling pathways, and mounting evidence now indicates that kinesins transport pre-assembled signaling modules as vesicular cargo. These findings bring together two fields, signal transduction and molecular motors, and lead to a deeper understanding of the interplay between trafficking, localization and intercellular communication.

A complete inventory of fungal kinesins in representative filamentous ascomycetes

Complete inventories of kinesins from three pathogenic filamentous ascomycetes, Botryotinia fuckeliana, Cochliobolus heterostrophus, and Gibberella moniliformis, are described. These protein sequences were compared with those of the filamentous saprophyte, Neurospora crassa and the two yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. Data mining and phylogenetic analysis of the motor domain yielded a constant set of 10 kinesins in the filamentous fungal species, compared with a smaller set in S. cerevisiae and S. pombe. The filamentous fungal kinesins fell into nine subfamilies when compared with well-characterized kinesins from other eukaryotes. A few putative kinesins (one in B. fuckeliana and two in C. heterostrophus) could not be defined as functional, due to unorthodox organization and lack of experimental data. The broad representation of filamentous fungal kinesins across most of the known subfamilies and the ease of gene manipulation make fungi ideal models for functional and evolutionary investigation of these proteins.

Molecular motors

Life implies movement. Most forms of movement in the living world are powered by tiny protein machines known as molecular motors. Among the best known are motors that use sophisticated intramolecular amplification mechanisms to take nanometre steps along protein tracks in the cytoplasm. These motors transport a wide variety of cargo, power cell locomotion, drive cell division and, when combined in large ensembles, allow organisms to move. Motor defects can lead to severe diseases or may even be lethal. Basic principles of motor design and mechanism have now been derived, and an understanding of their complex cellular roles is emerging.

Single fungal kinesin motor molecules move processively along microtubules

Conventional kinesins are two-headed molecular motors that move as single molecules micrometer-long distances on microtubules by using energy derived from ATP hydrolysis. The presence of two heads is a prerequisite for this processive motility, but other interacting domains, like the neck and K-loop, influence the processivity and are implicated in allowing some single-headed kinesins to move processively. Neurospora kinesin (NKin) is a phylogenetically distant, dimeric kinesin from Neurospora crassa with high gliding speed and an unusual neck domain. We quantified the processivity of NKin and compared it to human kinesin, HKin, using gliding and fluorescence-based processivity assays. Our data show that NKin is a processive motor. Single NKin molecules translocated microtubules in gliding assays on average 2.14 micro m (N = 46). When we tracked single, fluorescently labeled NKin motors, they moved on average 1.75 micro m (N = 182) before detaching from the microtubule, whereas HKin motors moved shorter distances (0.83 micro m, N = 229) under identical conditions. NKin is therefore at least twice as processive as HKin. These studies, together with biochemical work, provide a basis for experiments to dissect the molecular mechanisms of processive movement.

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.

A conserved tyrosine in the neck of a fungal kinesin regulates the catalytic motor core

The neck domain of fungal conventional kinesins displays characteristic properties which are reflected in a specific sequence pattern. The exchange of the strictly conserved Tyr 362, not present in animals, into Lys, Cys or Phe leads to a failure to dimerize. The destabilizing effect is confirmed by a lower coiled-coil propensity of mutant peptides. Whereas the Phe substitution has only a structural effect, the Lys and Cys replacements lead to dramatic kinetic changes. The steady state ATPase is 4- to 7-fold accelerated, which may be due to a faster microtubule-stimulated ADP release rate. These data suggest that an inhibitory effect of the fungal neck domain on the motor core is mediated by direct interaction of the aromatic ring of Tyr 362 with the head, whereas the OH group is essential for dimerization. This is the first demonstration of a direct influence of the kinesin neck region in regulation of the catalytic activity.

Kinesin light chain-independent function of the Kinesin heavy chain in cytoplasmic streaming and posterior localisation in the Drosophila oocyte

Microtubules and the Kinesin heavy chain, the force-generating component of the plus end-directed microtubule motor Kinesin I are required for the localisation of oskar mRNA to the posterior pole of the Drosophila oocyte, an essential step in the determination of the anteroposterior axis. We show that the Kinesin heavy chain is also required for the posterior localisation of Dynein, and for all cytoplasmic movements within the oocyte. Furthermore, the KHC localises transiently to the posterior pole in an oskar mRNA-independent manner. Surprisingly, cytoplasmic streaming still occurs in kinesin light chain null mutants, and both oskar mRNA and Dynein localise to the posterior pole. Thus, the Kinesin heavy chain can function independently of the light chain in the oocyte, indicating that it associates with its cargoes by a novel mechanism.

Identification of RanBP2- and kinesin-mediated transport pathways with restricted neuronal and subcellular localization

Ran-binding proteins, karyopherins, and RanGTPase mediate and impart directionality to nucleocytoplasmic transport processes. This biological process remains elusive in neurons. RanBP2 has been localized at the nuclear pore complexes and is very abundant in the neuroretina. RanBP2 mediates the assembly of a large complex comprising RanGTPase, CRM1/exportin-1, importin-beta, KIF5-motor proteins, components of the 19S cap of the 26S proteasome, ubc9 and opsin. Here, we show RanBP2 is abundant in the ellipsoid compartment of photoreceptors and RanGTPase-positive particles in cytoplasmic tracks extending away from the nuclear envelope of subpopulations of ganglion cells, suggesting RanBP2's release from nuclear pore complexes. KIF5C and KIF5B are specifically expressed in a subset of neuroretinal cells and differentially localize with RanBP2 and importin-beta in distinct compartments. The C-terminal domains of KIF5B and KIF5C, but not KIF5A, associate directly with importin-beta in a RanGTPase-dependent fashion in vivo and in vitro, indicating importin-beta is an endogenous cargo for a subset of KIF5s in retinal neurons. The KIF5 transport pathway is absent from the myoid region of a topographically distinct subclass of blue cones and the distribution of kinesin-light chains is largely distinct from its KIF5 partners. Altogether, the results identify the existence of neuronal- and subtype-specific kinesin-mediated transport pathways of importin-beta-bound cargoes to and/or from RanBP2 and indicate RanBP2 itself may also constitute a scaffold carrier for some of its associated partners. The implications of these findings in protein kinesis and pathogenesis of degenerative neuropathies 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.

Glutamate-receptor-interacting protein GRIP1 directly steers kinesin to dendrites

In cells, molecular motors operate in polarized sorting of molecules, although the steering mechanisms of motors remain elusive. In neurons, the kinesin motor conducts vesicular transport such as the transport of synaptic vesicle components to axons and of neurotransmitter receptors to dendrites, indicating that vesicles may have to drive the motor for the direction to be correct. Here we show that an AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate) receptor subunit--GluR2-interacting protein (GRIP1)--can directly interact and steer kinesin heavy chains to dendrites as a motor for AMPA receptors. As would be expected if this complex is functional, both gene targeting and dominant negative experiments of heavy chains of mouse kinesin showed abnormal localization of GRIP1. Moreover, expression of the kinesin-binding domain of GRIP1 resulted in accumulation of the endogenous kinesin predominantly in the somatodendritic area. This pattern was different from that generated by the overexpression of the kinesin-binding scaffold protein JSAP1 (JNK/SAPK-associated protein-1, also known as Mapk8ip3), which occurred predominantly in the somatoaxon area. These results indicate that directly binding proteins can determine the traffic direction of a motor protein.

Herpes simplex virus tegument protein US11 interacts with conventional kinesin heavy chain

Little is known about the mechanisms of transport of neurotropic herpesviruses, such as herpes simplex virus (HSV), varicella-zoster virus, and pseudorabies virus, within neurons. For these viruses, which replicate in the nucleus, anterograde transport from the cell body of dorsal root ganglion (DRG) neurons to the axon terminus occurs over long distances. In the case of HSV, unenveloped nucleocapsids in human DRG neurons cocultured with autologous skin were observed by immunoelectron microscopy to colocalize with conventional ubiquitous kinesin, a microtubule-dependent motor protein, in the cell body and axon during anterograde axonal transport. Subsequently, four candidate kinesin-binding structural HSV proteins were identified (VP5, VP16, VP22, and US11) using oligohistidine-tagged human ubiquitous kinesin heavy chain (uKHC) as bait. Of these viral proteins, a direct interaction between uKHC and US11 was identified. In vitro studies identified residues 867 to 894 as the US11-binding site in uKHC located within the proposed heptad repeat cargo-binding domain of uKHC. In addition, the uKHC-binding site in US11 maps to the C-terminal RNA-binding domain. US11 is consistently cotransported with kinetics similar to those of the capsid protein VP5 into the axons of dissociated rat neurons, unlike the other tegument proteins VP16 and VP22. These observations suggest a major role for the uKHC-US11 interaction in anterograde transport of unenveloped HSV nucleocapsids in axons.

Principles of cargo attachment to cytoplasmic motor proteins

The attachment of cytoskeletal motor proteins to cargo-laden vesicles is crucial for efficient transport in intracellular membrane trafficking pathways. Recent studies have identified specific kinesin-binding and dynein-binding proteins that could serve as membrane-associated 'receptor' proteins for the respective motors. New insights have also emerged about the cargo-binding domains of the motor proteins, and the regulation of motor binding to cargoes.

Glycogen synthase kinase 3 phosphorylates kinesin light chains and negatively regulates kinesin-based motility

Membrane-bounded organelles (MBOs) are delivered to different domains in neurons by fast axonal transport. The importance of kinesin for fast antero grade transport is well established, but mechanisms for regulating kinesin-based motility are largely unknown. In this report, we provide biochemical and in vivo evidence that kinesin light chains (KLCs) interact with and are in vivo substrates for glycogen synthase kinase 3 (GSK3). Active GSK3 inhibited anterograde, but not retrograde, transport in squid axoplasm and reduced the amount of kinesin bound to MBOs. Kinesin microtubule binding and microtubule-stimulated ATPase activities were unaffected by GSK3 phosphorylation of KLCs. Active GSK3 was also localized preferentially to regions known to be sites of membrane delivery. These data suggest that GSK3 can regulate fast anterograde axonal transport and targeting of cargos to specific subcellular domains in neurons.

Toucan protein is essential for the assembly of syncytial mitotic spindles in Drosophila melanogaster

The toc gene of Drosophila melanogaster encodes a 235-kD polypeptide with a coiled-coil domain, which is highly expressed during oogenesis (Grammont et al., 1997, 2000). We now report the localization of the Toucan protein during early embryonic development. The Toucan protein is present only during the syncytial stages and is associated with the nuclear envelope and the cytoskeletal structures of the syncytial embryo. In anaphase A, Toucan is concentrated at the spindle poles near the minus end of microtubules. This microtubule association is very dynamic during the nuclear cell cycle. Mutant embryos lacking the Toucan protein are blocked in a metaphase-like state. They display abnormal and nonfunctional spindles, characterized by broad poles, detachment of the centrosomes, and failure of migration of the chromosomes. These results strongly suggest that Toucan represents a factor essential for the assembly and the function of the syncytial mitotic spindles.