驱动蛋白Kin11调控嗜热四膜虫生殖系小核的减数分裂

驱动蛋白(kinesin)是分子马达蛋白质超家族成员,主要参与囊泡与细胞器的运输、纺锤体组装、有丝分裂和减数分裂等过程。在减数分裂期,不同驱动蛋白发挥功能的调控机制并不十分清楚。嗜热四膜虫(Tetrahymena thermophila)中含有14个驱动蛋白家族成员。其中,kinesin-6家族的唯一成员Kin11(TTHERM_00637750),在营养生长期低表达,饥饿期不表达,有性生殖期表达上调。Kin11编码1608个氨基酸,包含1个N端保守的马达蛋白结构域,C端卷曲螺旋(coiled-coil)结构域,并在N端和C端分别含有核定位信号NLS1和NLS2。Kin11在营养生长期和有性生殖期,定位在有丝分裂和减数分裂的小核和纺锤体上,并在有性生殖后期alignment阶段定位于小核上。Kin11与微管蛋白共定位于有丝分裂和减数分裂的纺锤体上。将Kin11的N端含有NLS1的1~400位氨基酸序列截短后,截断突变体定位在有性生殖减数分裂期的小核和纺锤体上。而将其C端含有NLS2的1008~1608位氨基酸残基截短后,截断突变体只能定位在有丝分裂和减数分裂后期的小核及有丝分裂的纺锤体上。敲除KIN11导致减数分裂过程中的纺锤体结构发生异常变化,小核染色体不均等分离与丢失,有性生殖发育停滞。结果表明,嗜热四膜虫驱动蛋白Kin11通过影响纺锤体结构,参与调控四膜虫生殖系小核在减数分裂过程中的正常分离。     

Functional modulation of IFT kinesins extends the sensory repertoire of ciliated neurons in Caenorhabditis elegans

The diversity of sensory cilia on Caenorhabditis elegans neurons allows the animal to detect a variety of sensory stimuli. Sensory cilia are assembled by intraflagellar transport (IFT) kinesins, which transport ciliary precursors, bound to IFT particles, along the ciliary axoneme for incorporation into ciliary structures. Using fluorescence microscopy of living animals and serial section electron microscopy of high pressure-frozen, freeze-substituted IFT motor mutants, we found that two IFT kinesins, homodimeric OSM-3 kinesin and heterotrimeric kinesin II, function in a partially redundant manner to build full-length amphid channel cilia but are completely redundant for building full-length amphid wing (AWC) cilia. This difference reflects cilia-specific differences in OSM-3 activity, which serves to extend distal singlets in channel cilia but not in AWC cilia, which lack such singlets. Moreover, AWC-specific chemotaxis assays reveal novel sensory functions for kinesin II in these wing cilia. We propose that kinesin II is a "canonical" IFT motor, whereas OSM-3 is an "accessory" IFT motor, and that subtle changes in the deployment or actions of these IFT kinesins can contribute to differences in cilia morphology, cilia function, and sensory perception.     

Characterization of the KLP68D kinesin-like protein in Drosophila: possible roles in axonal transport

This paper describes the molecular and biochemical properties of KLP68D, a new kinesin-like motor protein in Drosophila melanogaster. Sequence analysis of a full-length cDNA encoding KLP68D demonstrates that this protein has a domain that shares significant sequence identity with the entire 340-amin acid kinesin heavy chain motor domain. Sequences extending beyond the motor domain predict a region of alpha-helical coiled-coil followed by a globular "tail" region; there is significant sequence similarity between the alpha-helical coiled- coil region of the KLP68D protein and similar regions of the KIF3 protein of mouse and the KRP85 protein of sea urchin. This finding suggests that all three proteins may be members of the same family, and that they all perform related functions. KLP68D protein produced in Escherichia coli is, like kinesin itself, a plus-end directed microtubule motor. In situ hybridization analysis of KLP68D RNA in Drosophila embryos indicates that the KLP68D gene is expressed primarily in the central nervous system and in a subset of the peripheral nervous system during embryogenesis. Thus, KLP68D may be used for anterograde axonal transport and could conceivably move cargoes in fly neurons different than those moved by kinesin heavy chain or other plus-end directed motors.     

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

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

Nucleotide-dependent interaction of the N-terminal domain of MukB with microtubules

The MukB protein from Escherichia coli has a domain structure that is reminiscent of the eukaryotic motor proteins kinesin and myosin: N-terminal globular domains, a region of coiled-coil, and a specialised C-terminal domain. Sequence alignment of the N-terminal domain of MukB with the kinesin motor domain indicated an approximately 22% sequence identity. These observations raised the possibility that MukB might be a prokaryotic motor protein and, due to the sequence homology shared with kinesin, might bind to microtubules (Mts). We found that a construct encoding the first 342 residues of MukB (Muk342) binds specifically to Mts and shares a number of properties with the motor domain of kinesin. Visualisation of the Muk342 decorated Mt complexes using negative stain electron microscopy indicated that the Muk342 smoothly decorates the outside of Mts. Biochemical data demonstrate that Muk342 decorates Mts with a binding stoichiometry of one Muk342 monomer per tubulin monomer. These findings strongly suggest that MukB has a role in force generation and that it is a prokaryotic homologue of kinesin and myosin.     

Kinesin family in murine central nervous system

In neuronal axons, various kinds of membranous components are transported along microtubules bidirectionally. However, only two kinds of mechanochemical motor proteins, kinesin and brain dynein, had been identified as transporters of membranous organelles in mammalian neurons. Recently, a series of genes that encode proteins closely related to kinesin heavy chain were identified in several organisms including Schizosaccharomyces pombe, Aspergillus niddulans, Saccharomyces cerevisiae, Caenorhabditus elegans, and Drosophila. Most of these members of the kinesin family are implicated in mechanisms of mitosis or meiosis. To address the mechanism of intracellular organelle transport at a molecular level, we have cloned and characterized five different members (KIF1-5), that encode the microtubule-associated motor domain homologous to kinesin heavy chain, in murine brain tissue. Homology analysis of amino acid sequence indicated that KIF1 and KIF5 are murine counterparts of unc104 and kinesin heavy chain, respectively, while KIF2, KIF3, and KIF4 are as yet unidentified new species. Complete amino acid sequence of KIF3 revealed that KIF3 consists of NH2-terminal motor domain, central alpha-helical rod domain, and COOH-terminal globular domain. Complete amino acid sequence of KIF2 revealed that KIF2 consists of NH2-terminal globular domain, central motor domain, and COOH-terminal alpha-helical rod domain. This is the first identification of the kinesin-related protein which has its motor domain at the central part in its primary structure. Northern blot analysis revealed that KIF1, KIF3, and KIF5 are expressed almost exclusively in murine brain, whereas KIF2 and KIF4 are expressed in brain as well as in other tissues. All these members of the kinesin family are expressed in the same type of neurons, and thus each one of them may transport its specific organelle in the murine central nervous system.     

Microtubule-dependent control of cell shape and pseudopodial activity is inhibited by the antibody to kinesin motor domain

One of the major functions of cytoplasmic microtubules is their involvement in maintenance of asymmetric cell shape. Microtubules were considered to perform this function working as rigid structural elements. At the same time, microtubules play a critical role in intracellular organelle transport, and this fact raises the possibility that the involvement of microtubules in maintenance of cell shape may be mediated by directed transport of certain cellular components to a limited area of the cell surface (e.g., to the leading edge) rather than by their functioning as a mechanical support. To test this hypothesis we microinjected cultured human fibroblasts with the antibody (called HD antibody) raised against kinesin motor domain highly conserved among the different members of kinesin superfamily. As was shown before this antibody inhibits kinesin-dependent microtubule gliding in vitro and interferes with a number of microtubule-dependent transport processes in living cells. Preimmune IgG fraction was used for control experiments. Injections of fibroblasts with HD antibody but not with preimmune IgG significantly reduced their asymmetry, resulting in loss of long processes and elongated cell shape. In addition, antibody injection suppressed pseudopodial activity at the leading edge of fibroblasts moving into an experimentally made wound. Analysis of membrane organelle distribution showed that kinesin antibody induced clustering of mitochondria in perinuclear region and their withdrawal from peripheral parts of the cytoplasm. HD antibody does not affect either density or distribution of cytoplasmic microtubules. The results of our experiments show that many changes of phenotype induced in cells by microtubule-depolymerizing agents can be mimicked by the inhibition of motor proteins, and therefore microtubule functions in maintaining of the cell shape and polarity are mediated by motor proteins rather than by being provided by rigidity of tubulin polymer itself.     

A novel mouse kinesin of the UNC-104/KIF1 subfamily encoded by the Kif1b gene

Kinesin and kinesin-related proteins are microtubule-dependent motor proteins that transport organelles. We have cloned and sequenced a full-length 9924 bp mouse cDNA for a new kinesin of the UNC-104/KIF1 subfamily. Northern blot analysis of mouse RNAs detected high levels of a 10 kb mRNA in brain and eye, but lower levels in other tissues. Human RNA dot-blot analysis detected this mRNA in all tissues examined, although at different levels. The overall structure of the new kinesin (predicted size 204 kDa) was most similar to mouse KIF1A; however, 2.1 kb of the 5' portion of the cDNA were identical to the published sequence for KIF1B (Nangaku, M., Sato-Yoshitake, R., Okada, Y., Noda, Y., Takemura, R., Yamazaki, H., Hirokawa, N., 1994. KIF1B, a novel microtubule plus end-directed monomeric motor protein for transport of mitochondria. Cell 79, 1209-1220). We localized the Kif1b gene to the distal end of mouse Chromosome 4 by haplotype analysis of an interspecific backcross from The Jackson Laboratory. We had previously mapped the gene for the novel kinesin to the same location (Gong, T.-W.L., Burmeister, M., Lomax, M.I., 1996b. The novel gene D4Mille maps to mouse Chromosome 4 and human Chromosome 1p36. Mamm. Genome 7, 790-791). We conclude, therefore, that the Kif1b gene generates two major kinesin isoforms by alternative splicing. The shorter 7.8 kb mRNA encodes a 130 kDa kinesin, KIF1Bp130, whereas the 10 kb mRNA encodes a 204 kDa kinesin, KIF1Bp204. In addition, alternative splicing of two exons in the conserved region adjacent to the motor domain generates four different isoforms of each kinesin, leading to eight kinesin isoforms derived from the Kif1b gene.     

Molecular basis of dystrobrevin interaction with kinesin heavy chain: structural determinants of their binding

Dystrobrevins are a family of widely expressed dystrophin-associated proteins that comprises alpha and beta isoforms and displays significant sequence homology with several protein-binding domains of the dystrophin C-terminal region. The complex distribution of the multiple dystrobrevin isoforms suggests that the variability of their composition may be important in mediating their function. We have recently identified kinesin as a novel dystrobrevin-interacting protein and localized the dystrobrevin-binding site on the cargo-binding domain of neuronal kinesin heavy chain (Kif5A). In the present study, we assessed the kinetics of the dystrobrevin-Kif5A interaction by quantitative pull-down assay and surface plasmon resonance (SPR) analysis and found that beta-dystrobrevin binds to kinesin with high affinity (K(D) approximately 40 nM). Comparison of the sensorgrams obtained with alpha and beta-dystrobrevin at the same concentration of analyte showed a lower affinity of alpha compared to that of beta-dystrobrevin, despite their functional domain homology and about 70% sequence identity. Analysis of the contribution of single dystrobrevin domains to the interaction revealed that the deletion of either the ZZ domain or the coiled-coil region decreased the kinetics of the interaction, suggesting that the tertiary structure of dystrobrevin may play a role in regulating the interaction of dystrobrevin with kinesin. In order to understand if structural changes induced by post-translational modifications could affect dystrobrevin affinity for kinesin, we phosphorylated beta-dystrobrevin in vitro and found that it showed reduced binding capacity towards kinesin. The interaction between the adaptor/scaffolding protein dystrobrevin and the motor protein kinesin may play a role in the transport and targeting of components of the dystrophin-associated protein complex to specific sites in the cell, with the differences in the binding properties of dystrobrevin isoforms reflecting their functional diversity within the same cell type. Phosphorylation events could have a regulatory role in this context.     

Kinesin- and Myosin-driven Steps of Vesicle Recruitment for Ca2+-regulated Exocytosis

Kinesin and myosin have been proposed to transport intracellular organelles and vesicles to the cell periphery in several cell systems. However, there has been little direct observation of the role of these motor proteins in the delivery of vesicles during regulated exocytosis in intact cells. Using a confocal microscope, we triggered local bursts of Ca²⁺-regulated exocytosis by wounding the cell membrane and visualized the resulting individual exocytotic events in real time. Different temporal phases of the exocytosis burst were distinguished by their sensitivities to reagents targeting different motor proteins. The function blocking antikinesin antibody SUK4 as well as the stalk-tail fragment of kinesin heavy chain specifically inhibited a slow phase, while butanedione monoxime, a myosin ATPase inhibitor, inhibited both the slow and fast phases. The blockage of Ca²⁺/calmodulin-dependent protein kinase II with autoinhibitory peptide also inhibited the slow and fast phases, consistent with disruption of a myosin-actin– dependent step of vesicle recruitment. Membrane resealing after wounding was also inhibited by these reagents. Our direct observations provide evidence that in intact living cells, kinesin and myosin motors may mediate two sequential transport steps that recruit vesicles to the release sites of Ca²⁺-regulated exocytosis, although the identity of the responsible myosin isoform is not yet known. They also indicate the existence of three semistable vesicular pools along this regulated membrane trafficking pathway. In addition, our results provide in vivo evidence for the cargo-binding function of the kinesin heavy chain tail domain.     

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 Spitzenk?rper. 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.     

The Translocation Selectivity of the Kinesins that Mediate Neuronal Organelle Transport

Polarized kinesin‐driven transport is crucial for development and maintenance of neuronal polarity. Kinesins are thought to recognize biochemical differences between axonal and dendritic microtubules in order to deliver their cargoes to the appropriate domain. To identify kinesins that mediate polarized transport, we prepared constitutively active versions of all the kinesins implicated in vesicle transport and expressed them in cultured hippocampal neurons. Seven kinesins translocated preferentially to axons and five translocated into both axons and dendrites. None translocated selectively to dendrites. Highly homologous members of the same subfamily displayed distinctly different translocation preferences and were differentially regulated during development. By expressing chimeric kinesins, we identified two microtubule‐binding elements within the motor domain that are important for selective translocation. We also discovered elements in the dimerization domain of kinesin‐2 motors that contribute to their selective translocation. These observations indicate that selective interactions between kinesin motor domains and microtubules can account for polarized transport to the axon, but not for selective dendritic transport.     

Complete sequence of a gene encoding KAR3-related kinesin-like protein in Candida albicans

In contrast to Saccharomyces cerevisiae, little is known about the kinesin-like protein (KLP) in Candida albicans. The motor domain of kinesin, or KLP, contains a subregion, which is well conserved from yeast to humans. A similarity search, with the murine ubiquitous kinesin heavy chain region as a query, revealed 6 contigs that contain putative KLPs in the genome of C. albicans. Of these, the length of an open reading (ORF) of 375 amino acids, temporarily designated CaKAR3, was noticeably short compared with the closely related S. cerevisiae KAR3 (ScKAR3) of 729 amino acids. This finding prompted us to isolate a lambda genomic clone containing the complete CaKAR3 ORF, and here the complete sequence of CaKAR3 is reported. CaKAR3 is a C-terminus motor protein, of 687 amino acids, encoded by a non-disrupting gene. When compared with ScKAR3, the amino terminal region of 112 amino acids was unique, with the middle part of the 306 amino acids exhibiting 25% identity and 44% similarity, while the remaining C-terminal motor domain exhibited 64% identity and 78% similarity, and have been submitted to GeneBank under the accession number AY182242.     

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.     

Large conformational changes in a kinesin motor catalyzed by interaction with microtubules

Kinesin motor proteins release nucleotide upon interaction with microtubules (MTs), then bind and hydrolyze ATP to move along the MT. Although crystal structures of kinesin motors bound to nucleotides have been solved, nucleotide-free structures have not. Here, using cryomicroscopy and three-dimensional (3D) reconstruction, we report the structure of MTs decorated with a Kinesin-14 motor, Kar3, in the nucleotide-free state, as well as with ADP and AMPPNP, with resolution sufficient to show alpha helices. We find large structural changes in the empty motor, including melting of the switch II helix alpha4, closure of the nucleotide binding pocket, and changes in the central beta sheet reminiscent of those reported for nucleotide-free myosin crystal structures. We propose that the switch II region of the motor controls docking of the Kar3 neck by conformational changes in the central beta sheet, similar to myosin, rather than by rotation of the motor domain, as proposed for the Kif1A kinesin motor.     

Kinesin proteins: A phylum of motors for microtubule-based motility

The cellular processes of transport, division and, possibly, early development all involve microtubule-based motors. Recent work shows that, unexpectedly, many of these cellular functions are carried out by different types of kinesin and kinesin-related motor proteins. The kinesin proteins are a large and rapidly growing family of microtubule-motor proteins that share a 340-amino-acid motor domain. Phylogenetic analysis of the conserved motor domains groups the kinesin proteins into a number of subfamilies, the members of which exhibit a common molecular organization and related functions. The kinesin proteins that belong to different subfamilies differ in their rates and polarity of movement along microtubules, and probably in the particles/organelles that they transport. The kinesins arose early in eukaryotic evolution and gene duplication has allowed functional specialization to occur, resulting in a surprisingly large number of different classes of these proteins adapted for intracellular transport of vesicles and organelles, and for assembly and force generation in the meiotic and mitotic spindles.     

Unconventional motoring: an overview of the Kin C and Kin I kinesins

All kinesins share a conserved core motor domain implying a common mechanism for generating force from ATP hydrolysis. How is it then that kinesins exhibit such divergent activities: motility, microtubule cross‐linking and microtubule depolymerization? Although conventional motile kinesins have served as the paradigm for understanding kinesin function, the unconventional kinesins exploit variations on the motile theme to perform unexpected tasks. This review summarizes the biological functions and examines the possible molecular mechanisms of Kin C and Kin I unconventional kinesins. We also discuss the possible differences between the microtubule destabilization models proposed for Kar3 and Kin I kinesins .     

Crystal structure of the mitotic spindle kinesin Eg5 reveals a novel conformation of the neck-linker

Success of mitosis depends upon the coordinated and regulated activity of many cellular factors, including kinesin motor proteins, which are required for the assembly and function of the mitotic spindle. Eg5 is a kinesin implicated in the formation of the bipolar spindle and its movement prior to and during anaphase. We have determined the crystal structure of the Eg5 motor domain with ADP-Mg bound. This structure revealed a new intramolecular binding site of the neck-linker. In other kinesins, the neck-linker has been shown to be a critical mechanical element for force generation. The neck-linker of conventional kinesin is believed to undergo an ordered-to-disordered transition as it translocates along a microtubule. The structure of Eg5 showed an ordered neck-linker conformation in a position never observed previously. The docking of the neck-linker relies upon residues conserved only in the Eg5 subfamily of kinesin motors. Based on this new information, we suggest that the neck-linker of Eg5 may undergo an ordered-to-ordered transition during force production. This ratchet-like mechanism is consistent with the biological activity of Eg5.     

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 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.