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

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.

Evolution of cytokinesis-related protein localization during the emergence of multicellularity in volvocine green algae

BACKGROUND

The volvocine lineage, containing unicellular Chlamydomonas reinhardtii and differentiated multicellular Volvox carteri, is a powerful model for comparative studies aiming at understanding emergence of multicellularity. Tetrabaena socialis is the simplest multicellular volvocine alga and belongs to the family Tetrabaenaceae that is sister to more complex multicellular volvocine families, Goniaceae and Volvocaceae. Thus, T. socialis is a key species to elucidate the initial steps in the evolution of multicellularity. In the asexual life cycle of C. reinhardtii and multicellular volvocine species, reproductive cells form daughter cells/colonies by multiple fission. In embryogenesis of the multicellular species, daughter protoplasts are connected to one another by cytoplasmic bridges formed by incomplete cytokinesis during multiple fission. These bridges are important for arranging the daughter protoplasts in appropriate positions such that species-specific integrated multicellular individuals are shaped. Detailed comparative studies of cytokinesis between unicellular and simple multicellular volvocine species will help to elucidate the emergence of multicellularity from the unicellular ancestor. However, the cytokinesis-related genes between closely related unicellular and multicellular species have not been subjected to a comparative analysis.

RESULTS

Here we focused on dynamin-related protein 1 (DRP1), which is known for its role in cytokinesis in land plants. Immunofluorescence microscopy using an antibody against T. socialis DRP1 revealed that volvocine DRP1 was localized to division planes during cytokinesis in unicellular C. reinhardtii and two simple multicellular volvocine species T. socialis and Gonium pectorale. DRP1 signals were mainly observed in the newly formed division planes of unicellular C. reinhardtii during multiple fission, whereas in multicellular T. socialis and G. pectorale, DRP1 signals were observed in all division planes during embryogenesis.

CONCLUSIONS

These results indicate that the molecular mechanisms of cytokinesis may be different in unicellular and multicellular volvocine algae. The localization of DRP1 during multiple fission might have been modified in the common ancestor of multicellular volvocine algae. This modification may have been essential for the re-orientation of cells and shaping colonies during the emergence of multicellularity in this lineage.

H+- and Na+- elicited rapid changes of the microtubule cytoskeleton in the biflagellated green alga Chlamydomonas

Although microtubules are known for dynamic instability, the dynamicity is considered to be tightly controlled to support a variety of cellular processes. Yet diverse evidence suggests that this is not applicable to Chlamydomonas, a biflagellate fresh water green alga, but intense autofluorescence from photosynthesis pigments has hindered the investigation. By expressing a bright fluorescent reporter protein at the endogenous level, we demonstrate in real time discreet sweeping changes in algal microtubules elicited by rises of intracellular H⁺ and Na⁺. These results from this model organism with characteristics of animal and plant cells provide novel explanations regarding how pH may drive cellular processes; how plants may respond to, and perhaps sense stresses; and how organisms with a similar sensitive cytoskeleton may be susceptible to environmental changes.

Salinity-Induced Palmella Formation Mechanism in Halotolerant Algae Dunaliella salina Revealed by Quantitative Proteomics and Phosphoproteomics

Palmella stage is critical for some unicellular algae to survive in extreme environments. The halotolerant algae Dunaliella salina is a good single-cell model for studying plant adaptation to high salinity. To investigate the molecular adaptation mechanism in salinity shock-induced palmella formation, we performed a comprehensive physiological, proteomics and phosphoproteomics study upon palmella formation of D. salina using dimethyl labeling and Ti⁴⁺-immobilized metal ion affinity chromatography (IMAC) proteomic approaches. We found that 151 salinity-responsive proteins and 35 salinity-responsive phosphoproteins were involved in multiple signaling and metabolic pathways upon palmella formation. Taken together with photosynthetic parameters and enzyme activity analyses, the patterns of protein accumulation and phosphorylation level exhibited the mechanisms upon palmella formation, including dynamics of cytoskeleton and cell membrane curvature, accumulation and transport of exopolysaccharides, photosynthesis and energy supplying (i.e., photosystem II stability and activity, cyclic electron transport, and C4 pathway), nuclear/chloroplastic gene expression regulation and protein processing, reactive oxygen species homeostasis, and salt signaling transduction. The salinity-responsive protein-protein interaction (PPI) networks implied that signaling and protein synthesis and fate are crucial for modulation of these processes. Importantly, the 3D structure of phosphoprotein clearly indicated that the phosphorylation sites of eight proteins were localized in the region of function domain.

Kinesin-2 and kinesin-9 have atypical functions during ciliogenesis in the male gametophyte of Marsilea vestita

Background

Spermatogenesis in the semi-aquatic fern, Marsilea vestita, is a rapid, synchronous process that is initiated when dry microspores are placed in water. Development is post-transcriptionally driven and can be divided into two phases. The first phase consists of nine mitotic division cycles that produce 7 sterile cells and 32 spermatids. During the second phase, each spermatid differentiates into a corkscrew-shaped motile spermatozoid with ~140 cilia.

Results

Analysis of the transcriptome from the male gametophyte of Marsilea revealed that one kinesin-2 (MvKinesin-2) and two kinesin-9 s (MvKinesin-9A and MvKinesin-9B) are present during spermatid differentiation and ciliogenesis. RNAi knockdowns show that MvKinesin-2 is required for mitosis and cytokinesis in spermatogenous cells. Without MvKinesin-2, most spermatozoids contain two or more coiled microtubule ribbons with attached cilia and very large cell bodies. MvKinesin-9A is required for the correct placement of basal bodies along the organelle coil. Knockdowns of MvKinesin-9A have basal bodies and cilia that are irregularly positioned. Spermatozoid swimming behavior in MvKinesin-2 and -9A knockdowns is altered because of defects in axonemal placement or ciliogenesis. MvKinesin-2 knockdowns only quiver in place while MvKinesin-9A knockdowns swim erratically compared to controls. In contrast, spermatozoids produced after the silencing of MvKinesin-9B exhibit normal morphology and swimming behavior, though development is slower than normal for these gametes.

Conclusions

Our results show that MvKinesin-2 and MvKinesin-9A are required for ciliogenesis and motility in the Marsilea male gametophyte; however, these kinesins display atypical roles during these processes. MvKinesin-2 is required for cytokinesis, a role not typically associated with this protein, as well as for ciliogenesis during rapid development and MvKinesin-9A is needed for the correct orientation of basal bodies. Our results are the first to investigate the kinesin-linked mechanisms that regulate ciliogenesis in a land plant.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-016-0107-7) contains supplementary material, which is available to authorized users.

Plant Intracellular Transport: Tracing Functions of the Retrograde Kinesin

Adding to its varied repertoire of functions in cell morphogenesis and cell division, a molecular motor protein of the kinesin-14 class has recently been implicated in rapid retrograde transport along cellular tracks in moss.

The Chlamydomonas cell cycle

The position of Chlamydomonas within the eukaryotic phylogeny makes it a unique model in at least two important ways: as a representative of the critically important, early-diverging lineage leading to plants; and as a microbe retaining important features of the last eukaryotic common ancestor (LECA) that has been lost in the highly studied yeast lineages. Its cell biology has been studied for many decades and it has well-developed experimental genetic tools, both classical (Mendelian) and molecular. Unlike land plants, it is a haploid with very few gene duplicates, making it ideal for loss-of-function genetic studies. The Chlamydomonas cell cycle has a striking temporal and functional separation between cell growth and rapid cell division, probably connected to the interplay between diurnal cycles that drive photosynthetic cell growth and the cell division cycle; it also exhibits a highly choreographed interaction between the cell cycle and its centriole-basal body-flagellar cycle. Here, we review the current status of studies of the Chlamydomonas cell cycle. We begin with an overview of cell-cycle control in the well-studied yeast and animal systems, which has yielded a canonical, well-supported model. We discuss briefly what is known about similarities and differences in plant cell-cycle control, compared with this model. We next review the cytology and cell biology of the multiple-fission cell cycle of Chlamydomonas. Lastly, we review recent genetic approaches and insights into Chlamydomonas cell-cycle regulation that have been enabled by a new generation of genomics-based tools.

The nucleotide-binding proteins Nubp1 and Nubp2 are negative regulators of ciliogenesis

Nucleotide-binding proteins Nubp1 and Nubp2 are MRP/MinD-type P-loop NTPases with sequence similarity to bacterial division site-determining proteins and are conserved, essential proteins throughout the Eukaryotes. They have been implicated, together with their interacting minus-end directed motor protein KIFC5A, in the regulation of centriole duplication in mammalian cells. Here we show that Nubp1 and Nubp2 are integral components of centrioles throughout the cell cycle, recruited independently of KIFC5A. We further demonstrate their localization at the basal body of the primary cilium in quiescent vertebrate cells or invertebrate sensory cilia, as well as in the motile cilia of mouse cells and in the flagella of Chlamydomonas. RNAi-mediated silencing of nubp-1 in C. elegans causes the formation of morphologically aberrant and additional cilia in sensory neurons. Correspondingly, downregulation of Nubp1 or Nubp2 in mouse quiescent NIH 3T3 cells markedly increases the number of ciliated cells, while knockdown of KIFC5A dramatically reduces ciliogenesis. Simultaneous double silencing of Nubp1 + KIFC5A restores the percentage of ciliated cells to control levels. We document the normal ciliary recruitment, during these silencing regimes, of basal body proteins critical for ciliogenesis, namely CP110, CEP290, cenexin, Chibby, AurA, Rab8, and BBS7. Interestingly, we uncover novel interactions of Nubp1 with several members of the CCT/TRiC molecular chaperone complex, which we find enriched at the basal body and recruited independently of the Nubps or KIFC5A. Our combined results for Nubp1, Nubp2, and KIFC5A and their striking effects on cilium formation suggest a central regulatory role for these proteins, likely involving CCT/TRiC chaperone activity, in ciliogenesis.

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 Ca(2+), 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.

CMF22 is a broadly conserved axonemal protein and is required for propulsive motility in Trypanosoma brucei

The eukaryotic flagellum (or cilium) is a broadly conserved organelle that provides motility for many pathogenic protozoa and is critical for normal development and physiology in humans. Therefore, defining core components of motile axonemes enhances understanding of eukaryotic biology and provides insight into mechanisms of inherited and infectious diseases in humans. In this study, we show that component of motile flagella 22 (CMF22) is tightly associated with the flagellar axoneme and is likely to have been present in the last eukaryotic common ancestor. The CMF22 amino acid sequence contains predicted IQ and ATPase associated with a variety of cellular activities (AAA) motifs that are conserved among CMF22 orthologues in diverse organisms, hinting at the importance of these domains in CMF22 function. Knockdown by RNA interference (RNAi) and rescue with an RNAi-immune mRNA demonstrated that CMF22 is required for propulsive cell motility in Trypanosoma brucei. Loss of propulsive motility in CMF22-knockdown cells was due to altered flagellar beating patterns, rather than flagellar paralysis, indicating that CMF22 is essential for motility regulation and likely functions as a fundamental regulatory component of motile axonemes. CMF22 association with the axoneme is weakened in mutants that disrupt the nexin-dynein regulatory complex, suggesting potential interaction with this complex. Our results provide insight into the core machinery required for motility of eukaryotic flagella.

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 140 kDa 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.

Genetic and cytological analysis of a novel type of low temperature-dependent intrasubspecific hybrid weakness in rice

Hybrid weakness (HW) is an important postzygotic isolation which occurs in both intra- and inter-specific crosses. In this study, we described a novel low temperature-dependent intrasubspecific hybrid weakness in the F₁ plants derived from the cross between two indica rice varieties Taifeng A and V1134. HW plants showed growth retardation, reduced panicle number and pale green leaves with chlorotic spots. Cytological assay showed that there were reduced cell numbers, larger intercellular spaces, thicker cell walls, and abnormal development of chloroplast and mitochondria in the mature leaves from HW F₁ plants in comparison with that from both of the parental lines. Genetic analysis revealed that HW was controlled by two complementary dominant genes Hw3 from V1134 and Hw4 from Taifeng A. Hw3 was mapped in a 136 kb interval between the markers Indel1118 and Indel1117 on chromosome 11, and Hw4 was mapped in the region of about 15 cM between RM182 and RM505 on chromosome 7, respectively. RT-PCR analysis revealed that only LOC_Os11g44310, encoding a putative calmodulin-binding protein (OsCaMBP), differentially expressed among Taifeng A, V1134 and their HW F₁. No recombinant was detected using the markers designed based on the sequence of LOC_Os11g44310 in the BC₁F₂ (Taifeng A//Taifeng A/V1134) population. Hence, LOC_Os11g44310 was probably the candidate gene of Hw3. Gene amplification suggested that LOC_Os11g44310 was present in V1134 and absent in Taifeng A. BLAST search revealed that LOC_Os11g44310 had one copy in the japonica genomic sequence of Nipponbare, and no homologous sequence in the indica reference sequence of 9311. Our results indicate that Hw3 is a novel gene for inducing hybrid weakness in rice.

De novo transcriptomic analysis of hydrogen production in the green alga Chlamydomonas moewusii through RNA-Seq

BACKGROUND

Microalgae can make a significant contribution towards meeting global renewable energy needs in both carbon-based and hydrogen (H2) biofuel. The development of energy-related products from algae could be accelerated with improvements in systems biology tools, and recent advances in sequencing technology provide a platform for enhanced transcriptomic analyses. However, these techniques are still heavily reliant upon available genomic sequence data. Chlamydomonas moewusii is a unicellular green alga capable of evolving molecular H2 under both dark and light anaerobic conditions, and has high hydrogenase activity that can be rapidly induced. However, to date, there is no systematic investigation of transcriptomic profiling during induction of H2 photoproduction in this organism.

RESULTS

In this work, RNA-Seq was applied to investigate transcriptomic profiles during the dark anaerobic induction of H2 photoproduction. 156 million reads generated from 7 samples were then used for de novo assembly after data trimming. BlastX results against NCBI database and Blast2GO results were used to interpret the functions of the assembled 34,136 contigs, which were then used as the reference contigs for RNA-Seq analysis. Our results indicated that more contigs were differentially expressed during the period of early and higher H2 photoproduction, and fewer contigs were differentially expressed when H2-photoproduction rates decreased. In addition, C. moewusii and C. reinhardtii share core functional pathways, and transcripts for H2 photoproduction and anaerobic metabolite production were identified in both organisms. C. moewusii also possesses similar metabolic flexibility as C. reinhardtii, and the difference between C. moewusii and C. reinhardtii on hydrogenase expression and anaerobic fermentative pathways involved in redox balancing may explain their different profiles of hydrogenase activity and secreted anaerobic metabolites.

CONCLUSIONS

Herein, we have described a workflow using commercial software to analyze RNA-Seq data without reference genome sequence information, which can be applied to other unsequenced microorganisms. This study provided biological insights into the anaerobic fermentation and H2 photoproduction of C. moewusii, and the first transcriptomic RNA-Seq dataset of C. moewusii generated in this study also offer baseline data for further investigation (e.g. regulatory proteins related to fermentative pathway discussed in this study) of this organism as a H2-photoproduction strain.

Independent localization of plasma membrane and chloroplast components during eyespot assembly

Like many algae, Chlamydomonas reinhardtii is phototactic, using two anterior flagella to swim toward light optimal for photosynthesis. The flagella are responsive to signals initiated at the photosensory eyespot, which comprises photoreceptors in the plasma membrane and layers of pigment granules in the chloroplast. Phototaxis depends on placement of the eyespot at a specific asymmetric location relative to the flagella, basal bodies, and bundles of two or four highly acetylated microtubules, termed rootlets, which extend from the basal bodies toward the posterior of the cell. Previous work has shown that the eyespot is disassembled prior to cell division, and new eyespots are assembled in daughter cells adjacent to the nascent four-membered rootlet associated with the daughter basal body (D4), but the chronology of these assembly events has not been determined. Here we use immunofluorescence microscopy to follow assembly and acetylation of the D4 rootlet, localization of individual eyespot components in the plasma membrane or chloroplast envelope, and flagellar emergence during and immediately following cell division. We find that the D4 rootlet is assembled before the initiation of eyespot assembly, which occurs within the same time frame as rootlet acetylation and flagellar outgrowth. Photoreceptors in the plasma membrane are correctly localized in eyespot mutant cells lacking pigment granule layers, and chloroplast components of the eyespot assemble in mutant cells in which photoreceptor localization is retarded. The data suggest that plasma membrane and chloroplast components of the eyespot are independently responsive to a cytoskeletal positioning cue.

Intraflagellar transport drives flagellar surface motility

The assembly and maintenance of all cilia and flagella require intraflagellar transport (IFT) along the axoneme. IFT has been implicated in sensory and motile ciliary functions, but the mechanisms of this relationship remain unclear. Here, we used Chlamydomonas flagellar surface motility (FSM) as a model to test whether IFT provides force for gliding of cells across solid surfaces. We show that IFT trains are coupled to flagellar membrane glycoproteins (FMGs) in a Ca²⁺-dependent manner. IFT trains transiently pause through surface adhesion of their FMG cargos, and dynein-1b motors pull the cell towards the distal tip of the axoneme. Each train is transported by at least four motors, with only one type of motor active at a time. Our results demonstrate the mechanism of Chlamydomonas gliding motility and suggest that IFT plays a major role in adhesion-induced ciliary signaling pathways.

DOI:http://dx.doi.org/10.7554/eLife.00744.001

Cilia and flagella protrude like bristles from the cell surface. They share the same basic ‘9+2’ axoneme structure, being made up of nine microtubule doublets that surround a central pair of singlet microtubules. Flagella are generally involved in cell propulsion, whereas motile cilia help to move fluids over cell surfaces.

Maintaining cilia and flagella is a challenge for cells, which must find a way to send new proteins all the way along the axoneme to the site of assembly at the flagellar tip. Cells achieve this via a process called intraflagellar transport, in which proteins are carried back and forth by kinesin and dynein motors along the axonemal doublet microtubules. Intraflagellar transport has been proposed to influence other functions of cilia and flagella, including the propulsion of cells over surfaces. However, the details of these interactions are unclear.

Through a combination of biophysical and microscopy approaches, Shih et al. describe the mechanism that the green alga Chalmydomonas uses to power flagellar gliding over surfaces. By tracking single fluorescently tagged molecules, Shih et al. observed that flagellar membrane glycoproteins are carried along the axoneme by the intraflagellar transport machinery. During transport, flagellar membrane glycoproteins transiently adhere to the surface, and dynein motors that were previously engaged in carrying these glycoproteins now transmit force that moves the axonemal microtubules. This process, which is dependent on the concentration of calcium ions in the extracellular environment, generates the force that propels the alga's flagella along the surface.

Gliding motility is thought to have been one of the initial driving forces for the evolution of cilia and flagella. How the intricate mechanism of flagellar beat motility could have evolved has been the subject of much discussion, as it would require the flagellum to have evolved first. In demonstrating that gliding motility is powered by the same intraflagellar transport mechanism that is required for flagellar assembly, Shih et al. provide strong evidence for the evolution of primitive flagella before the evolution of flagellar beating. Furthermore, since algal flagella have essentially the same structure as the cilia of human cells, these findings could ultimately aid in the development of treatments for diseases that result from defects in intraflagellar transport, including polycystic kidney disease and retinal degeneration.

DOI:http://dx.doi.org/10.7554/eLife.00744.002

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.

PF19 encodes the p60 catalytic subunit of katanin and is required for assembly of the flagellar central apparatus in Chlamydomonas

For all eukaryotic cilia the basal bodies provide a template for the assembly of the doublet microtubules, and intraflagellar transport provides a mechanism for transport of axonemal components into the growing cilium. What is not known is how the central pair of microtubules is nucleated or how their associated polypeptides are assembled. Here we report that the Chlamydomonas pf19 mutation results in a single amino acid change within the p60 catalytic subunit of katanin, and that this mutation prevents microtubule severing activity. The pf19 mutant has paralyzed flagella that lack the central apparatus. Using a combination of mutant analysis, RNAi-mediated reduction of protein expression and in vitro assays, we demonstrate that the p60 catalytic subunit of the microtubule severing protein katanin is required for central apparatus assembly in Chlamydomonas. In addition, we show that in Chlamydomonas the microtubule severing activity of p60 katanin is not required for stress-induced deflagellation or cell cycle progression as has been previously reported.

Flagellar kinesins in protists

Cilia and flagella are organelles of the cell body present in many eukaryotic cells. Although their basic structure is well conserved from unicellular organisms to mammals, they show amazing diversity in number, structure, molecular composition, disposition and function. These complex organelles are generally assembled by the action of intraflagellar transport, which is powered by kinesin and dynein motor proteins. Several types of kinesins can function in flagella. They all have a well-conserved motor domain with characteristic signatures, but display exhaustive diversification of some domains. This diversity can be explained by the multitude of functions fulfilled by these proteins (transport of cargoes along microtubules, polymerization and depolymerization of microtubules). Functional and phylogenetic analyses reveal that at least seven kinesin families are involved in flagellum assembly and function. In protists, where cilia and flagella fulfill many essential roles, this diversity of function is also observed.

Sperm flagella: comparative and phylogenetic perspectives of protein components

Sperm motility is necessary for the transport of male DNA to eggs in species with both external and internal fertilization. Flagella comprise several proteins for generating and regulating motility. Central cytoskeletal structures called axonemes have been well conserved through evolution. In mammalian sperm flagella, two accessory structures (outer dense fiber and the fibrous sheath) surround the axoneme. The axonemal bend movement is based on the active sliding of axonemal doublet microtubules by the molecular motor dynein, which is divided into outer and inner arm dyneins according to positioning on the doublet microtubule. Outer and inner arm dyneins play different roles in the production and regulation of flagellar motility. Several regulatory mechanisms are known for both dyneins, which are important in motility activation and chemotaxis at fertilization. Although dynein itself has certain properties that contribute to the formation and propagation of flagellar bending, other axonemal structures-specifically, the radial spoke/central pair apparatus-have essential roles in the regulation of flagellar bending. Recent genetic and proteomic studies have explored several new components of axonemes and shed light on the generation and regulation of sperm motility during fertilization.

Biochemical characterization of the novel rice kinesin K23 and its kinetic study using fluorescence resonance energy transfer between an intrinsic tryptophan residue and a fluorescent ATP analogue

We previously demonstrated that the rice kinesin K16, which belongs to the kinesin-7 subfamily, has unique enzymatic properties and atomic structure within key functional regions. In this study, we focused on a novel rice plant kinesin, K23, which also belongs to the kinesin-7 subfamily. The biochemical characterization of the K23 motor domain (K23MD) was studied and compared with the rice kinesin K16 and other related kinesins. K23 exhibits ∼45-fold (1.3 Pi mol(-1) site mol(-1) s(-1)) lower microtubule-dependent ATPase activity than conventional kinesins, whereas its affinity for microtubules is comparable with conventional kinesins. MgADP-free K23 is unstable compared with the unusually stable MgADP-free K16MD. The enzymatic properties of K23MD are somewhat different from those of K16. We used a fluorescent ATP analogue 2'(3')-O-(N'-methylanthraniloyl)-ATP (mant-ATP) for the kinetic characterization of K23. The fluorescence of mant-ATP was not significantly altered during its hydrolysis by K23. However, significant fluorescence resonance energy transfer (FRET) between mant-ATP and W21 in the motor domain was observed. The kinetic study using FRET revealed that K23 has unique kinetic characteristics when compared with other kinesins.

Characterization of a novel rice kinesin O12 with a calponin homology domain

Genomic analysis predicted that the rice (Oryza sativa var. japonica) genome encodes at least 41 kinesin-like proteins including the novel kinesin O12, which is classified as a kinesin-14 family member. O12 has a calponin homology (CH) domain that is known as an actin-binding domain. In this study, we expressed the functional domains of O12 in Escherichia coli and determined its enzymatic characteristics compared with other kinesins. The microtubule-dependent ATPase activity of recombinant O12 containing the motor and CH domains was significantly reduced in the presence of actin. Interestingly, microtubule-dependent ATPase activity of the motor domain was also affected by actin in the absence of the CH domain. Our findings suggest that the motor activity of the rice plant-specific kinesin O12 may be regulated by actin.

Influence of centriole number on mitotic spindle length and symmetry

The functional role of centrioles or basal bodies in mitotic spindle assembly and function is currently unclear. Although supernumerary centrioles have been associated with multipolar spindles in cancer cells, suggesting centriole number might dictate spindle polarity, bipolar spindles are able to assemble in the complete absence of centrioles, suggesting a level of centriole-independence in the spindle assembly pathway. In this report we perturb centriole number using mutations in Chlamydomonas reinhardtii, and measure the response of the mitotic spindle to these perturbations in centriole number. Although altered centriole number increased the frequency of monopolar and multipolar spindles, the majority of spindles remained bipolar regardless of the centriole number. But even when spindles were bipolar, abnormal centriole numbers led to asymmetries in tubulin distribution, half-spindle length and spindle pole focus. Half spindle length correlated directly with number of centrioles at a pole, such that an imbalance in centriole number between the two poles of a bipolar spindle correlated with increased asymmetry between half spindle lengths. These results are consistent with centrioles playing an active role in regulating mitotic spindle length. Mutants with centriole number alteration also show increased cytokinesis defects, but these do not correlate with centriole number in the dividing cell and may therefore reflect downstream consequences of defects in preceding cell divisions.

Comparison of the chloroplast peroxidase system in the chlorophyte Chlamydomonas reinhardtii, the bryophyte Physcomitrella patens, the lycophyte Selaginella moellendorffii and the seed plant Arabidopsis thaliana

BACKGROUND

Oxygenic photosynthesis is accompanied by the formation of reactive oxygen species (ROS), which damage proteins, lipids, DNA and finally limit plant yield. The enzymes of the chloroplast antioxidant system are exclusively nuclear encoded. During evolution, plastid and mitochondrial genes were post-endosymbiotically transferred to the nucleus, adapted for eukaryotic gene expression and post-translational protein targeting and supplemented with genes of eukaryotic origin.

RESULTS

Here, the genomes of the green alga Chlamydomonas reinhardtii, the moss Physcomitrella patens, the lycophyte Selaginella moellendorffii and the seed plant Arabidopsis thaliana were screened for ORFs encoding chloroplast peroxidases. The identified genes were compared for their amino acid sequence similarities and gene structures. Stromal and thylakoid-bound ascorbate peroxidases (APx) share common splice sites demonstrating that they evolved from a common ancestral gene. In contrast to most cormophytes, our results predict that chloroplast APx activity is restricted to the stroma in Chlamydomonas and to thylakoids in Physcomitrella. The moss gene is of retrotransposonal origin.The exon-intron-structures of 2CP genes differ between chlorophytes and streptophytes indicating an independent evolution. According to amino acid sequence characteristics only the A-isoform of Chlamydomonas 2CP may be functionally equivalent to streptophyte 2CP, while the weakly expressed B- and C-isoforms show chlorophyte specific surfaces and amino acid sequence characteristics. The amino acid sequences of chloroplast PrxII are widely conserved between the investigated species. In the analyzed streptophytes, the genes are unspliced, but accumulated four introns in Chlamydomonas. A conserved splice site indicates also a common origin of chlorobiont PrxQ.The similarity of splice sites also demonstrates that streptophyte glutathione peroxidases (GPx) are of common origin. Besides a less related cysteine-type GPx, Chlamydomonas encodes two selenocysteine-type GPx. The latter were lost prior or during streptophyte evolution.

CONCLUSION

Throughout plant evolution, there was a strong selective pressure on maintaining the activity of all three investigated types of peroxidases in chloroplasts. APx evolved from a gene, which dates back to times before differentiation of chlorobionts into chlorophytes and streptophytes, while Prx and presumably also GPx gene patterns may have evolved independently in the streptophyte and chlorophyte branches.

Multiple roles of the furrow deepening Ca2+ transient during cytokinesis in zebrafish embryos

The generation of a required series of localized Ca(2+) transients during cytokinesis in zebrafish embryos suggests that Ca(2+) plays a necessary role in regulating this process. Here, we report that cortical actin remodeling, characterized by the reorganization of the contractile band and the formation during furrow deepening of pericleavage F-actin enrichments (PAEs), requires a localized increase in intracellular Ca(2+), which is released from IP(3)-sensitive stores. We demonstrate that VAMP-2 vesicle fusion at the deepening furrow also requires Ca(2+) released via IP(3) receptors, as well as the presence of PAEs and the action of calpains. Finally, by expressing a dominant-negative form of the kinesin-like protein, kif23, we demonstrate that its recruitment to the furrow region is required for VAMP-2 vesicle transport; and via FRAP analysis, that kif23 localization is also Ca(2+)-dependent. Collectively, our data demonstrate that a localized increase in intracellular Ca(2+) is involved in regulating several key events during furrow deepening and subsequent apposition.

Intraflagellar transport motors in cilia: moving along the cell's antenna

Intraflagellar transport (IFT), the motor-dependent movement of IFT particles along the axoneme, is critical for the assembly, maintenance, and function of motile and sensory cilia, and, consequently, this process underlies ciliary motility, cilium-based signaling, and ciliopathies. Here, I present my perspective on IFT as a model system for studying motor-driven cargo transport. I review evidence that kinesin-2 motors physically transport IFT particles as cargo and hypothesize that several accessory kinesins confer cilia-specific functions by augmenting the action of the two core IFT motors, kinesin-2 and dynein 1b, which assemble the cilium foundation.