Kinesin-like Calmodulin-binding Protein Research Articles

Multiplex gene editing reveals cucumber MILDEW RESISTANCE LOCUS O family roles in powdery mildew resistance

Abstract Powdery mildew (PM) is one of the most widespread and prevalent diseases that affects a wide range of crops. In cucumber (Cucumis sativus L.), previous forward genetic studies have identified MILDEW RESISTANCE LOCUS O 8 (CsMLO8) as necessary but alone insufficient for cucumber PM resistance (PMR) and suggested the involvement of other members of the CsMLO family. However, the function of other CsMLO family members in cucumber remains largely unknown. Here, we developed a highly efficient multiplex gene editing system in cucumber to generate a series of Csmlo mutants from all the 13 family members. Systematic analysis of these mutants revealed growth effects of these CsMLO family members on development and PMR. Importantly, we obtained the Csmlo1/8/11 triple mutant with complete resistance to PM. Transcriptome and proteome analysis of PM-resistant Csmlo mutants suggested that the kinesin-like calmodulin-binding protein (KCBP)-interacting Ca2+-binding protein (CsKIC), calmodulin-like protein 28 (CsCML28), and Ca2+-dependent protein kinase 11 (CsCPK11)-mediated calcium signaling pathway is involved in PMR. CsMLO8 interacted directly with CsKIC, and the simultaneous silencing of both genes resulted in a phenotype that resembled the silencing of CsKIC alone. Silencing CsCML28 and CsCPK11 increased susceptibility to PM, whereas overexpressing CsCPK11 through genetic transformation enhanced cucumber's PMR, demonstrating their positive regulatory roles in PMR. Given the importance of PMR for cucurbit crops, this research provides unprecedented insights into the function of the proteins encoded by the CsMLO gene family as well as the plant defense response to PM pathogen.

A legume kinesin controls vacuole morphogenesis for rhizobia endosymbiosis.

Symbioses between legumes and rhizobia require establishment of the plant-derived symbiosome membrane, which surrounds the rhizobia and accommodates the symbionts by providing an interface for nutrient and signal exchange. The host cytoskeleton and endomembrane trafficking systems play central roles in the formation of a functional symbiotic interface for rhizobia endosymbiosis; however, the underlying mechanisms remain largely unknown. Here we demonstrate that the nodulation-specific kinesin-like calmodulin-binding protein (nKCBP), a plant-specific microtubule-based kinesin motor, controls central vacuole morphogenesis in symbiotic cells in Medicago truncatula. Phylogenetic analysis further indicated that nKCBP duplication occurs solely in legumes of the clade that form symbiosomes. Knockout of nKCBP results in central vacuole deficiency, defective symbiosomes and abolished nitrogen fixation. nKCBP decorates linear particles along microtubules, and crosslinks microtubules with the actin cytoskeleton, to control central vacuole formation by modulating vacuolar vesicle fusion in symbiotic cells. Together, our findings reveal that rhizobia co-opted nKCBP to achieve symbiotic interface formation by regulating cytoskeletal assembly and central vacuole morphogenesis during nodule development.

Molecular chaperone Hsp90 protects KCBP from degradation by proteasome in Dunaliella salina cells.

Kinesin-like calmodulin-binding protein (KCBP) is a unique kinesin with half kinesin and half myosin, with kinesin motor domain at C-terminus and myosin tail homology region 4 (MyTH4) and band 4.1, ezrin, radixin, moesin (FERM) domains at N-terminus. The special structure endows KCBP multi-intracellular functions, including cell division, trichome morphogenesis in plants, and flagellar function in algae. However, little is known about the molecular mechanism underlying these functions. Here, we identified a molecular chaperone Hsp90 as a novel binding partner with KCBP in Dunaliella salina using a yeast two-hybrid screen. Further analysis showed that Hsp90 interacted with both the N-terminal and C-terminal of DsKCBP. Since Hsp90 was involved in the stability and proteolytic turnover of numerous proteins, whether Hsp90 regulated the degradation of DsKCBP was investigated. Our results showed that both Hsp90 and DsKCBP presented in the purified proteasome, and the interaction of DsKCBP-Hsp90 was inhibited upon Hsp90 inhibitor geldanamycin treatment. The level of DsKCBP proteins was diminished remarkably indicating that the disassociation of DsKCBP from Hsp90 accelerated the degradation of the former. Furthermore, immunofluorescence results showed that the localization of DsKCBP at basal body and flagella was disappeared by Hsp90 inhibition. The increased mRNA level of DsKCBP during flagellar assembly was not obvious by geldanamycin treatment. These data provided evidence that Hsp90 protected DsKCBP from degradation by proteasome and was involved in the role of DsKCBP in flagellar assembly.

Orchestration of microtubules and the actin cytoskeleton in trichome cell shape determination by a plant-unique kinesin.

Microtubules (MTs) and actin filaments (F-actin) function cooperatively to regulate plant cell morphogenesis. However, the mechanisms underlying the crosstalk between these two cytoskeletal systems, particularly in cell shape control, remain largely unknown. In this study, we show that introduction of the MyTH4-FERM tandem into KCBP (kinesin-like calmodulin-binding protein) during evolution conferred novel functions. The MyTH4 domain and the FERM domain in the N-terminal tail of KCBP physically bind to MTs and F-actin, respectively. During trichome morphogenesis, KCBP distributes in a specific cortical gradient and concentrates at the branching sites and the apexes of elongating branches, which lack MTs but have cortical F-actin. Further, live-cell imaging and genetic analyses revealed that KCBP acts as a hub integrating MTs and actin filaments to assemble the required cytoskeletal configuration for the unique, polarized diffuse growth pattern during trichome cell morphogenesis. Our findings provide significant insights into the mechanisms underlying cytoskeletal regulation of cell shape determination.

Clustering of a kinesin-14 motor enables processive retrograde microtubule-based transport in plants.

The molecular motors kinesin and dynein drive bidirectional motility along microtubules (MTs) in most eukaryotic cells. Land plants, however, are a notable exception, because they contain a large number of kinesins but lack cytoplasmic dynein, the foremost processive retrograde transporter. It remains unclear how plants achieve retrograde cargo transport without dynein. Here, we have analysed the motility of the six members of minus-end-directed kinesin-14 motors in the moss Physcomitrella patens and found that none are processive as native dimers. However, when artificially clustered into as little as dimer of dimers, the type-VI kinesin-14 (a homologue of Arabidopsis KCBP (kinesin-like calmodulin binding protein)) exhibited highly processive and fast motility (up to 0.6 μm s-1). Multiple kin14-VI dimers attached to liposomes also induced transport of this membrane cargo over several microns. Consistent with these results, in vivo observations of green fluorescent protein-tagged kin14-VI in moss cells revealed fluorescent punctae that moved processively towards the minus-ends of the cytoplasmic MTs. These data suggest that clustering of a kinesin-14 motor serves as a dynein-independent mechanism for retrograde transport in plants.

Arabidopsis KCBP interacts with AIR9 but stays in the cortical division zone throughout mitosis via its MyTH4-FERM domain.

The preprophase band of microtubules performs the crucial function of marking the plane of cell division. Although the preprophase band depolymerises at the onset of mitosis, the division plane is 'memorized' by a cortical division zone to which the phragmoplast is attracted during cytokinesis. Proteins have been discovered that are part of the molecular memory but little is known about how they contribute to phragmoplast guidance. Previously, we found that the microtubule-associated protein AIR9 is found in the cortical division zone at preprophase and returns during cell plate insertion but is absent from the cortex during the intervening mitosis. To identify new components of the preprophase memory, we searched for proteins that interact with AIR9. We detected the kinesin-like calmodulin-binding protein, KCBP, which can be visualized at the predicted cortical site throughout division. A truncation study of KCBP indicates that its MyTH4-FERM domain is required for linking the motor domain to the cortex. These results suggest a mechanism by which minus-end-directed KCBP helps guide the centrifugally expanding phragmoplast to the cortical division site.

PERK-KIPK-KCBP signalling negatively regulates root growth in Arabidopsis thaliana.

The Arabidopsis proline-rich, extensin-like receptor-like kinases (PERKs) are a small group of receptor-like kinases that are thought to act as sensors at the cell wall through their predicted proline-rich extracellular domains. In this study, we focused on the characterization of a subclade of three Arabidopsis predicted PERK genes, PERK8, -9, and -10, for which no functions were known. Yeast two-hybrid interaction studies were conducted with the PERK8,- 9, and -10 cytosolic kinase domains, and two members of the Arabidopsis AGC VIII kinase family were identified as interacting proteins: AGC1-9 and the closely related kinesin-like calmodulin-binding protein (KCBP)-interacting protein kinase (KIPK). As KIPK has been identified previously as an interactor of KCBP, these interactions were also examined further and confirmed in this study. Finally, T-DNA mutants for each gene were screened for altered phenotypes under different conditions, and from these screens, a role for the PERK, KIPK, and KCBP genes in negatively regulating root growth was uncovered.

Characterization of the microtubule-binding activity of kinesin-like calmodulin binding protein from Dunaliella salina

Although the C-terminal motor and the N-terminal myosin-like domains of KCBP in Dunaliella salina (DsKCBP) are implicated in interaction with the microtubules, its microtubule binding property has not been addressed. It has been shown that several calmodulin isoforms suppress the microtubule binding activity of KCBP, but whether the calmodulin-like protein (CLP) has this ability remains unknown. The results of our previous study showed that there are two microtubule binding sites in DsKCBP, motor domain at the C-terminus and MyTH4-FREM at the N-terminus. In the present study, MyTH4, without the companion of FERM, was identified as the minimal domain responsible for interaction with the microtubules in the N-terminal of DsKCBP. CLP interacted with the calmodulin-binding domain of DsKCBP in the presence of Ca2+, and inhibited the microtubule-binding activity of motor domain but not MyTH4 domain. Furthermore, MyTH4 domain in the N-terminus of DsKCBP was responsible for binding to the microtubules, and had 10-fold weaker affinity to the microtubules than the motor domain.

Plant Kinesin-Like Calmodulin Binding Protein Employs Its Regulatory Domain for Dimerization

Kinesin-like calmodulin binding protein (KCBP), a Kinesin-14 family motor protein, is involved in the structural organization of microtubules during mitosis and trichome morphogenesis in plants. The molecular mechanism of microtubule bundling by KCBP remains unknown. KCBP binding to microtubules is regulated by Ca2+-binding proteins that recognize its C-terminal regulatory domain. In this work, we have discovered a new function of the regulatory domain. We present a crystal structure of an Arabidopsis KCBP fragment showing that the C-terminal regulatory domain forms a dimerization interface for KCBP. This dimerization site is distinct from the dimerization interface within the N-terminal domain. Side chains of hydrophobic residues of the calmodulin binding helix of the regulatory domain form the C-terminal dimerization interface. Biochemical experiments show that another segment of the regulatory domain located beyond the dimerization interface, its negatively charged coil, is unexpectedly and absolutely required to stabilize the dimers. The strong microtubule bundling properties of KCBP are unaffected by deletion of the C-terminal regulatory domain. The slow minus-end directed motility of KCBP is also unchanged in vitro. Although the C-terminal domain is not essential for microtubule bundling, we suggest that KCBP may use its two independent dimerization interfaces to support different types of bundled microtubule structures in cells. Two distinct dimerization sites may provide a mechanism for microtubule rearrangement in response to Ca2+ signaling since Ca2+- binding proteins can disengage KCBP dimers dependent on its C-terminal dimerization interface.

Polarized cell growth, organelle motility, and cytoskeletal organization in conifer pollen tube tips are regulated by KCBP, the calmodulin-binding kinesin

Kinesin-like calmodulin-binding protein (KCBP), a member of the Kinesin 14 family, is a minus end directed C-terminal motor unique to plants and green algae. Its motor activity is negatively regulated by calcium/calmodulin binding, and its tail region contains a secondary microtubule-binding site. It has been identified but not functionally characterized in the conifer Picea abies. Conifer pollen tubes exhibit polarized growth as organelles move into the tip in an unusual fountain pattern directed by microfilaments but uniquely organized by microtubules. We demonstrate here that PaKCBP and calmodulin regulate elongation and motility. PaKCBP is a 140kDa protein immunolocalized to the elongating tip, coincident with microtubules. This localization is lost when microtubules are disrupted with oryzalin, which also reorganizes microfilaments into bundles. Colocalization of PaKCBP along microtubules is enhanced when microfilaments are disrupted with latrunculin B, which also disrupts the fine network of microtubules throughout the tip while preserving thicker microtubule bundles. Calmodulin inhibition by W-12 perfusion reversibly slows pollen tube elongation, alters organelle motility, promotes microfilament bundling, and microtubule bundling coincident with increased PaKCBP localization. The constitutive activation of PaKCBP by microinjection of an antibody that displaces calcium/calmodulin and activates microtubule bundling repositions vacuoles in the tip before rapidly stopping organelle streaming and pollen tube elongation. We propose that PaKCBP is one of the target proteins in conifer pollen modulated by calmodulin inhibition leading to microtubule bundling, which alters microtubule and microfilament organization, repositions vacuoles and slows organelle motility and pollen tube elongation.

The degradation of kinesin-like calmodulin binding protein of D. salina (DsKCBP) is mediated by the ubiquitin–proteasome system

Kinesin-like calmodulin binding protein (KCBP) is a member of kinesin-14 subfamily with unconventional domains distinct from other kinesins. This unique kinesin has the myosin tail homology 4 domain (MyTH4) and band4.1, ezrin, radixin and moesin domain (FERM) at the N-terminal which interact with several cytoskeleton proteins. Although KCBP is implicated in several microtubule-related cellular processes, studies on the KCBP of Dunaliella salina (DsKCBP) have not been reported. In this study, the roles of DsKCBP in flagella and cytoskeleton were investigated and the results showed that DsKCBP was present in flagella and upregulated during flagellar assembly indicting that it may be a flagellar kinesin and plays a role in flagellar assembly. A MyTH4-FERM domain of the DsKCBP was identified as a microtubule and actin interacting site. The interaction of DsKCBP with both microtubules and actin microfilaments suggests that this kinesin may be employed to coordinate these two cytoskeleton elements in algal cells. To gain more insights into the cellular function of the kinesin, DsKCBP-interacting proteins were examined using yeast two-hybrid screen. A 26S proteasome subunit Rpn8 was identified as a novel interacting partner of DsKCBP and the MyTH4-FERM domain was necessary for the interaction of DsKCBP with Rpn8. Furthermore, the DsKCBP was polyubiquitinated and up-regulated by proteasome inhibitor and degraded by ubiquitin-proteasome system indicating that proteasome is related to kinesin degradation.

Structural dynamics of the microtubule binding and regulatory elements in the kinesin-like calmodulin binding protein

Kinesins are molecular motors that power cell division and transport of various proteins and organelles. Their motor activity is driven by ATP hydrolysis and depends on interactions with microtubule tracks. Essential steps in kinesin movement rely on controlled alternate binding to and detaching from the microtubules. The conformational changes in the kinesin motors induced by nucleotide and microtubule binding are coordinated by structural elements within their motor domains. Loop L11 of the kinesin motor domain interacts with the microtubule and is implicated in both microtubule binding and sensing nucleotide bound to the active site of kinesin. Consistent with its proposed role as a microtubule sensor, loop L11 is rarely seen in crystal structures of unattached kinesins. Here, we report four structures of a regulated plant kinesin, the kinesin-like calmodulin binding protein (KCBP), determined by X-ray crystallography. Although all structures reveal the kinesin motor in the ATP-like conformation, its loop L11 is observed in different conformational states, both ordered and disordered. When structured, loop L11 adds three additional helical turns to the N-terminal part of the following helix alpha4. Although interactions with protein neighbors in the crystal support the ordering of loop L11, its observed conformation suggests the conformation for loop L11 in the microtubule-bound kinesin. Variations in the positions of other features of these kinesins were observed. A critical regulatory element of this kinesin, the calmodulin binding helix positioned at the C-terminus of the motor domain, is thought to confer negative regulation of KCBP. Calmodulin binds to this helix and inserts itself between the motor and the microtubule. Comparison of five independent structures of KCBP shows that the positioning of the calmodulin binding helix is not decided by crystal packing forces but is determined by the conformational state of the motor. The observed variations in the position of the calmodulin binding helix fit the regulatory mechanism previously proposed for this kinesin motor.

Microtubule-associated proteins in higher plants

A variety of microtubule-associated proteins (MAPs) have been reported in higher plants. Microtubule (MT) polymerization starts from the gamma-tubulin complex (gammaTuC), a component of the MT nucleation site. MAP200/MOR1 and katanin regulate the length of the MT by promoting the dynamic instability of MTs and cutting MTs, respectively. In construction of different MT structures, MTs are bundled or are associated with other components--actin filaments, the plasma membrane, and organelles. The MAP65 family and some of kinesin family are important in bundling MTs. MT plus-end-tracking proteins (+TIPs) including end-binding protein 1 (EB1), Arabidopsis thaliana kinesin 5 (ATK5), and SPIRAL 1 (SPR1) localize to the plus end of MTs. It has been suggested that +TIPs are involved in binding of MT to other structures. Phospholipase D (PLD) is a possible candidate responsible for binding of MTs to the plasma membrane. Many candidates have been reported as actin-binding MAPs, for example calponin-homology domain (KCH) family kinesin, kinesin-like calmodulin-binding protein (KCBP), and MAP190. RNA distribution and translation depends on MT structures, and several RNA-related MAPs have been reported. This article gives an overview of predicted roles of these MAPs in higher plants.

A kinesin-like calmodulin-binding protein inChlamydomonas: evidence for a role in cell division and flagellar functions

Kinesin-like calmodulin-binding protein, KCBP, is a novel member of the C-kinesin superfamily first discovered in flowering plants. This minus-end-directed kinesin exhibits Ca(2+)-calmodulin-sensitive motor activity in vitro and has been implicated in trichome morphogenesis and cell division. A homologue of KCBP is also found in the unicellular, biflagellate green alga Chlamydomonas reinhardtii (CrKCBP). Unlike plant cells, Chlamydomonas cells do not form trichomes and do not assemble a phragmoplast before cell division. To test whether CrKCBP is involved in additional microtubule-based processes not observed in plants, we generated antibodies against the putative calmodulin-binding domain and used these antibodies in biochemical and localization studies. In interphase cells CrKCBP primarily localizes near the base of the flagella, although surprisingly, a small fraction also localizes along the length of the flagella. CrKCBP is bound to isolated axonemes in an ATP-dependent fashion and is not a component of the dynein arms, radial spokes or central apparatus. During mitosis, CrKCBP appears concentrated at the centrosomes during prophase and metaphase. However, during telophase and cytokinesis CrKCBP co-localizes with the microtubules associated with the phycoplast. These studies implicate CrKCBP in flagellar functions as well as cell division.

Genetic engineering of a Ca 2+ dependent chemical switch into the linear biomotor kinesin

Kinesin is a linear motor protein driven by energy released by ATP hydrolysis. In the present work, we genetically installed an M13 peptide sequence into Loop 12 of kinesin, which is one of the major microtubule binding regions of the protein. Because the M13 sequence has high affinity for Ca 2+-calmodulin, the association of the engineered kinesin with microtubules showed a steep Ca 2+-dependency in ATPase activity at Ca 2+ concentrations of pCa 6.5–8. The calmodulin-binding domain of plant kinesin-like calmodulin-binding protein is also known to confer Ca 2+-calmodulin regulation to kinesins. Unlike this plant kinesin, however, our novel engineered kinesin achieves this regulation while maintaining the interaction between kinesin and microtubules. The engineered kinesin is switched on/off reversibly by an external signal (i.e., Ca 2+-calmodulin) and, thus, can be used as a model system for a bio/nano-actuator.

Origin and evolution of Kinesin-like calmodulin-binding protein.

Kinesin-like calmodulin-binding protein (KCBP), a member of the Kinesin-14 family, is a C-terminal microtubule motor with three unique domains including a myosin tail homology region 4 (MyTH4), a talin-like domain, and a calmodulin-binding domain (CBD). The MyTH4 and talin-like domains (found in some myosins) are not found in other reported kinesins. A calmodulin-binding kinesin called kinesin-C (SpKinC) isolated from sea urchin (Strongylocentrotus purpuratus) is the only reported kinesin with a CBD. Analysis of the completed genomes of Homo sapiens, Saccharomyces cerevisiae, Caenorhabditis elegans, Drosophila melanogaster, and a red alga (Cyanidioschyzon merolae 10D) did not reveal the presence of a KCBP. This prompted us to look at the origin of KCBP and its relationship to SpKinC. To address this, we isolated KCBP from a gymnosperm, Picea abies, and a green alga, Stichococcus bacillaris. In addition, database searches resulted in identification of KCBP in another green alga, Chlamydomonas reinhardtii, and several flowering plants. Gene tree analysis revealed that the motor domain of KCBPs belongs to a clade within the Kinesin-14 (C-terminal motors) family. Only land plants and green algae have a kinesin with the MyTH4 and talin-like domains of KCBP. Further, our analysis indicates that KCBP is highly conserved in green algae and land plants. SpKinC from sea urchin, which has the motor domain similar to KCBP and contains a CBD, lacks the MyTH4 and talin-like regions. Our analysis indicates that the KCBPs, SpKinC, and a subset of the kinesin-like proteins are all more closely related to one another than they are to any other kinesins, but that either KCBP gained the MyTH4 and talin-like domains or SpKinC lost them.

Crystal Structure of Kinesin Regulated by Ca2+-Calmodulin

Kinesins orchestrate cell division by controlling placement of chromosomes. Kinesins must be precisely regulated or else cell division fails. Calcium, a universal second messenger in eukaryotes, and calmodulin regulate some kinesins by causing the motor to dissociate from its biological track, the microtubule. Our focus was the mechanism of calcium regulation of kinesin at atomic resolution. Here we report the crystal structure of kinesin-like calmodulin-binding protein (KCBP) from potato, which was resolved to 2.3 A. The structure reveals three subdomains of the regulatory machinery located at the C terminus extension of the kinesin motor. Calmodulin that is activated by Ca2+ ions binds to an alpha-helix positioned on the microtubule-binding face of kinesin. A negatively charged segment following this helix competes with microtubules. A mimic of the conventional kinesin neck, connecting the calmodulin-binding helix to the KCBP motor core, links the regulatory machine to the kinesin catalytic cycle. Together with biochemical data, the crystal structure suggests that Ca(2+)-calmodulin inhibits the binding of KCBP to microtubules by blocking the microtubule-binding sites on KCBP.

KIC, a novel Ca2+ binding protein with one EF-hand motif, interacts with a microtubule motor protein and regulates trichome morphogenesis.

Kinesin-like calmodulin binding protein (KCBP) is a microtubule motor protein involved in the regulation of cell division and trichome morphogenesis. Genetic studies have shown that KCBP is likely to interact with several other proteins. To identify KCBP-interacting proteins, we used the C-terminal region of KCBP in a yeast two-hybrid screen. This screening resulted in the isolation of a novel KCBP-interacting Ca2+ binding protein (KIC). KIC, with its single EF-hand motif, bound Ca2+ at a physiological concentration. Coprecipitation with bacterially expressed protein and native KCBP, gel-mobility shift studies, and ATPase assays with the KCBP motor confirmed that KIC interacts with KCBP in a Ca2+-dependent manner. Interestingly, although both Ca2+-KIC and Ca2+-calmodulin were able to interact with KCBP and inhibit its microtubule binding activity, the concentration of Ca2+ required to inhibit the microtubule-stimulated ATPase activity of KCBP by KIC was threefold less than that required for calmodulin. Two KIC-related Ca2+ binding proteins and a centrin from Arabidopsis, which contain one and four EF-hand motifs, respectively, bound Ca2+ but did not affect microtubule binding and microtubule-stimulated ATPase activities of KCBP, indicating the specificity of Ca2+ sensors in regulating their targets. Overexpression of KIC in Arabidopsis resulted in trichomes with reduced branch number resembling the zwichel/kcbp phenotype. These results suggest that KIC modulates the activity of KCBP in response to changes in cytosolic Ca2+ and regulates trichome morphogenesis.

Isolation and analyses of genes preferentially expressed during early cotton fiber development by subtractive PCR and cDNA array.

Cotton fibers are differentiated epidermal cells originating from the outer integuments of the ovule. To identify genes involved in cotton fiber elongation, we performed subtractive PCR using cDNA prepared from 10 days post anthesis (d.p.a.) wild-type cotton fiber as tester and cDNA from a fuzzless-lintless (fl) mutant as driver. We recovered 280 independent cDNA fragments including most of the previously published cotton fiber-related genes. cDNA macroarrays showed that 172 genes were significantly up-regulated in elongating cotton fibers as confirmed by in situ hybridization in representative cases. Twenty-nine cDNAs, including a putative vacuolar (H+)-ATPase catalytic subunit, a kinesin-like calmodulin binding protein, several arabinogalactan proteins and key enzymes involved in long chain fatty acid biosynthesis, accumulated to greater than 50-fold in 10 d.p.a. fiber cells when compared to that in 0 d.p.a. ovules. Various upstream pathways, such as auxin signal transduction, the MAPK pathway and profilin- and expansin-induced cell wall loosening, were also activated during the fast fiber elongation period. This report constitutes the first systematic analysis of genes involved in cotton fiber development. Our results suggest that a concerted mechanism involving multiple cellular pathways is responsible for cotton fiber elongation.

The cotton kinesin-like calmodulin-binding protein associates with cortical microtubules in cotton fibers.

Microtubules in interphase plant cells form a cortical array, which is critical for plant cell morphogenesis. Genetic studies imply that the minus end-directed microtubule motor kinesin-like calmodulin-binding protein (KCBP) plays a role in trichome morphogenesis in Arabidopsis. However, it was not clear whether this motor interacted with interphase microtubules. In cotton (Gossypium hirsutum) fibers, cortical microtubules undergo dramatic reorganization during fiber development. In this study, cDNA clones of the cotton KCBP homolog GhKCBP were isolated from a cotton fiber-specific cDNA library. During cotton fiber development from 10 to 21 DPA, the GhKCBP protein level gradually decreases. By immunofluorescence, GhKCBP was detected as puncta along cortical microtubules in fiber cells of different developmental stages. Thus our results provide evidence that GhKCBP plays a role in interphase cell growth likely by interacting with cortical microtubules. In contrast to fibers, in dividing cells of cotton, GhKCBP localized to the nucleus, the microtubule preprophase band, mitotic spindle, and the phragmoplast. Therefore KCBP likely exerts multiple roles in cell division and cell growth in flowering plants.