Kinesin II and regulated intraflagellar transport of Chlamydomonas aurora protein kinase

Assembly of FAP93 at the proximal axoneme in Chlamydomonas cilia

To identify proteins specific to the proximal ciliary axoneme, we used iTRAQ to compare short (~2 μm) and full-length (~11 μm) axonemes of Chlamydomonas. Known components of the proximal axoneme such as minor dynein heavy chains and LF5 kinase as well as the ciliary tip proteins FAP256 (CEP104) and EB1 were enriched in short axonemes whereas proteins present along the length of the axoneme were of similar abundance in both samples. The iTRAQ analysis revealed that FAP93, a protein of unknown function, and protein phosphatase 2A (PP2A) are enriched in the short axonemes. Consistently, immunoblots show enrichment of FAP93 and PP2A in short axonemes and immunofluorescence confirms the localization of FAP93 and enrichment of PP2A at the proximal axoneme. Ciliary regeneration reveals that FAP93 assembles continuously but more slowly than other axonemal structures and terminates at 1.03 μm in steady-state axonemes. The length of FAP93 assembly correlates with ciliary length, demonstrating ciliary length-dependent assembly of FAP93. Dikaryon rescue experiments show that FAP93 can assemble independently of IFT transport. In addition, FRAP analysis of GFP-tagged FAP93 demonstrates that FAP93 is stably anchored in the axoneme. FAP93 may function as a scaffold for assembly of other specific proteins at the proximal axoneme.

Permanent deconstruction of intracellular primary cilia in differentiating granule cell neurons

Primary cilia on granule cell neuron progenitors in the developing cerebellum detect sonic hedgehog to facilitate proliferation. Following differentiation, cerebellar granule cells become the most abundant neuronal cell type in the brain. While granule cell cilia are essential during early developmental stages, they become infrequent upon maturation. Here, we provide nanoscopic resolution of cilia in situ using large-scale electron microscopy volumes and immunostaining of mouse cerebella. In many granule cells, we found intracellular cilia, concealed from the external environment. Cilia were disassembled in differentiating granule cell neurons-in a process we call cilia deconstruction-distinct from premitotic cilia resorption in proliferating progenitors. In differentiating granule cells, cilia deconstruction involved unique disassembly intermediates, and, as maturation progressed, mother centriolar docking at the plasma membrane. Unlike ciliated neurons in other brain regions, our results show the deconstruction of concealed cilia in differentiating granule cells, which might prevent mitogenic hedgehog responsiveness. Ciliary deconstruction could be paradigmatic of cilia removal during differentiation in other tissues.

Regulation of ciliary homeostasis by intraflagellar transport-independent kinesins

Cilia are highly conserved eukaryotic organelles that protrude from the cell surface and are involved in sensory perception, motility, and signaling. Their proper assembly and function rely on the bidirectional intraflagellar transport (IFT) system, which involves motor proteins, including antegrade kinesins and retrograde dynein. Although the role of IFT-mediated transport in cilia has been extensively studied, recent research has highlighted the contribution of IFT-independent kinesins in ciliary processes. The coordinated activities and interplay between IFT kinesins and IFT-independent kinesins are crucial for maintaining ciliary homeostasis. In this comprehensive review, we aim to delve into the specific contributions and mechanisms of action of the IFT-independent kinesins in cilia. By shedding light on their involvement, we hope to gain a more holistic perspective on ciliogenesis and ciliopathies.

Permanent deconstruction of intracellular primary cilia in differentiating granule cell neurons

Primary cilia on granule cell neuron progenitors in the developing cerebellum detect sonic hedgehog to facilitate proliferation. Following differentiation, cerebellar granule cells become the most abundant neuronal cell type in the brain. While essential during early developmental stages, the fate of granule cell cilia is unknown. Here, we provide nanoscopic resolution of ciliary dynamics in situ by studying developmental changes in granule cell cilia using large-scale electron microscopy volumes and immunostaining of mouse cerebella. We found that many granule cell primary cilia were intracellular and concealed from the external environment. Cilia were disassembed in differentiating granule cell neurons in a process we call cilia deconstruction that was distinct from pre-mitotic cilia resorption in proliferating progenitors. In differentiating granule cells, ciliary loss involved unique disassembly intermediates, and, as maturation progressed, mother centriolar docking at the plasma membrane. Cilia did not reform from the docked centrioles, rather, in adult mice granule cell neurons remained unciliated. Many neurons in other brain regions require cilia to regulate function and connectivity. In contrast, our results show that granule cell progenitors had concealed cilia that underwent deconstruction potentially to prevent mitogenic hedgehog responsiveness. The ciliary deconstruction mechanism we describe could be paradigmatic of cilia removal during differentiation in other tissues.

IFT20: An Eclectic Regulator of Cellular Processes beyond Intraflagellar Transport

Initially discovered as the smallest component of the intraflagellar transport (IFT) system, the IFT20 protein has been found to be implicated in several unconventional mechanisms beyond its essential role in the assembly and maintenance of the primary cilium. IFT20 is now considered a key player not only in ciliogenesis but also in vesicular trafficking of membrane receptors and signaling proteins. Moreover, its ability to associate with a wide array of interacting partners in a cell-type specific manner has expanded the function of IFT20 to the regulation of intracellular degradative and secretory pathways. In this review, we will present an overview of the multifaceted role of IFT20 in both ciliated and non-ciliated cells.

FBB18 participates in preassembly of almost all axonemal dyneins independent of R2TP complex

Assembly of dynein arms requires cytoplasmic processes which are mediated by dynein preassembly factors (DNAAFs). CFAP298, which is conserved in organisms with motile cilia, is required for assembly of dynein arms but with obscure mechanisms. Here, we show that FBB18, a Chlamydomonas homologue of CFAP298, localizes to the cytoplasm and functions in folding/stabilization of almost all axonemal dyneins at the early steps of dynein preassembly. Mutation of FBB18 causes no or short cilia accompanied with partial loss of both outer and inner dynein arms. Comparative proteomics using 15N labeling suggests partial degradation of almost all axonemal dynein heavy chains (DHCs). A mutant mimicking a patient variant induces particular loss of DHCα. FBB18 associates with 9 DNAAFs and 14 out of 15 dynein HCs but not with IC1/IC2. FBB18 interacts with RuvBL1/2, components of the HSP90 co-chaperone R2TP complex but not the holo-R2TP complex. Further analysis suggests simultaneous formation of multiple DNAAF complexes involves dynein folding/stability and thus provides new insights into axonemal dynein preassembly.

Cilium Length and Intraflagellar Transport Regulation by Kinases PKG-1 and GCK-2 in Caenorhabditis elegans Sensory Neurons

To understand how ciliopathies such as polycystic kidney disease or Bardet-Biedl syndrome develop, we need to understand the basic molecular mechanisms underlying cilium development. Cilium growth depends on the presence of functional intraflagellar transport (IFT) machinery, and we hypothesized that various kinases and phosphatases might be involved in this regulatory process. A candidate screen revealed two kinases, PKG-1 (a cGMP-dependent protein kinase) and GCK-2 (a mitogen-activated protein kinase kinase kinase kinase 3 [MAP4K3] kinase involved in mTOR signaling), significantly affecting dye filling, chemotaxis, cilium morphology, and IFT component distribution. PKG-1 and GCK-2 show similar expression patterns in Caenorhabditis elegans cilia and colocalize with investigated IFT machinery components. In pkg-1 mutants, a high level of accumulation of kinesin-2 OSM-3 in distal segments was observed in conjunction with an overall reduction of anterograde and retrograde IFT particle A transport, likely as a function of reduced tubulin acetylation. In contrast, in gck-2 mutants, both kinesin-2 motility and IFT particle A motility were significantly elevated in the middle segments, in conjunction with increased tubulin acetylation, possibly the cause of longer cilium growth. Observed effects in mutants can be also seen in manipulating upstream and downstream effectors of the respective cGMP and mTOR pathways. Importantly, transmission electron microscopy (TEM) analysis revealed no structural changes in cilia of pkg-1 and gck-2 mutants.

IFT54 regulates IFT20 stability but is not essential for tubulin transport during ciliogenesis

Intraflagellar transport (IFT) is required for ciliogenesis by ferrying ciliary components using IFT complexes as cargo adaptors. IFT54 is a component of the IFT-B complex and is also associated with cytoplasmic microtubules (MTs). Loss of IFT54 impairs cilia assembly as well as cytoplasmic MT dynamics. The N-terminal calponin homology (CH) domain of IFT54 interacts with tubulins/MTs and has been proposed to transport tubulin during ciliogenesis, whereas the C-terminal coiled-coil (CC) domain binds IFT20. However, the precise function of these domains in vivo is not well understood. We showed that in Chlamydomonas, loss of IFT54 completely blocks ciliogenesis but does not affect spindle formation and proper cell cycle progression, even though IFT54 interacts with mitotic MTs. Interestingly, IFT54 lacking the CH domain allows proper flagellar assembly. The CH domain is required for the association of IFT54 with the axoneme but not with mitotic MTs, and also regulates the flagellar import of IFT54 but not IFT81 and IFT46. The C-terminal CC domain is essential for IFT54 to bind IFT20, and for its recruitment to the basal body and incorporation into IFT complexes. Complete loss of IFT54 or the CC domain destabilizes IFT20. ift54 mutant cells expressing the CC domain alone rescue the stability of IFT20 and form stunted flagella with accumulation of both IFT-A component IFT43 and IFT-B component IFT46, indicating that IFT54 also functions in IFT turn-around at the flagellar tip.

Functional exploration of the IFT-A complex in intraflagellar transport and ciliogenesis

Intraflagellar transport (IFT) particles or trains are composed of IFT-A and IFT-B complexes. To assess the working mechanism of the IFT-A complex in IFT and ciliogenesis, we have analyzed ift43 mutants of Chlamydomnonas in conjunction with mutants of the other IFT-A subunits. An ift43 null mutant or a mutant with a partial deletion of the IFT43 conserved domain has no or short flagella. The mutants accumulate not only IFT-B but also IFT-Ain the short flagella, which is in contrast to an ift140 null mutant. The IFT43 conserved domain is necessary and sufficient for the function of IFT43. IFT43 directly interacts with IFT121 and loss of IFT43 results in instability of IFT-A. A construct with a partial deletion of the IFT43 conserved domain is sufficient to rescue the instability phenotype of IFT-A, but results in diminishing of IFT-A at the peri-basal body region. We have further provided evidence for the direct interactions within the IFT-A complex and shown that the integrity of IFT-A is important for its stability and cellular localization. Finally, we show that both IFT43 and IFT140 are involved in mobilizing ciliary precursors from the cytoplasmic pool during flagellar regeneration, suggesting a novel role of IFT-A in transporting ciliary components in the cytoplasm to the peri-basal body region.

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.

Intraflagellar transport is essential for mammalian spermiogenesis but is absent in mature sperm

Intraflagellar transport (IFT) is necessary for the assembly and maintenance of most cilia, with the exception of gametic flagella in some organisms. IFT is required for assembly of mouse sperm flagella, and defects in IFT lead to male infertility. However, mature sperm lack IFT proteins and thus do not require IFT for maintenance of the axoneme.

Drosophila sperm are unusual in that they do not require the intraflagellar transport (IFT) system for assembly of their flagella. In the mouse, the IFT proteins are very abundant in testis, but we here show that mature sperm are completely devoid of them, making the importance of IFT to mammalian sperm development unclear. To address this question, we characterized spermiogenesis and fertility in the Ift88^Tg737Rpw mouse. This mouse has a hypomorphic mutation in the gene encoding the IFT88 subunit of the IFT particle. This mutation is highly disruptive to ciliary assembly in other organs. Ift88^−/− mice are completely sterile. They produce ∼350-fold fewer sperm than wild-type mice, and the remaining sperm completely lack or have very short flagella. The short flagella rarely have axonemes but assemble ectopic microtubules and outer dense fibers and accumulate improperly assembled fibrous sheath proteins. Thus IFT is essential for the formation but not the maintenance of mammalian sperm flagella.

Regulation of flagellar biogenesis by a calcium dependent protein kinase in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii, a bi-flagellated green alga, is a model organism for studies of flagella or cilia related activities including cilia-based signaling, flagellar motility and flagellar biogenesis. Calcium has been shown to be a key regulator of these cellular processes whereas the signaling pathways linking calcium to these cellular functions are less understood. Calcium-dependent protein kinases (CDPKs), which are present in plants but not in animals, are also present in ciliated microorganisms which led us to examine their possible functions and mechanisms in flagellar related activities. By in silico analysis of Chlamydomonas genome we have identified 14 CDPKs and studied one of the flagellar localized CDPKs – CrCDPK3. CrCDPK3 was a protein of 485 amino acids and predicted to have a protein kinase domain at the N-terminus and four EF-hand motifs at the C-terminus. In flagella, CrCDPK3 was exclusively localized in the membrane matrix fraction and formed an unknown 20 S protein complex. Knockdown of CrCDPK3 expression by using artificial microRNA did not affect flagellar motility as well as flagellar adhesion and mating. Though flagellar shortening induced by treatment with sucrose or sodium pyrophosphate was not affected in RNAi strains, CrCDPK3 increased in the flagella, and pre-formed protein complex was disrupted. During flagellar regeneration, CrCDPK3 also increased in the flagella. When extracellular calcium was lowered to certain range by the addition of EGTA after deflagellation, flagellar regeneration was severely affected in RNAi cells compared with wild type cells. In addition, during flagellar elongation induced by LiCl, RNAi cells exhibited early onset of bulbed flagella. This work expands new functions of CDPKs in flagellar activities by showing involvement of CrCDPK3 in flagellar biogenesis in Chlamydomonas.

Endothelial cells from humans and mice with polycystic kidney disease are characterized by polyploidy and chromosome segregation defects through survivin down-regulation

Autosomal-dominant polycystic kidney disease (ADPKD) is the most common hereditary and systemic disorder associated with various cardiovascular complications. It has been implicated with dysfunction in primary cilia. We and others have shown that the immediate function of endothelial cilia is to sense extracellular signal. The long-term function of cilia is hypothesized to regulate cell cycle. Here, we show that ciliary function (polycystins) and structure (polaris) are required for proper cellular division. Cilia mutant cells undergo abnormal cell division with apparent defects in mitotic spindle formation, cellular spindle assembly checkpoint and centrosome amplification. Down-regulation of the chromosomal passenger survivin contributes to these abnormalities, which further result in cell polyploidy. Re-expression of survivin restores a competent spindle assembly checkpoint and reduces polyploidy. Aged animals show a more severe phenotype in cellular division, consistent with progression of cardiovascular complications seen in older ADPKD patients. For the first time, we show that structure and function of mechanosensory cilia are crucial in maintaining proper cellular proliferation. Furthermore, developmental aging plays a crucial role in the progression of these abnormal cellular phenotypes. We propose that abnormal function or structure of primary cilia not only causes failure to transmit extracellular signals, but also is associated with cytokinesis defects in both mice and humans with polycystic kidney disease.

Composition and sensory function of the trypanosome flagellar membrane

A cilium is an extension of the cell that contains an axonemal complex of microtubules and associated proteins bounded by a membrane which is contiguous with the cell body membrane. Cilia may be nonmotile or motile, the latter having additional specific roles in cell or fluid movement. The term flagellum refers to the motile cilium of free-living single cells (e.g. bacteria, archaea, spermatozoa, and protozoa). In eukaryotes, both nonmotile and motile cilia possess sensory functions. The ciliary interior (cilioplasm) is separated from the cytoplasm by a selective barrier that prevents passive diffusion of molecules between the two domains. The sensory functions of cilia reside largely in the membrane and signals generated in the cilium are transduced into a variety of cellular responses. In this review we discuss the structure and biogenesis of the cilium, with special attention to the trypanosome flagellar membrane, its lipid and protein composition and its proposed roles in sensing and signaling.

Molecular mechanisms of protein and lipid targeting to ciliary membranes

Cilia are specialized surface regions of eukaryotic cells that serve a variety of functions, ranging from motility to sensation and to regulation of cell growth and differentiation. The discovery that a number of human diseases, collectively known as ciliopathies, result from defective cilium function has expanded interest in these structures. Among the many properties of cilia, motility and intraflagellar transport have been most extensively studied. The latter is the process by which multiprotein complexes associate with microtubule motors to transport structural subunits along the axoneme to and from the ciliary tip. By contrast, the mechanisms by which membrane proteins and lipids are specifically targeted to the cilium are still largely unknown. In this Commentary, we review the current knowledge of protein and lipid targeting to ciliary membranes and outline important issues for future study. We also integrate this information into a proposed model of how the cell specifically targets proteins and lipids to the specialized membrane of this unique organelle.

Posttranslational protein modifications in cilia and flagella

Tubulin and other flagellar and ciliary proteins are the substrates for a host of posttranslational modifications (PTMs), many of which have been highly conserved over evolutionary time. In addition to the binding of MAPs (microtubule-associated proteins) that provide a specific functionality, or the use of different tubulin isotypes to convey a specific function, most cells rely on an array of PTMs. These include phosphorylation, acetylation, glycylation, glutamylation, and methylation. The first and the last of this list are not unique to the tubulin in cilia and flagella, while the others are. This chapter will review briefly these varying modifications and will conclude with detailed methods for their detection and localization at the limit of resolution provided by electron microscopy.

Primary cilia and the cell cycle

Cilia are microtubule-based structures that protrude from the cell surface and function as sensors for mechanical and chemical environmental cues that regulate cellular differentiation or division. In metazoans, ciliary signaling is important during organismal development and in the homeostasis controls of adult tissues, with receptors for the Hedgehog, platelet derived growth factor (PDGF), Wnt, and other signaling cascades arrayed and active along the ciliary membrane. In normal cells, cilia are dynamically regulated during cell cycle progression: present in G0 and G1 cells, and usually in S/G2 cells, but almost invariably resorbed before mitotic entry, to reappear post-cytokinesis. This periodic resorption and reassembly of cilia, specified by the intrinsic cell cycle the intrinsic cell cycle machinery, influences the susceptibility of cells to the influence of extrinsic signals with cilia-associated receptors. Pathogenic conditions of mammals associated with loss of or defects in ciliary integrity include a number of developmental disorders, cystic syndromes in adults, and some cancers. With the continuing expansion of the list of human diseases associated with ciliary abnormalities, the identification of the cellular mechanisms regulating ciliary growth and disassembly has become a topic of intense research interest. Although these mechanisms are far from being understood, a number of recent studies have begun to identify key regulatory factors that may begin to offer insight into disease pathogenesis and treatment. In this chapter we will discuss the current state of knowledge regarding cell cycle control of ciliary dynamics, and provide general methods that can be applied to investigate cell cycle-dependent ciliary growth and disassembly.

Analysis of cargo transport by IFT and GFP imaging of IFT in Chlamydomonas

Chlamydomonas reinhardtii is the organism in which intraflagellar transport (IFT) was first visualized and in which the composition of IFT particles was originally elucidated. As the universality of IFT among ciliated/flagellated cells was uncovered, the diversity of organisms used to study IFT has grown. Still, because of the ease of isolation of flagella from Chlamydomonas and the battery of temperature-sensitive mutants affecting IFT proteins and motors, this unicellular alga remains the principal model for biochemical studies of IFT motors and cargo; furthermore, the long, exposed flagella of this cell are ideally suited for observing IFT in real time with GFP-tagged components of IFT.

The Golgin GMAP210/TRIP11 anchors IFT20 to the Golgi complex

Eukaryotic cells often use proteins localized to the ciliary membrane to monitor the extracellular environment. The mechanism by which proteins are sorted, specifically to this subdomain of the plasma membrane, is almost completely unknown. Previously, we showed that the IFT20 subunit of the intraflagellar transport particle is localized to the Golgi complex, in addition to the cilium and centrosome, and hypothesized that the Golgi pool of IFT20 plays a role in sorting proteins to the ciliary membrane. Here, we show that IFT20 is anchored to the Golgi complex by the golgin protein GMAP210/Trip11. Mice lacking GMAP210 die at birth with a pleiotropic phenotype that includes growth restriction, ventricular septal defects of the heart, omphalocele, and lung hypoplasia. Cells lacking GMAP210 have normal Golgi structure, but IFT20 is no longer localized to this organelle. GMAP210 is not absolutely required for ciliary assembly, but cilia on GMAP210 mutant cells are shorter than normal and have reduced amounts of the membrane protein polycystin-2 localized to them. This work suggests that GMAP210 and IFT20 function together at the Golgi in the sorting or transport of proteins destined for the ciliary membrane.

Sensory signaling-dependent remodeling of olfactory cilia architecture in C. elegans

Nonmotile primary cilia are sensory organelles composed of a microtubular axoneme and a surrounding membrane sheath that houses signaling molecules. Optimal cellular function requires the precise regulation of axoneme assembly, membrane biogenesis, and signaling protein targeting and localization via as yet poorly understood mechanisms. Here, we show that sensory signaling is required to maintain the architecture of the specialized AWB olfactory neuron cilia in C. elegans. Decreased sensory signaling results in alteration of axoneme length and expansion of a membraneous structure, thereby altering the topological distribution of a subset of ciliary transmembrane signaling molecules. Signaling-regulated alteration of ciliary structures can be bypassed by modulation of intracellular cGMP or calcium levels and requires kinesin-II-driven intraflagellar transport (IFT), as well as BBS- and RAB8-related proteins. Our results suggest that compensatory mechanisms in response to altered levels of sensory activity modulate AWB cilia architecture, revealing remarkable plasticity in the regulation of cilia structure.

The primary cilium coordinates signaling pathways in cell cycle control and migration during development and tissue repair

Cell cycle control and migration are critical processes during development and maintenance of tissue functions. Recently, primary cilia were shown to take part in coordination of the signaling pathways that control these cellular processes in human health and disease. In this review, we present an overview of the function of primary cilia and the centrosome in the signaling pathways that regulate cell cycle control and migration with focus on ciliary signaling via platelet-derived growth factor receptor alpha (PDGFRalpha). We also consider how the primary cilium and the centrosome interact with the extracellular matrix, coordinate Wnt signaling, and modulate cytoskeletal changes that impinge on both cell cycle control and cell migration.

Protein networks and complexes in photoreceptor cilia

Vertebrate photoreceptor cells are ciliated sensory cells specialized for single photon detection. The photoreceptor outer segment corresponds to the ciliary shaft of a prototypic cilium. In the outer segment compartment, the ciliary membrane is highly modified into membranous disks which are enveloped by the plasma membrane in rod cells. At these outer segment disks, the visual transduction cascade--a prototypical G-protein coupled receptor transduction pathway is arranged. The light sensitive outer segments are linked by the socalled connecting cilium with the inner segment, the photoreceptor compartment which contains all organelles necessary for cell metabolism. The connecting cilium correlates with the transition zone, the short junction between the basal body and the axoneme of a prototypic cilium. The connecting cilium and the calycal processes, including the periciliary ridge complex, as well as the basal body complex are in close functional association with each other. In the latter ciliary compartments, the export and import from/into the outer segment of the photoreceptor cell are controlled and regulated. In all subciliary compartments, proteins are arranged in functional multiprotein complexes. In the outer segment, signaling components are arranged into complexes which provide specificity and speed for the signaling and serve in adaptation. Centrin-G-protein complexes may regulate the light driven translocation of the visual G-protein transducin through the connecting cilium. Intraflagellar transport (IFT) complexes may serve in intersegmental exchange of molecules. The import/export of molecules is thought to be regulated by proteins arranged in networks at the basal body complex. Proteins of the interactome related to the human Usher syndrome are localized in the connecting cilium and may participate in the ciliary transport, but are also arranged at interfaces between the inner segment and the connecting cilium where they probably control the cargo handover between the transport systems of the inner segment and these of the cilium. Furthermore, USH protein complexes may further provide mechanical stabilization to membrane specializations of the calycal processes and the connecting cilium. The protein complex in which the retinitis pigmentosa GTPase regulator (RPGR) participates in the ciliary compartments also plays a key role in the function and maintenance of photoreceptor cells. It further associates through the presumed scaffolding protein RPGRIP1 with the nephrocystin protein network. Although many of these proteins have been also found in prototypic cilia or primary cilia, the arrangements of the proteins in complexes can be specific for vertebrate photoreceptor cells. Defects of proteins in these complexes lead to photoreceptor cell death and retinal degeneration, underlying syndromic and non-syndromic blindness.

The intraflagellar transport protein IFT20 is associated with the Golgi complex and is required for cilia assembly

Eukaryotic cilia are assembled via intraflagellar transport (IFT) in which large protein particles are motored along ciliary microtubules. The IFT particles are composed of at least 17 polypeptides that are thought to contain binding sites for various cargos that need to be transported from their site of synthesis in the cell body to the site of assembly in the cilium. We show here that the IFT20 subunit of the particle is localized to the Golgi complex in addition to the basal body and cilia where all previous IFT particle proteins had been found. In living cells, fluorescently tagged IFT20 is highly dynamic and moves between the Golgi complex and the cilium as well as along ciliary microtubules. Strong knock down of IFT20 in mammalian cells blocks ciliary assembly but does not affect Golgi structure. Moderate knockdown does not block cilia assembly but reduces the amount of polycystin-2 that is localized to the cilia. This work suggests that IFT20 functions in the delivery of ciliary membrane proteins from the Golgi complex to the cilium.

The primary cilium as the cell's antenna: signaling at a sensory organelle

Almost every vertebrate cell has a specialized cell surface projection called a primary cilium. Although these structures were first described more than a century ago, the full scope of their functions remains poorly understood. Here, we review emerging evidence that in addition to their well-established roles in sight, smell, and mechanosensation, primary cilia are key participants in intercellular signaling. This new appreciation of primary cilia as cellular antennae that sense a wide variety of signals could help explain why ciliary defects underlie such a wide range of human disorders, including retinal degeneration, polycystic kidney disease, Bardet-Biedl syndrome, and neural tube defects.

Intraflagellar Transport Particles Participate Directly in Cilium-Generated Signaling in Chlamydomonas

Primary cilia are widely used for signal transduction during development and in homeostasis and are assembled and maintained by intraflagellar transport (IFT). Here, we have dissected the role of IFT in signaling within the flagella (structural and functional counterparts of cilia) of the biflagellated green alga Chlamydomonas. Using a conditional IFT mutant enables us to deplete the IFT machinery from intact, existing flagella. We identify a cGMP-dependent protein kinase (CrPKG) within flagella as the substrate of a protein tyrosine kinase activated by flagellar adhesion during fertilization. We demonstrate that flagellar adhesion stimulates association of CrPKG with a new flagellar compartment. Moreover, formation of the compartment requires IFT, and IFT particles themselves are part of the compartment. Our results lead to a model in which the IFT machinery is required not only for assembling cilia and flagella but also for organizing a signaling pathway within the organelles during cilium-generated signaling.

Evolution of intraflagellar transport from coated vesicles and autogenous origin of the eukaryotic cilium

The cilium/flagellum is a sensory-motile organelle ancestrally present in eukaryotic cells. For assembly cilia universally rely on intraflagellar transport (IFT), a specialised bidirectional transport process mediated by the ancestral and conserved IFT complex. Based on the homology of IFT complex proteins to components of coat protein I (COPI) and clathrin-coated vesicles, we propose that the non- vesicular, membrane-bound IFT evolved as a specialised form of coated vesicle transport from a protocoatomer complex. IFT thus shares common ancestry with all protocoatomer derivatives, including all vesicle coats and the nuclear pore complex (NPC). This has major implications for the evolutionary origin of the cilium. First, it reinforces the tenet that duplication and divergence of pre-existing structures, rather than symbiosis, were the major themes during cilium evolution. Second, it suggests that the initial step in the autogenous origin of the cilium was the establishment of a membrane patch with transmembrane proteins transported by the ancestral vesicle-coating IFT complex. We propose a scenario for how the initial membrane patch gradually protruded to enhance exposure to the environment, then started to move, and finally compartmentalised to render receptor signalling and ciliary beating more efficient.

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.

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.

The intraflagellar transport machinery of Chlamydomonas reinhardtii

First discovered in the green alga, Chlamydomonas, intraflagellar transport (IFT) is the bidirectional movement of protein particles along the length of eukaryotic cilia and flagella. Composed of approximately 16 different proteins, IFT particles are moved out to the distal tip of the organelle by kinesin-II and are brought back to the cell body by cytoplasmic dynein 1b. Mutant analysis of the IFT motor and particle proteins using diverse organisms has revealed a conserved and essential role for IFT in the assembly and maintenance of cilia and flagella. IFT is thought to mediate this assembly through the delivery of axonemal precursors out to the distal tip of the growing organelle. Consistent with this model, the IFT particle proteins are rich in protein-protein binding motifs, suggesting that the particles may act as scaffolds for the binding of multiple cargoes. With most of the IFT proteins now identified at the level of the gene, this review will briefly examine both the structure and function of the IFT machinery of Chlamydomonas reinhardtii.