Temperature-sensitive mutations affecting flagellar assembly and function in Chlamydomonas reinhardtii

Tubulin glycylation controls ciliary motility through modulation of outer-arm dyneins

Tubulins undergo several kinds of posttranslational modifications (PTMs) including glutamylation and glycylation. The contribution of these PTMs to the motilities of cilia and flagella is still unclear. Here, we investigated the role of tubulin glycylation by examining a novel Chlamydomonas mutant lacking TTLL3, an enzyme responsible for initiating glycylation. Immunostaining of cells and flagella revealed that glycylation is only restricted to the axonemal tubulin composing the outer-doublet but not the central-pair microtubules. Furthermore, the flagellar localization of TTLL3 was found to be dependent on intraflagellar transport. The mutant, ttll3(ex5), completely lacks glycylation and consequently exhibits slower swimming velocity compared with the wild-type strain. By combining the ttll3(ex5) mutation with multiple axonemal dynein-deficient mutants, we found that the lack of glycylation does not affect the motility of the outer-arm dynein lacking mutations. Sliding disintegration assay using isolated axonemes revealed that the lack of glycylation decreases microtubule sliding velocity in the normal axoneme but not in the axoneme lacking the outerarm dyneins. Based on our recent study that glycylation occurs exclusively on β-tubulin in Chlamydomonas, these findings suggest that tubulin glycylation controls flagellar motility through modulating outer-arm dyneins, presumably by neutralizing the negative charges of glutamate residues at the C-terminus region of β-tubulin.

The purpose and ubiquity of turnover

Turnover-constant component production and destruction-is ubiquitous in biology. Turnover occurs across organisms and scales, including for RNAs, proteins, membranes, macromolecular structures, organelles, cells, hair, feathers, nails, antlers, and teeth. For many systems, turnover might seem wasteful when degraded components are often fully functional. Some components turn over with shockingly high rates and others do not turn over at all, further making this process enigmatic. However, turnover can address fundamental problems by yielding powerful properties, including regeneration, rapid repair onset, clearance of unpredictable damage and errors, maintenance of low constitutive levels of disrepair, prevention of stable hazards, and transitions. I argue that trade-offs between turnover benefits and metabolic costs, combined with constraints on turnover, determine its presence and rates across distinct contexts. I suggest that the limits of turnover help explain aging and that turnover properties and the basis for its levels underlie this fundamental component of life.

Differential modulation of sensory response dynamics by cilia structure and intraflagellar transport within and across chemosensory neurons

Sensory neurons contain morphologically diverse primary cilia that are built by intraflagellar transport (IFT) and house sensory signaling molecules. Since both ciliary structural and signaling proteins are trafficked via IFT, it has been challenging to decouple the contributions of IFT and cilia structure to neuronal responses. By acutely inhibiting IFT without altering cilia structure and vice versa , here we describe the differential roles of ciliary trafficking and sensory ending morphology in shaping chemosensory responses in C. elegans. We show that a minimum cilium length but not continuous IFT is necessary for a subset of responses in the ASH nociceptive neurons. In contrast, neither cilia nor continuous IFT are necessary for odorant responses in the AWA olfactory neurons. Instead, continuous IFT differentially modulates response dynamics in AWA. Upon acute inhibition of IFT, cilia-destined odorant receptors are shunted to ectopic branches emanating from the cilia base. Spatial segregation of receptors in these branches from a cilia-restricted regulatory kinase results in odorant desensitization defects, highlighting the importance of precise organization of signaling molecules at sensory endings in regulating response dynamics. We also find that adaptation of AWA responses upon repeated exposure to an odorant is mediated by IFT-driven removal of its cognate receptor, whereas adaptation to a second odorant is regulated via IFT-independent mechanisms. Our results reveal unexpected complexity in the contribution of IFT and cilia organization to the regulation of responses even within a single chemosensory neuron type, and establish a critical role for these processes in the precise modulation of olfactory behaviors.

Testing the ion-current model for flagellar length sensing and IFT regulation

Eukaryotic cilia and flagella are microtubule-based organelles whose relatively simple shape makes them ideal for investigating the fundamental question of organelle size regulation. Most of the flagellar materials are transported from the cell body via an active transport process called intraflagellar transport (IFT). The rate of IFT entry into flagella, known as IFT injection, has been shown to negatively correlate with flagellar length. However, it remains unknown how the cell measures the length of its flagella and controls IFT injection. One of the most-discussed theoretical models for length sensing to control IFT is the ion-current model, which posits that there is a uniform distribution of Ca²⁺ channels along the flagellum and that the Ca²⁺ current from the flagellum into the cell body increases linearly with flagellar length. In this model, the cell uses the Ca²⁺ current to negatively regulate IFT injection. The recent discovery that IFT entry into flagella is regulated by the phosphorylation of kinesin through a calcium-dependent protein kinase has provided further impetus for the ion-current model. To test this model, we measured and manipulated the levels of Ca²⁺ inside of Chlamydomonas flagella and quantified IFT injection. Although the concentration of Ca²⁺ inside of flagella was weakly correlated with the length of flagella, we found that IFT injection was reduced in calcium-deficient flagella, rather than increased as the model predicted, and that variation in IFT injection was uncorrelated with the occurrence of flagellar Ca²⁺ spikes. Thus, Ca²⁺ does not appear to function as a negative regulator of IFT injection, hence it cannot form the basis of a stable length control system.

Signaling in the primary cilium through the lens of the Hedgehog pathway

Cilia are microtubule-based, cell-surface projections whose machinery is evolutionarily conserved. In vertebrates, cilia are observed on almost every cell type and are either motile or immotile. Immotile sensory, or primary cilia, are responsive to extracellular ligands and signals. Cilia can be thought of as compartments, functionally distinct from the cell that provides an environment for signaling cascades. Hedgehog is a critical developmental signaling pathway which is functionally linked to primary cilia in vertebrates. The major components of the vertebrate Hedgehog signaling pathway dynamically localize to the ciliary compartment and ciliary membrane. Critically, G-protein coupled receptor (GPCR) Smoothened, the obligate transducer of the pathway, is enriched and activated in the cilium. While Smoothened is the most intensely studied ciliary receptor, many GPCRs localize within cilia. Understanding the link between Smoothened and cilia defines common features, and distinctions, of GPCR signaling within the primary cilium. This article is categorized under: Signaling Pathways > Global Signaling Mechanisms Signaling Pathways > Cell Fate Signaling.

Regulation of ciliary membrane protein trafficking and signalling by kinesin motor proteins

Primary cilia are antenna-like sensory organelles that regulate a substantial number of cellular signalling pathways in vertebrates, both during embryonic development as well as in adulthood, and mutations in genes coding for ciliary proteins are causative of an expanding group of pleiotropic diseases known as ciliopathies. Cilia consist of a microtubule-based axoneme core, which is subtended by a basal body and covered by a bilayer lipid membrane of unique protein and lipid composition. Cilia are dynamic organelles, and the ability of cells to regulate ciliary protein and lipid content in response to specific cellular and environmental cues is crucial for balancing ciliary signalling output. Here we discuss mechanisms involved in regulation of ciliary membrane protein trafficking and signalling, with main focus on kinesin-2 and kinesin-3 family members.

Intraflagellar transport is deeply integrated in hedgehog signaling

The vertebrate hedgehog pathway is organized in primary cilia, and hedgehog components relocate into or out of cilia during signaling. Defects in intraflagellar transport (IFT) typically disrupt ciliary assembly and attenuate hedgehog signaling. Determining whether IFT drives the movement of hedgehog components is difficult due to the requirement of IFT for building cilia. Unlike most IFT proteins, IFT27 is dispensable for cilia formation but affects hedgehog signaling similarly to other IFTs, allowing us to examine its role in the dynamics of signaling. Activating signaling at points along the pathway in Ift27 mutant cells showed that IFT is extensively involved in the pathway. Similar analysis of Bbs mutant cells showed that BBS proteins participate at many levels of signaling but are not needed to concentrate Gli transcription factors at the ciliary tip. Our analysis showed that smoothened delivery to cilia does not require IFT27, but the role of other IFTs is not known. Using a rapamycin-induced dimerization system to sequester IFT-B proteins at the mitochondria in cells with fully formed cilia did not affect the delivery of Smo to cilia, suggesting that this membrane protein may not require IFT-B for delivery.

Cell-Type Transcriptomes of the Multicellular Green Alga Volvox carteri Yield Insights into the Evolutionary Origins of Germ and Somatic Differentiation Programs

Germ-soma differentiation is a hallmark of complex multicellular organisms, yet its origins are not well understood. Volvox carteri is a simple multicellular green alga that has recently evolved a simple germ-soma dichotomy with only two cell-types: large germ cells called gonidia and small terminally differentiated somatic cells. Here, we provide a comprehensive characterization of the gonidial and somatic transcriptomes of V. carteri to uncover fundamental differences between the molecular and metabolic programming of these cell-types. We found extensive transcriptome differentiation between cell-types, with somatic cells expressing a more specialized program overrepresented in younger, lineage-specific genes, and gonidial cells expressing a more generalist program overrepresented in more ancient genes that shared striking overlap with stem cell-specific genes from animals and land plants. Directed analyses of different pathways revealed a strong dichotomy between cell-types with gonidial cells expressing growth-related genes and somatic cells expressing an altruistic metabolic program geared toward the assembly of flagella, which support organismal motility, and the conversion of storage carbon to sugars, which act as donors for production of extracellular matrix (ECM) glycoproteins whose secretion enables massive organismal expansion. V. carteri orthologs of diurnally controlled genes from C. reinhardtii, a single-celled relative, were analyzed for cell-type distribution and found to be strongly partitioned, with expression of dark-phase genes overrepresented in somatic cells and light-phase genes overrepresented in gonidial cells- a result that is consistent with cell-type programs in V. carteri arising by cooption of temporal regulons in a unicellular ancestor. Together, our findings reveal fundamental molecular, metabolic, and evolutionary mechanisms that underlie the origins of germ-soma differentiation in V. carteri and provide a template for understanding the acquisition of germ-soma differentiation in other multicellular lineages.

DRC2/CCDC65 is a central hub for assembly of the nexin-dynein regulatory complex and other regulators of ciliary and flagellar motility

DRC2 is a subunit of the nexin–dynein regulatory complex linked to primary ciliary dyskinesia. Little is known about the impact of drc2 mutations on axoneme composition and structure. We used proteomic and structural approaches to reveal that DRC2 coassembles with DRC1 to attach the N-DRC to the A-tubule and mediate interactions with other regulatory structures.

The nexin–dynein regulatory complex (N-DRC) plays a central role in the regulation of ciliary and flagellar motility. In most species, the N-DRC contains at least 11 subunits, but the specific function of each subunit is unknown. Mutations in three subunits (DRC1, DRC2/CCDC65, DRC4/GAS8) have been linked to defects in ciliary motility in humans and lead to a ciliopathy known as primary ciliary dyskinesia (PCD). Here we characterize the biochemical, structural, and motility phenotypes of two mutations in the DRC2 gene of Chlamydomonas. Using high-resolution proteomic and structural approaches, we find that the C-terminal region of DRC2 is critical for the coassembly of DRC2 and DRC1 to form the base plate of N-DRC and its attachment to the outer doublet microtubule. Loss of DRC2 in drc2 mutants disrupts the assembly of several other N-DRC subunits and also destabilizes the assembly of several closely associated structures such as the inner dynein arms, the radial spokes, and the calmodulin- and spoke-associated complex. Our study provides new insights into the range of ciliary defects that can lead to PCD.

Maintaining centrosomes and cilia

Centrosomes and cilia are present in organisms from all branches of the eukaryotic tree of life. These structures are composed of microtubules and various other proteins, and are required for a plethora of cell processes such as structuring the cytoskeleton, sensing the environment, and motility. Deregulation of centrosome and cilium components leads to a wide range of diseases, some of which are incompatible with life. Centrosomes and cilia are thought to be very stable and can persist over long periods of time. However, these structures can disappear in certain developmental stages and diseases. Moreover, some centrosome and cilia components are quite dynamic. While a large body of knowledge has been produced regarding the biogenesis of these structures, little is known about how they are maintained. In this Review, we propose the existence of specific centrosome and cilia maintenance programs, which are regulated during development and homeostasis, and when deregulated can lead to disease.

Building a multipurpose insertional mutant library for forward and reverse genetics in Chlamydomonas

BACKGROUND

The unicellular green alga, Chlamydomonas reinhardtii, is a classic model for studying flagella and biofuel. However, precise gene editing, such as Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and CRISPR-associated protein (Cas9) system, is not widely used in this organism. Screening of random insertional mutant libraries by polymerase chain reaction provides an alternate strategy to obtain null mutants of individual gene. But building, screening, and maintaining such a library was time-consuming and expensive.

RESULTS

By selecting a suitable parental strain, keeping individual mutants using the agar plate, and designing an insertion cassette-specific primer for library screening, we successfully generated and maintained ~150,000 insertional mutants of Chlamydomonas, which was used for both reverse and forward genetics analysis. We obtained 26 individual mutants corresponding to 20 genes and identified 967 motility-defect mutants including 10 mutants with defective accumulation of intraflagellar transport complex at the basal body. We also obtained 929 mutants defective in oil droplet assembly after nitrogen deprivation. Furthermore, a new insertion cassette with splicing donor sequences at both ends was also constructed, which increased the efficiency of gene interruption.

CONCLUSION

In summary, this library provides a multifunctional platform both for obtaining mutants of interested genes and for screening of mutants with specific phenotype.

The Intraflagellar Transport Machinery

Eukaryotic cilia and flagella are evolutionarily conserved organelles that protrude from the cell surface. The unique location and properties of cilia allow them to function in vital processes such as motility and signaling. Ciliary assembly and maintenance rely on intraflagellar transport (IFT), the bidirectional movement of a multicomponent transport system between the ciliary base and tip. Since its initial discovery more than two decades ago, considerable effort has been invested in dissecting the molecular mechanisms of IFT in a variety of model organisms. Importantly, IFT was shown to be essential for mammalian development, and defects in this process cause a number of human pathologies known as ciliopathies. Here, we review current knowledge of IFT with a particular emphasis on the IFT machinery and specific mechanisms of ciliary cargo recognition and transport.

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.

Intraflagellar transport is required for the maintenance of the trypanosome flagellum composition but not its length

Intraflagellar transport (IFT) is required for construction of most cilia and flagella. Here, we used electron microscopy, immunofluorescence and live video microscopy to show that IFT is absent or arrested in the mature flagellum of Trypanosoma brucei upon RNA interference (RNAi)-mediated knockdown of IFT88 and IFT140, respectively. Flagella assembled prior to RNAi did not shorten, showing that IFT is not essential for the maintenance of flagella length. Although the ultrastructure of the axoneme was not visibly affected, flagellar beating was strongly reduced and the distribution of several flagellar components was drastically modified. The R subunit of the protein kinase A was no longer concentrated in the flagellum but was largely found in the cell body whereas the kinesin 9B motor was accumulating at the distal tip of the flagellum. In contrast, the distal tip protein FLAM8 was dispersed along the flagellum. This reveals that IFT also functions in maintaining the distribution of some flagellar proteins after construction of the organelle is completed.

Mechanism of ciliary disassembly

As motile organelles and sensors, cilia play pivotal roles in cell physiology, development and organ homeostasis. Ciliary defects are associated with a class of cilia-related diseases or developmental disorders, termed ciliopathies. Even though the presence of cilia is required for diverse functions, cilia can be removed through ciliary shortening or resorption that necessitates disassembly of the cilium, which occurs normally during cell cycle progression, cell differentiation and in response to cellular stress. The functional significance of ciliary resorption is highlighted in controlling the G1-S transition during cell cycle progression. Internal or external cues that trigger ciliary resorption initiate signaling cascades that regulate several downstream events including depolymerization of axonemal microtubules, dynamic changes in actin and the ciliary membrane, regulation of intraflagellar transport and posttranslational modifications of ciliary proteins. To ensure ciliary resorption, both the active disassembly of the cilium and the simultaneous inhibition of ciliary assembly must be coordinately regulated.

Analysis of Axonemal Assembly During Ciliary Regeneration in Chlamydomonas

Chlamydomonas reinhardtii is an outstanding model genetic organism for study of assembly of cilia. Here, methods are described for synchronization of ciliary regeneration in Chlamydomonas to analyze the sequence in which ciliary proteins assemble. In addition, the methods described allow analysis of the mechanisms involved in regulation of ciliary length, the proteins required for ciliary assembly, and the temporal expression of genes encoding ciliary proteins. Ultimately, these methods can contribute to discovery of conserved genes that when defective lead to abnormal ciliary assembly and human disease.

Genetic and genomic approaches to identify genes involved in flagellar assembly in Chlamydomonas reinhardtii

Flagellar assembly requires intraflagellar transport of components from the cell body to the flagellar tip for assembly. The understanding of flagellar assembly has been aided by the ease of biochemistry and the availability of mutants in the unicellular green alga, Chlamydomonas reinhardtii. In this chapter, we discuss means to identify genes involved in these processes using forward and reverse genetics. In particular, the ease and low cost of whole genome sequencing (WGS) will help to make gene identification easier and promote the understanding of this important process.

New mutations in flagellar motors identified by whole genome sequencing in Chlamydomonas

Background

The building of a cilium or flagellum requires molecular motors and associated proteins that allow the relocation of proteins from the cell body to the distal end and the return of proteins to the cell body in a process termed intraflagellar transport (IFT). IFT trains are carried out by kinesin and back to the cell body by dynein.

Methods

We used whole genome sequencing to identify the causative mutations for two temperature-sensitive flagellar assembly mutants in Chlamydomonas and validated the changes using reversion analysis. We examined the effect of these mutations on the localization of IFT81, an IFT complex B protein, the cytoplasmic dynein heavy chain (DHC1b), and the dynein light intermediate chain (D1bLIC).

Results

The strains, fla18 and fla24, have mutations in kinesin-2 and cytoplasmic dynein, respectively. The fla18 mutation alters the same glutamic acid (E₂₄G) mutated in the fla10-14 allele (E₂₄K). The fla18 strain loses flagella at 32?C more rapidly than the E₂₄K allele but less rapidly than the fla10-1 allele. The fla18 mutant loses its flagella by detachment rather than by shortening. The fla24 mutation falls in cytoplasmic dynein and changes a completely conserved amino acid (L₃₂₄₃P) in an alpha helix in the AAA5 domain. The fla24 mutant loses its flagella by shortening within 6 hours at 32?C. DHC1b protein is reduced by 18-fold and D1bLIC is reduced by 16-fold at 21?C compared to wild-type cells. We identified two pseudorevertants (L₃₂₄₃S and L₃₂₄₃R), which remain flagellated at 32?C. Although fla24 cells assemble full-length flagella at 21?C, IFT81 protein localization is dramatically altered. Instead of localizing at the basal body and along the flagella, IFT81 is concentrated at the proximal end of the flagella. The pseudorevertants show wild-type IFT81 localization at 21?C, but proximal end localization of IFT81 at 32?C.

Conclusions

The change in the AAA5 domain of the cytoplasmic dynein in fla24 may block the recycling of IFT trains after retrograde transport. It is clear that different alleles in the flagellar motors reveal different functions and roles. Multiple alleles will be important for understanding structure-function relationships.

Whole genome sequencing identifies a deletion in protein phosphatase 2A that affects its stability and localization in Chlamydomonas reinhardtii

Whole genome sequencing is a powerful tool in the discovery of single nucleotide polymorphisms (SNPs) and small insertions/deletions (indels) among mutant strains, which simplifies forward genetics approaches. However, identification of the causative mutation among a large number of non-causative SNPs in a mutant strain remains a big challenge. In the unicellular biflagellate green alga Chlamydomonas reinhardtii, we generated a SNP/indel library that contains over 2 million polymorphisms from four wild-type strains, one highly polymorphic strain that is frequently used in meiotic mapping, ten mutant strains that have flagellar assembly or motility defects, and one mutant strain, imp3, which has a mating defect. A comparison of polymorphisms in the imp3 strain and the other 15 strains allowed us to identify a deletion of the last three amino acids, Y313F314L315, in a protein phosphatase 2A catalytic subunit (PP2A3) in the imp3 strain. Introduction of a wild-type HA-tagged PP2A3 rescues the mutant phenotype, but mutant HA-PP2A3 at Y313 or L315 fail to rescue. Our immunoprecipitation results indicate that the Y313, L315, or YFLΔ mutations do not affect the binding of PP2A3 to the scaffold subunit, PP2A-2r. In contrast, the Y313, L315, or YFLΔ mutations affect both the stability and the localization of PP2A3. The PP2A3 protein is less abundant in these mutants and fails to accumulate in the basal body area as observed in transformants with either wild-type HA-PP2A3 or a HA-PP2A3 with a V310T change. The accumulation of HA-PP2A3 in the basal body region disappears in mated dikaryons, which suggests that the localization of PP2A3 may be essential to the mating process. Overall, our results demonstrate that the terminal YFL tail of PP2A3 is important in the regulation on Chlamydomonas mating.

Kinesin-2: a family of heterotrimeric and homodimeric motors with diverse intracellular transport functions

Kinesin-2 was first purified as a heterotrimeric, anterograde, microtubule-based motor consisting of two distinct kinesin-related subunits and a novel associated protein (KAP) that is currently best known for its role in intraflagellar transport and ciliogenesis. Subsequent work, however, has revealed diversity in the oligomeric state of different kinesin-2 motors owing to the combinatorial heterodimerization of its subunits and the coexistence of both heterotrimeric and homodimeric kinesin-2 motors in some cells. Although the functional significance of the homo- versus heteromeric organization of kinesin-2 motor subunits and the role of KAP remain uncertain, functional studies suggest that cooperation between different types of kinesin-2 motors or between kinesin-2 and a member of a different motor family can generate diverse patterns of anterograde intracellular transport. Moreover, despite being restricted to ciliated eukaryotes, kinesin-2 motors are now known to drive diverse transport events outside cilia. Here, I review the organization, assembly, phylogeny, biological functions, and motility mechanism of this diverse family of intracellular transport motors.

Chemical screening methods for flagellar phenotypes in Chlamydomonas

Cilia and flagella are important organelles used for sensing the external cellular environment or for motility. Abnormalities in ciliary structure or function can have devastating pathological consequences ranging from sinusitis and obesity to polycystic kidney disease, retinal degeneration, and mental retardation. Chlamydomonas flagella are excellent models to study the regulation and normal function of cilia. We utilized the 1280 compound Sigma LOPAC annotated library to screen for phenotypes in Chlamydomonas flagellar length, motility, deflagellation, and cellular toxicity. Phenotypes were assessed by quantitation from direct microscopic visualization and custom-designed motility/viability assays. Compounds were clustered based on data across all assays to facilitate the identification of novel pathways regulating flagella in Chlamydomonas. These methods can both aid our understanding of the basic biology of flagellar regulation and provide useful points of therapeutic intervention for cilia-related disorders.

The RABL5 homolog IFT22 regulates the cellular pool size and the amount of IFT particles partitioned to the flagellar compartment in Chlamydomonas reinhardtii

Cilia and flagella, sensory and motile structures protruding from the cell body, rely on the continuous bidirectional traffic of intraflagellar transport (IFT) particles to ferry flagellar precursors into flagella for assembly. Cells synthesize a large pool of IFT particle proteins in the cell body, but only a small portion engages in active transport within the flagella at any given time. The atypical small G protein Rab-like 5 (RABL5) has been shown to move in an IFT-like manner in the flagella, but its function in ciliogenesis is controversial. In this report, we demonstrate that IFT22, the Chlamydomonas reinhardtii homolog of RABL5, is a bona fide IFT particle complex B subunit. Although the amount of IFT22 remains unaffected by depletion of either complex A or B, depletion of IFT22 leads to a smaller pool of both complex A and B. Strikingly, the smaller cellular pool of IFT particles does not lead to a reduced distribution of IFT particles to flagella. Instead, the amount of IFT particle proteins, including IFT22 itself, increase in the flagella. Moreover, cells over-expressing IFT22 also accumulate IFT particles in their flagella. Taken together, these data indicate that, in C. reinhardtii, IFT22 controls the cellular levels of both complex A and B, thus plays a critical role in determining the cellular availability of IFT particles. In addition, although IFT22 may not directly carry any precursors for flagellar assembly, it controls how many IFT particles participate in ferrying precursors into flagella.

Kinesin-3 KLP-6 regulates intraflagellar transport in male-specific cilia of Caenorhabditis elegans

Cilia are cellular sensory organelles whose integrity of structure and function are important to human health. All cilia are assembled and maintained by kinesin-2 motors in a process termed intraflagellar transport (IFT), but they exhibit great variety of morphology and function. This diversity is proposed to be conferred by cell-specific modulation of the core IFT by additional factors, but examples of such IFT modulators are limited. Here we demonstrate that the cell-specific kinesin-3 KLP-6 acts as a modulator of both IFT dynamics and length in the cephalic male (CEM) cilia of Caenorhabditis elegans. Live imaging of GFP-tagged kinesins in CEM cilia shows partial uncoupling of the IFT motors of the kinesin-2 family, kinesin-II and OSM-3/KIF17, with a portion of OSM-3 moving independently of the IFT complex. KLP-6 moves independently of the kinesin-2 motors and acts to reduce the velocity of OSM-3 and IFT. Additionally, kinesin-II mutants display a novel CEM cilia elongation phenotype that is partially dependent on OSM-3 and KLP-6. Our observations illustrate modulation of the general kinesin-2-driven IFT process by a cell-specific kinesin-3 in cilia of C. elegans male neurons.

Mechanoreception in motile flagella of Chlamydomonas

Ciliates and flagellates temporarily swim backwards on collision by generating a mechanoreceptor potential. Although this potential has been shown to be associated with cilia in Paramecium, the molecular entity of the mechanoreceptor has remained unknown. Here we show that Chlamydomonas cells express TRP11, a member of the TRP (transient receptor potential) subfamily V, in the proximal region of the flagella, and that suppression of TRP11 expression results in loss of the avoiding reaction. The results indicate that Chlamydomonas flagella exhibit mechanosensitivity, despite constant motility, by localizing the mechanoreceptor in the proximal region, where active bending is restricted.

Molecular genetics and pathogenic mechanisms for the severe ciliopathies: insights into neurodevelopment and pathogenesis of neural tube defects

Meckel-Gruber syndrome (MKS) is a severe autosomal recessively inherited disorder characterized by developmental defects of the central nervous system that comprise neural tube defects that most commonly present as occipital encephalocele. MKS is considered to be the most common syndromic form of neural tube defect. MKS is genetically heterogeneous with six known disease genes: MKS1, MKS2/TMEM216, MKS3/TMEM67, RPGRIP1L, CEP290, and CC2D2A with the encoded proteins all implicated in the correct function of primary cilia. Primary cilia are microtubule-based organelles that project from the apical surface of most epithelial cell types. Recent progress has implicated the involvement of cilia in the Wnt and Shh signaling pathways and has led to an understanding of their role in normal mammalian neurodevelopment. The aim of this review is to provide an overview of the molecular genetics of the human disorder, and to assess recent insights into the etiology and molecular cell biology of severe ciliopathies from mammalian animal models of MKS.

Chlamydomonas IFT70/CrDYF-1 is a core component of IFT particle complex B and is required for flagellar assembly

DYF-1 is a highly conserved protein essential for ciliogenesis in several model organisms. In Caenorhabditis elegans, DYF-1 serves as an essential activator for an anterograde motor OSM-3 of intraflagellar transport (IFT), the ciliogenesis-required motility that mediates the transport of flagellar precursors and removal of turnover products. In zebrafish and Tetrahymena DYF-1 influences the cilia tubulin posttranslational modification and may have more ubiquitous function in ciliogenesis than OSM-3. Here we address how DYF-1 biochemically interacts with the IFT machinery by using the model organism Chlamydomonas reinhardtii, in which the anterograde IFT does not depend on OSM-3. Our results show that this protein is a stoichiometric component of the IFT particle complex B and interacts directly with complex B subunit IFT46. In concurrence with the established IFT protein nomenclature, DYF-1 is also named IFT70 after the apparent size of the protein. IFT70/CrDYF-1 is essential for the function of IFT in building the flagellum because the flagella of IFT70/CrDYF-1-depleted cells were greatly shortened. Together, these results demonstrate that IFT70/CrDYF-1 is a canonical subunit of IFT particle complex B and strongly support the hypothesis that the IFT machinery has species- and tissue-specific variations with functional ramifications.

Total internal reflection fluorescence (TIRF) microscopy of Chlamydomonas flagella

The eukaryotic flagellum is host to a variety of dynamic behaviors, including flagellar beating, the motility of glycoproteins in the flagellar membrane, and intraflagellar transport (IFT), the bidirectional traffic of protein particles between the flagellar base and tip. IFT is of particular interest, as it plays integral roles in flagellar length control, cell signaling, development, and human disease. However, our ability to understand dynamic flagellar processes such as IFT is limited in large part by the fidelity with which we can image these behaviors in living cells. This chapter introduces the application of total internal reflection fluorescence (TIRF) microscopy to visualize the flagella of Chlamydomonas reinhardtii. The advantages and challenges of TIRF are discussed in comparison to confocal and differential interference contrast techniques. This chapter also reviews current IFT insights gleaned from TIRF microscopy of Chlamydomonas and provides an outlook on the future of the technique, with particular emphasis on combining TIRF with other emerging imaging technologies.

Intraflagellar transport particle size scales inversely with flagellar length: revisiting the balance-point length control model

Chlamydomonas reinhardtii IFT particle trains, important for flagella maintenance and assembly, are observed to decrease in size as a function of cilia length.

The assembly and maintenance of eukaryotic flagella are regulated by intraflagellar transport (IFT), the bidirectional traffic of IFT particles (recently renamed IFT trains) within the flagellum. We previously proposed the balance-point length control model, which predicted that the frequency of train transport should decrease as a function of flagellar length, thus modulating the length-dependent flagellar assembly rate. However, this model was challenged by the differential interference contrast microscopy observation that IFT frequency is length independent. Using total internal reflection fluorescence microscopy to quantify protein traffic during the regeneration of Chlamydomonas reinhardtii flagella, we determined that anterograde IFT trains in short flagella are composed of more kinesin-associated protein and IFT27 proteins than trains in long flagella. This length-dependent remodeling of train size is consistent with the kinetics of flagellar regeneration and supports a revised balance-point model of flagellar length control in which the size of anterograde IFT trains tunes the rate of flagellar assembly.

Retrograde intraflagellar transport mutants identify complex A proteins with multiple genetic interactions in Chlamydomonas reinhardtii

The intraflagellar transport machinery is required for the assembly of cilia. It has been investigated by biochemical, genetic, and computational methods that have identified at least 21 proteins that assemble into two subcomplexes. It has been hypothesized that complex A is required for retrograde transport. Temperature-sensitive mutations in FLA15 and FLA17 show defects in retrograde intraflagellar transport (IFT) in Chlamydomonas. We show that IFT144 and IFT139, two complex A proteins, are encoded by FLA15 and FLA17, respectively. The fla15 allele is a missense mutation in a conserved cysteine and the fla17 allele is an in-frame deletion of three exons. The flagellar assembly defect of each mutant is rescued by the respective transgenes. In fla15 and fla17 mutants, bulges form in the distal one-third of the flagella at the permissive temperature and this phenotype is also rescued by the transgenes. These bulges contain the complex B component IFT74/72, but not alpha-tubulin or p28, a component of an inner dynein arm, which suggests specificity with respect to the proteins that accumulate in these bulges. IFT144 and IFT139 are likely to interact with each other and other proteins on the basis of three distinct genetic tests: (1) Double mutants display synthetic flagellar assembly defects at the permissive temperature, (2) heterozygous diploid strains exhibit second-site noncomplemention, and (3) transgenes confer two-copy suppression. Since these tests show different levels of phenotypic sensitivity, we propose they illustrate different gradations of gene interaction between complex A proteins themselves and with a complex B protein (IFT172).

Genetic and phenotypic analysis of flagellar assembly mutants in Chlamydomonas reinhardtii

Conditional mutants for flagellar assembly (fla) provide a useful tool to study intraflagellar transport (IFT) at the molecular level, and provide a unique set of tools to analyze cilia. The analysis of IFT phenotypes of fla mutants at the permissive temperature by a quantitative image analysis approach identified four distinct phases of the IFT cycle and directly demonstrated structural and functional remodeling of IFT particles at both axonemal extremities. In addition, the genetic analysis of fla mutants reveal interesting interactions among genes involved in flagellar assembly that help to provide information about the structure and function of IFT particles and their motors. This chapter provides protocols to isolate, characterize, and identify conditional Chlamydomonas flagellar assembly mutants and their genes and to test genetic interactions among proteins encoded by these genes.

Assembly of primary cilia

Primary cilia are microtubule-based, hair-like sensory organelles present on the surface of most growth-arrested cells in our body. Recent research has demonstrated a crucial role for primary cilia in regulating vertebrate developmental pathways and tissue homeostasis, and defects in genes involved in primary cilia assembly or function have been associated with a panoply of disorders and diseases, including polycystic kidney disease, left-right asymmetry defects, hydrocephalus, and Bardet Biedl Syndrome. Here we provide an up-to-date review focused on the molecular mechanisms involved in the assembly of primary cilia in vertebrate cells. We present an overview of the early stages of the cilia assembly process, as well as a description of the intraflagellar transport (IFT) system. IFT is a highly conserved process required for assembly of almost all eukaryotic cilia and flagella, and much of our current knowledge about IFT is based on studies performed in Chlamydomonas and Caenorhabditis elegans. Therefore, our review of the IFT literature includes studies performed in these two model organisms. The role of several non-IFT proteins (e.g., centrosomal proteins) in the ciliary assembly process is also discussed.

Chlamydomonas FAP133 is a dynein intermediate chain associated with the retrograde intraflagellar transport motor

Intraflagellar transport (IFT) is the bi-directional movement of particles along the length of axonemal outer doublet microtubules and is needed for the assembly and maintenance of eukaryotic cilia and flagella. Retrograde IFT requires cytoplasmic dynein 1b, a motor complex whose organization, structural composition and regulation is poorly understood. We have characterized the product of the Chlamydomonas FAP133 gene that encodes a new WD-repeat protein similar to dynein intermediate chains and homologous to the uncharacterized vertebrate protein WD34. FAP133 is located at the peri-basal body region as well as in punctate structures along the flagella. This protein is associated with the IFT machinery because it is specifically depleted from the flagella of cells with defects in anterograde IFT. Fractionation of flagellar matrix proteins indicates that FAP133 associates with both the LC8 dynein light chain and the IFT dynein heavy chain and light intermediate chain (DHC1b-D1bLIC) motor complex. In the absence of DHC1b or D1bLIC, FAP133 fails to localize at the peri-basal body region but, rather, is concentrated in a region of the cytoplasm near the cell center. Furthermore, we found that FAP133, LC8, DHC1b, D1bLIC, the FLA10 kinesin-2 necessary for anterograde IFT and other IFT scaffold components associate to form a large macromolecular assembly.

Functional analysis of an individual IFT protein: IFT46 is required for transport of outer dynein arms into flagella

Intraflagellar transport (IFT), which is the bidirectional movement of particles within flagella, is required for flagellar assembly. IFT particles are composed of ∼16 proteins, which are organized into complexes A and B. We have cloned Chlamydomonas reinhardtii and mouse IFT46, and show that IFT46 is a highly conserved complex B protein in both organisms. A C. reinhardtii insertional mutant null for IFT46 has short, paralyzed flagella lacking dynein arms and with central pair defects. The mutant has greatly reduced levels of most complex B proteins, indicating that IFT46 is necessary for complex B stability. A partial suppressor mutation restores flagellar length to the ift46 mutant. IFT46 is still absent, but levels of the other IFT particle proteins are largely restored, indicating that complex B is stabilized in the suppressed strain. Axonemal ultrastructure is restored, except that the outer arms are still missing, although outer arm subunits are present in the cytoplasm. Thus, IFT46 is specifically required for transporting outer arms into the flagellum.

Dissecting the molecular mechanisms of intraflagellar transport in chlamydomonas

BACKGROUND

The assembly and maintenance of eukaryotic cilia and flagella are mediated by intraflagellar transport (IFT), a bidirectional microtubule (MT)-based transport system. The IFT system consists of anterograde (kinesin-2) and retrograde (cDynein1b) motor complexes and IFT particles comprising two complexes, A and B. In the current model for IFT, kinesin-2 carries cDynein1b, IFT particles, and axonemal precursors from the flagellar base to the tip, and cDynein1b transports kinesin-2, IFT particles, and axonemal turnover products from the tip back to the base. Most of the components of the IFT system have been identified and characterized, but the mechanisms by which these different components are coordinated and regulated at the flagellar base and tip are unclear.

RESULTS

Using a variety of Chlamydomonas mutants, we confirm that cDynein1b requires kinesin-2 for transport toward the tip and show that during retrograde IFT, kinesin-2 can exit the flagella independent of the cDynein1b light intermediate chain (LIC) and IFT particles. Furthermore, using biochemical approaches, we find that IFT complex B can associate with cDynein1b independent of complex A and cDynein1b LIC. Finally, using electron microscopy, we show that the IFT tip turnaround point most likely is localized distal to the plus end of the outer-doublet B MTs.

CONCLUSION

Our results support a model for IFT in which tip turnaround involves (1) dissociation of IFT complexes A and B and release of inactive cDynein1b from complex B, (2) binding of complex A to active cDynein1b, and (3) reassociation of complex B with A prior to retrograde IFT.

Elucidating the function of primary cilia by conditional gene inactivation

PURPOSE OF REVIEW

This review discusses recent experimental approaches to determine the function of primary cilia by conditional inactivation of genes crucial for cilia formation.

RECENT FINDINGS

A functional role in the sensing of fluid flow was recently assigned to the primary cilia. This discovery shed light onto how cells sense dynamic fluid movements. Conditional inactivation of primary cilia formation in later ontogenic stages demonstrated the crucial role renal primary cilia play in the control of cell proliferation.

SUMMARY

Primary cilia can act as flow sensors, transmitting signals by means of calcium influx into the cells. Structures based on primary cilia are also crucial for the function of photoreceptor cells and it can be expected that additional functions of these organelles will be determined in the future. An important experimental approach to elucidate the involvement of primary cilia in other physiological processes is to specifically inactivate genes crucial for formation of primary cilia. Morphological and physiological changes induced by the loss of primary cilia will help determine additional roles primary cilia play in physiology and organ development.

Chlamydomonas shortens its flagella by activating axonemal disassembly, stimulating IFT particle trafficking, and blocking anterograde cargo loading

Almost all eukaryotic cells form cilia/flagella, maintain them at their genetically specified lengths, and shorten them. Here, we define the cellular mechanisms that bring about shortening of flagella prior to meiotic cell division and in response to environmental cues in the biflagellated green alga Chlamydomonas. We show that the flagellar shortening pathway is distinct from the one that enforces transient shortening essential for length control. During flagellar shortening, disassembly of the axoneme is stimulated over the basal rate, and the rate of entry into flagella of intraflagellar transport (IFT) particles is increased. Moreover, the particles entering the disassembling flagella lack cargo. Thus, flagellar shortening depends on the interplay between dynamic properties of the axoneme and the IFT machinery; a cell triggered to shorten its flagellum activates disassembly of the axoneme and stimulates entry into the flagellum of IFT particles possessing empty cargo binding sites available to retrieve the disassembled components.

Mutant kinesin-2 motor subunits increase chromosome loss

The Chlamydomonas anterograde intraflagellar transport motor, kinesin-2, is isolated as a heterotrimeric complex containing two motor subunits and a nonmotor subunit known as kinesin-associated polypeptide or KAP. One of the two motor subunits is encoded by the FLA10 gene. The sequence of the second motor subunit was obtained by mass spectrometry and sequencing. It shows 46.9% identity with the Fla10 motor subunit and the gene maps to linkage group XII/XIII near RPL9. The temperature-sensitive flagellar assembly mutants fla1 and fla8 are linked to this kinesin-2 motor subunit. In each strain, a unique single point mutation gives rise to a unique single amino acid substitution within the motor domain. The fla8 strain is named fla8-1 and the fla1 strain is named fla8-2. The fla8 and fla10 alleles show a chromosome loss phenotype. To analyze this chromosome loss phenotype, intragenic revertants of fla8-1, fla8-2, and fla10-14 were generated. The analysis of the mutants and the revertants demonstrates the importance of a pocket in the amino terminus of these motor subunits for both motor activity and for a novel, dominant effect on the fidelity of chromosome segregation.

The FLA3 KAP subunit is required for localization of kinesin-2 to the site of flagellar assembly and processive anterograde intraflagellar transport

Intraflagellar transport (IFT) is a bidirectional process required for assembly and maintenance of cilia and flagella. Kinesin-2 is the anterograde IFT motor, and Dhc1b/Dhc2 drives retrograde IFT. To understand how either motor interacts with the IFT particle or how their activities might be coordinated, we characterized a ts mutation in the Chlamydomonas gene encoding KAP, the nonmotor subunit of Kinesin-2. The fla3-1 mutation is an amino acid substitution in a conserved C-terminal domain. fla3-1 strains assemble flagella at 21 degrees C, but cannot maintain them at 33 degrees C. Although the Kinesin-2 complex is present at both 21 and 33 degrees C, the fla3-1 Kinesin-2 complex is not efficiently targeted to or retained in the basal body region or flagella. Video-enhanced DIC microscopy of fla3-1 cells shows that the frequency of anterograde IFT particles is significantly reduced. Anterograde particles move at near wild-type velocities, but appear larger and pause more frequently in fla3-1. Transformation with an epitope-tagged KAP gene rescues all of the fla3-1 defects and results in preferential incorporation of tagged KAP complexes into flagella. KAP is therefore required for the localization of Kinesin-2 at the site of flagellar assembly and the efficient transport of anterograde IFT particles within flagella.

Localization of the blue-light receptor phototropin to the flagella of the green alga Chlamydomonas reinhardtii

Blue light controls the sexual life cycle of Chlamydomonas, mediated by phototropin, a UV-A/blue-light receptor that plays a prominent role in multiple photoresponses. By using fractionation experiments and immunolocalization studies, this blue-light receptor, in addition to its known localization to the cell bodies, also was detected in flagella. Within the flagella, it was completely associated with the axonemes, in striking contrast to the situation in higher plants and the Chlamydomonas cell body where phototropin was observed in the plasma membrane. Its localization was not perturbed in mutants lacking several prominent structural components of the axoneme. This led to the conclusion that phototropin may be associated with the outer doublet microtubules. Analysis of a mutant (fla10) in which intraflagellar transport is compromised suggested that phototropin is a cargo for intraflagellar transport. The blue-light receptor thus seems to be an integral constituent of the flagella of this green alga, extending the list of organisms that harbor sensory molecules within this organelle to unicellular algae.

A short flagella mutant of Dunaliella salina (Chlorophyta, Chlorophyceae)

Dunaliella salina (Chlorophyta, Chlorophyceae) is a unicellular wall-less biflagellate alga. In this paper we describe a spontaneous mutant of D. salina, isolated from wild type cultures, which is characterized by very short flagella. The ultrastructure showed the basic 9 + 2 organization of wild-type flagella. Immunofluorescence localization of tubulin in this mutant confirmed the normal construction of the axoneme. Although, the mutant does not swim, still it is able to move and perform photobehavior. As shown by track reconstruction, and rotation movements, observed by means of reflection microscopy, this mutant can move, probably gliding by means of its stumpy flagella. A possible model to explain the mutant motion pattern is discussed.

An aurora kinase is essential for flagellar disassembly in Chlamydomonas

Cilia and flagella play key roles in development and sensory transduction, and several human disorders, including polycystic kidney disease, are associated with the failure to assemble cilia. Here, we show that the aurora protein kinase CALK in the biflagellated alga Chlamydomonas has a central role in two pathways for eliminating flagella. Cells rendered deficient in CALK were defective in regulated flagellar excision and regulated flagellar disassembly. Exposure of cells to altered ionic conditions, the absence of a centriole/basal body for nucleating flagellar assembly, cessation of delivery of flagellar components to their tip assembly site, and formation of zygotes all led to activation of the regulated disassembly pathway as indicated by phosphorylation of CALK and the absence of flagella. We propose that cells have a sensory pathway that detects conditions that are inappropriate for possession of a flagellum, and that CALK is a key effector of flagellar disassembly in that pathway.

Intraflagellar transport

It has been a decade since a novel form of microtubule (MT)-based motility, i.e., intraflagellar transport (IFT), was discovered in Chlamydomonas flagella. Subsequent research has supported the hypothesis that IFT is required for the assembly and maintenance of all cilia and flagella and that its underlying mechanism involves the transport of nonmembrane-bound macromolecular protein complexes (IFT particles) along axonemal MTs beneath the ciliary membrane. IFT requires the action of the anterograde kinesin-II motors and the retrograde IFT-dynein motors to transport IFT particles in opposite directions along the MT polymer lattice from the basal body to the tip of the axoneme and back again. A rich diversity of biological processes has been shown to depend upon IFT, including flagellar length control, cell swimming, mating and feeding, photoreception, animal development, sensory perception, chemosensory behavior, and lifespan control. These processes reflect the varied roles of cilia and flagella in motility and sensory signaling.

Cellular Deflagellation

Deciliation, also known as deflagellation, flagellar autotomy, flagellar excision, or flagellar shedding, refers to the process whereby eukaryotic cells shed their cilia or flagella, often in response to stress. Used for many decades as a tool for scientists interested in the structure, function, and genesis of cilia, deciliation itself is a process worthy of scientific investigation. Deciliation has numerous direct medical implications, but more profoundly, intriguing relationships between deciliation, ciliogenesis, and the cell cycle indicate that understanding the mechanism of deciliation will contribute to a deeper understanding of broad aspects of cell biology. This review provides a critical examination of diverse data bearing on this problem. It also highlights current deficiencies in our understanding of the mechanism of deciliation.

Defective flagellar assembly and length regulation in LF3 null mutants in Chlamydomonas

Four long-flagella (LF) genes are important for flagellar length control in Chlamydomonas reinhardtii. Here, we characterize two new null lf3 mutants whose phenotypes are different from previously identified lf3 mutants. These null mutants have unequal-length flagella that assemble more slowly than wild-type flagella, though their flagella can also reach abnormally long lengths. Prominent bulges are found at the distal ends of short, long, and regenerating flagella of these mutants. Analysis of the flagella by electron and immunofluorescence microscopy and by Western blots revealed that the bulges contain intraflagellar transport complexes, a defect reported previously (for review see Cole, D.G., 2003. Traffic. 4:435-442) in a subset of mutants defective in intraflagellar transport. We have cloned the wild-type LF3 gene and characterized a hypomorphic mutant allele of LF3. LF3p is a novel protein located predominantly in the cell body. It cosediments with the product of the LF1 gene in sucrose density gradients, indicating that these proteins may form a functional complex to regulate flagellar length and assembly.

IFT20 links kinesin II with a mammalian intraflagellar transport complex that is conserved in motile flagella and sensory cilia

Intraflagellar transport (IFT) is an evolutionarily conserved mechanism thought to be required for the assembly and maintenance of all eukaryotic cilia and flagella. Although IFT proteins are present in cells with sensory cilia, the organization of IFT protein complexes in those cells has not been analyzed. To determine whether the IFT complex is conserved in the sensory cilia of photo-receptors, we investigated protein interactions among four mammalian IFT proteins: IFT88/Polaris, IFT57/Hippi, IFT52/NGD5, and IFT20. We demonstrate that IFT proteins extracted from bovine photoreceptor outer segments, a modified sensory cilium, co-fractionate at approximately 17 S, similar to IFT proteins extracted from mouse testis. Using antibodies to IFT88 and IFT57, we demonstrate that all four IFT proteins co-immunoprecipitate from lysates of mouse testis, kidney, and retina. We also extended our analysis to interactions outside of the IFT complex and demonstrate an ATP-regulated co-immunoprecipitation of heterotrimeric kinesin II with the IFT complex. The internal architecture of the IFT complex was investigated using the yeast two-hybrid system. IFT20 exhibited a strong interaction with IFT57/Hippi and the kinesin II subunit, KIF3B. Our data indicate that all four mammalian IFT proteins are part of a highly conserved complex in multiple ciliated cell types. Furthermore, IFT20 appears to bridge kinesin II with the IFT complex.

Intraflagellar Transport Is Required in Drosophila to Differentiate Sensory Cilia but Not Sperm

BACKGROUND

Intraflagellar transport (IFT) uses kinesin II to carry a multiprotein particle to the tips of eukaryotic cilia and flagella and a nonaxonemal dynein to return it to the cell body. IFT particle proteins and motors are conserved in ciliated eukaryotes, and IFT-deficient mutants in algae, nematodes, and mammals fail to extend or maintain cilia and flagella, including sensory cilia. In Drosophila, the only ciliated cells are sensory neurons and sperm. no mechanoreceptor potential (nomp) mutations have been isolated that affect the differentiation and function of ciliated sense organs. The nompB gene is here shown to encode an IFT protein. Its mutant phenotypes reveal the consequences of an IFT defect in an insect.

RESULTS

Mechanosensory and olfactory neurons in nompB mutants have missing or defective cilia. nompB encodes the Drosophila homolog of the IFT complex B protein IFT88/Polaris/OSM-5. nompB is expressed in the ciliated sensory neurons, and a functional, tagged NOMPB protein is located in sensory cilia and around basal bodies. Surprisingly, nompB mutant males produce normally elongated, motile sperm. Neuronally restricted expression and male germline mosaic experiments show that nompB-deficient sperm are fully functional in transfer, competition, and fertilization.

CONCLUSIONS

NOMPB, the Drosophila homolog of IFT88, is required for the assembly of sensory cilia but not for the extension or function of the sperm flagellum. Assembly of this extremely long axoneme is therefore independent of IFT.

Flagellar adhesion between mating type plus and mating type minus gametes activates a flagellar protein-tyrosine kinase during fertilization in Chlamydomonas

When Chlamydomonas gametes of opposite mating type are mixed together, flagellar adhesion through sex-specific adhesion molecules triggers a transient elevation of intracellular cAMP, leading to gamete activation in preparation for cell-cell fusion and zygote formation. Here, we have identified a protein-tyrosine kinase (PTK) activity that is stimulated by flagellar adhesion. We determined that the protein-tyrosine kinase inhibitor genistein inhibited fertilization, and that fertilization was rescued by dibutyryl cAMP, indicating that the genistein-sensitive step was upstream of the increase in cAMP. Incubation with ATP of flagella isolated from non-adhering and adhering gametes followed by SDS-PAGE and immunoblotting with anti-phosphotyrosine antibodies showed that adhesion activated a flagellar PTK that phosphorylated a 105-kDa flagellar protein. Assays using an exogenous protein-tyrosine kinase substrate confirmed that the activated PTK could be detected only in flagella isolated from adhering gametes. Our results indicate that stimulation of the PTK is a very early event during fertilization. Activation of the PTK was blocked when gametes underwent flagellar adhesion in the presence of the protein kinase inhibitor staurosporine, but not in the presence of the cyclic nucleotide-dependent protein kinase inhibitor, H8, which (unlike staurosporine) does not block the increases in cAMP. In addition, incubation of gametes of a single mating type in dibutyryl cAMP failed to activate the PTK. Finally, flagella adhesion between plus and minus fla10-1 gametes, which have a temperature-sensitive lesion in the microtubule motor protein kinesin-II, failed to activate the PTK at elevated temperatures. Our results show that kinesin-II is essential for coupling flagellar adhesion to activation of a flagellar PTK and cAMP generation during fertilization in Chlamydomonas.