Polyubiquitylation of α-tubulin at K304 is required for flagellar disassembly in Chlamydomonas

Efficient methods for multiple types of precise gene-editing in Chlamydomonas

Precise gene-editing using CRISPR/Cas9 technology remains a long-standing challenge, especially for genes with low expression and no selectable phenotypes in Chlamydomonas reinhardtii, a classic model for photosynthesis and cilia research. Here, we developed a multi-type and precise genetic manipulation method in which a DNA break was generated by Cas9 nuclease and the repair was mediated using a homologous DNA template. The efficacy of this method was demonstrated for several types of gene editing, including inactivation of two low-expression genes (CrTET1 and CrKU80), the introduction of a FLAG-HA epitope tag into VIPP1, IFT46, CrTET1 and CrKU80 genes, and placing a YFP tag into VIPP1 and IFT46 for live-cell imaging. We also successfully performed a single amino acid substitution for the FLA3, FLA10 and FTSY genes, and documented the attainment of the anticipated phenotypes. Lastly, we demonstrated that precise fragment deletion from the 3'-UTR of MAA7 and VIPP1 resulted in a stable knock-down effect. Overall, our study has established efficient methods for multiple types of precise gene editing in Chlamydomonas, enabling substitution, insertion and deletion at the base resolution, thus improving the potential of this alga in both basic research and industrial applications.

The ancestral ESCRT protein TOM1L2 selects ubiquitinated cargoes for retrieval from cilia

Many G protein-coupled receptors (GPCRs) reside within cilia of mammalian cells and must undergo regulated exit from cilia for the appropriate transduction of signals such as hedgehog morphogens. Lysine 63-linked ubiquitin (UbK63) chains mark GPCRs for regulated removal from cilia, but the molecular basis of UbK63 recognition inside cilia remains elusive. Here, we show that the BBSome-the trafficking complex in charge of retrieving GPCRs from cilia-engages the ancestral endosomal sorting factor target of Myb1-like 2 (TOM1L2) to recognize UbK63 chains within cilia of human and mouse cells. TOM1L2 directly binds to UbK63 chains and the BBSome, and targeted disruption of the TOM1L2/BBSome interaction results in the accumulation of TOM1L2, ubiquitin, and the GPCRs SSTR3, Smoothened, and GPR161 inside cilia. Furthermore, the single-cell alga Chlamydomonas also requires its TOM1L2 ortholog in order to clear ubiquitinated proteins from cilia. We conclude that TOM1L2 broadly enables the retrieval of UbK63-tagged proteins by the ciliary trafficking machinery.

Tubulin Post-Translational Modifications: The Elusive Roles of Acetylation

Microtubules (MTs), dynamic polymers of α/β-tubulin heterodimers found in all eukaryotes, are involved in cytoplasm spatial organization, intracellular transport, cell polarity, migration and division, and in cilia biology. MTs functional diversity depends on the differential expression of distinct tubulin isotypes and is amplified by a vast number of different post-translational modifications (PTMs). The addition/removal of PTMs to α- or β-tubulins is catalyzed by specific enzymes and allows combinatory patterns largely enriching the distinct biochemical and biophysical properties of MTs, creating a code read by distinct proteins, including microtubule-associated proteins (MAPs), which allow cellular responses. This review is focused on tubulin-acetylation, whose cellular roles continue to generate debate. We travel through the experimental data pointing to α-tubulin Lys40 acetylation role as being a MT stabilizer and a typical PTM of long lived MTs, to the most recent data, suggesting that Lys40 acetylation enhances MT flexibility and alters the mechanical properties of MTs, preventing MTs from mechanical aging characterized by structural damage. Additionally, we discuss the regulation of tubulin acetyltransferases/desacetylases and their impacts on cell physiology. Finally, we analyze how changes in MT acetylation levels have been found to be a general response to stress and how they are associated with several human pathologies.

A targeted multi-proteomics approach generates a blueprint of the ciliary ubiquitinome

Establishment and maintenance of the primary cilium as a signaling-competent organelle requires a high degree of fine tuning, which is at least in part achieved by a variety of post-translational modifications. One such modification is ubiquitination. The small and highly conserved ubiquitin protein possesses a unique versatility in regulating protein function via its ability to build mono and polyubiquitin chains onto target proteins. We aimed to take an unbiased approach to generate a comprehensive blueprint of the ciliary ubiquitinome by deploying a multi-proteomics approach using both ciliary-targeted ubiquitin affinity proteomics, as well as ubiquitin-binding domain-based proximity labelling in two different mammalian cell lines. This resulted in the identification of several key proteins involved in signaling, cytoskeletal remodeling and membrane and protein trafficking. Interestingly, using two different approaches in IMCD3 and RPE1 cells, respectively, we uncovered several novel mechanisms that regulate cilia function. In our IMCD3 proximity labeling cell line model, we found a highly enriched group of ESCRT-dependent clathrin-mediated endocytosis-related proteins, suggesting an important and novel role for this pathway in the regulation of ciliary homeostasis and function. In contrast, in RPE1 cells we found that several structural components of caveolae (CAV1, CAVIN1, and EHD2) were highly enriched in our cilia affinity proteomics screen. Consistently, the presence of caveolae at the ciliary pocket and ubiquitination of CAV1 specifically, were found likely to play a role in the regulation of ciliary length in these cells. Cilia length measurements demonstrated increased ciliary length in RPE1 cells stably expressing a ubiquitination impaired CAV1 mutant protein. Furthermore, live cell imaging in the same cells revealed decreased CAV1 protein turnover at the cilium as the possible cause for this phenotype. In conclusion, we have generated a comprehensive list of cilia-specific proteins that are subject to regulation via ubiquitination which can serve to further our understanding of cilia biology in health and disease.

The role of exercise intensity on fatty liver in rats

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease and is often caused by obesity. Currently, moderate-intensity continuous training (MICT) and high-intensity interval training (HIIT) are two effective treatments for reducing fat mass in patients with obesity and NAFLD. However, the comparative fat-reducing effects and underlying molecular mechanisms of MICT and HIIT remain unclear. This comprehensive study was performed on male Wistar rats treated with standard diet, high-fat diet, MICT, and HIIT to explore their comparative fat-reducing effects and corresponding molecular mechanisms. HIIT had a greater effect on hepatic vacuolation density and lipid content reduction than MICT, and triglyceride and total cholesterol levels in the serum and the liver demonstrated different sensitivities to different exercise training programs. At the molecular level, both MICT and HIIT altered the processes of fatty acid synthesis, fatty acid transport, fatty acid β-oxidation, and cholesterol synthesis, wherein the transcriptional and translational levels of signaling molecules peroxisome proliferator-activated receptors (PPARs) regulating fatty acid and cholesterol synthesis were strongly changed. Moreover, the metabolic pathways of amino acids, bile acids, and carbohydrates were also affected according to transcriptome analysis, and the changes in the above-mentioned processes in the HIIT group were greater than those in the MICT group. In combination with the search tool for the retrieval of interacting genes/proteins (STRING) analysis and the role of PPARs in lipid metabolism, as well as the expression pattern of PPARs in the MICT and HIIT groups, the MICT-and HIIT-induced fat loss was mediated by the PPAR pathway, causing feedback responses in fatty acid, steroid, amino acid, bile acid, and carbohydrate metabolism, and HIIT had a better fat-reducing effect, which may be initiated by PPAR-α. This study provides a theoretical basis for targeted therapy of patients with obesity and NAFLD.

Regulation of Host-Pathogen Interactions via the Ubiquitin System

Ubiquitination is a posttranslational modification that regulates a multitude of cellular functions. Pathogens, such as bacteria and viruses, have evolved sophisticated mechanisms that evade or counteract ubiquitin-dependent host responses, or even exploit the ubiquitin system to their own advantage. This is largely done by numerous pathogen virulence factors that encode E3 ligases and deubiquitinases, which are often used as weapons in pathogen-host cell interactions. Moreover, upon pathogen attack, host cellular signaling networks undergo major ubiquitin-dependent changes to protect the host cell, including coordination of innate immunity, remodeling of cellular organelles, reorganization of the cytoskeleton, and reprogramming of metabolic pathways to restrict growth of the pathogen. Here we provide mechanistic insights into ubiquitin regulation of host-pathogen interactions and how it affects bacterial and viral pathogenesis and the organization and response of the host cell.

Molecular Evolution of Tubulins in Diatoms

Microtubules are formed by α- and β-tubulin heterodimers nucleated with γ-tubulin. Tubulins are conserved eukaryotic proteins. Previously, it was shown that microtubules are involved in diatom silica frustule morphogenesis. Diatom frustules are varied, and their morphology is species-specific. Despite the attractiveness of the problem of elucidating the molecular mechanisms of genetically programmed morphogenesis, the structure and evolution of diatom tubulins have not been studied previously. Based on available genomic and transcriptome data, we analyzed the phylogeny of the predicted amino acid sequences of diatom α-, β- and γ-tubulins and identified five groups for α-tubulins, six for β-tubulins and four for γ-tubulins. We identified characteristic amino acids of each of these groups and also analyzed possible posttranslational modification sites of diatom tubulins. According to our results, we assumed what changes occurred in the diatom tubulin structures during their evolution. We also identified which tubulin groups are inherent in large diatom taxa. The similarity between the evolution of diatom tubulins and the evolution of diatoms suggests that molecular changes in α-, β- and γ-tubulins could be one of the factors in the formation of a high morphological diversity of diatoms.

Intraflagellar Transport Proteins as Regulators of Primary Cilia Length

Primary cilia are small, antenna-like organelles that detect and transduce chemical and mechanical cues in the extracellular environment, regulating cell behavior and, in turn, tissue development and homeostasis. Primary cilia are assembled via intraflagellar transport (IFT), which traffics protein cargo bidirectionally along a microtubular axoneme. Ranging from 1 to 10 μm long, these organelles typically reach a characteristic length dependent on cell type, likely for optimum fulfillment of their specific roles. The importance of an optimal cilia length is underscored by the findings that perturbation of cilia length can be observed in a number of cilia-related diseases. Thus, elucidating mechanisms of cilia length regulation is important for understanding the pathobiology of ciliary diseases. Since cilia assembly/disassembly regulate cilia length, we review the roles of IFT in processes that affect cilia assembly/disassembly, including ciliary transport of structural and membrane proteins, ectocytosis, and tubulin posttranslational modification. Additionally, since the environment of a cell influences cilia length, we also review the various stimuli encountered by renal epithelia in healthy and diseased states that alter cilia length and IFT.

Phosphorylation and Ubiquitylation Regulate Protein Trafficking, Signaling, and the Biogenesis of Primary Cilia

The primary cilium is a solitary, microtubule-based membrane protrusion extending from the surface of quiescent cells that senses the cellular environment and triggers specific cellular responses. The functions of primary cilia require not only numerous different components but also their regulated interplay. The cilium performs highly dynamic processes, such as cell cycle-dependent assembly and disassembly as well as delivery, modification, and removal of signaling components to perceive and process external signals. On a molecular level, these processes often rely on a stringent control of key modulatory proteins, of which the activity, localization, and stability are regulated by post-translational modifications (PTMs). While an increasing number of PTMs on ciliary components are being revealed, our knowledge on the identity of the modifying enzymes and their modulation is still limited. Here, we highlight recent findings on cilia-specific phosphorylation and ubiquitylation events. Shedding new light onto the molecular mechanisms that regulate the sensitive equilibrium required to maintain and remodel primary cilia functions, we discuss their implications for cilia biogenesis, protein trafficking, and cilia signaling processes.

Ubiquitin links smoothened to intraflagellar transport to regulate Hedgehog signaling

In the absence of Hedgehog ligand, patched-1 (Ptch1) localizes to cilia and prevents ciliary accumulation and activation of smoothened (Smo). Upon ligand binding, Ptch1 is removed from cilia, and Smo is derepressed and accumulates in cilia where it activates signaling. The mechanisms regulating these dynamic movements are not well understood, but defects in intraflagellar transport components, including Ift27 and the BBSome, cause Smo to accumulate in cilia without pathway activation. We find that in the absence of ligand-induced pathway activation, Smo is ubiquitinated and removed from cilia, and this process is dependent on Ift27 and BBSome components. Activation of Hedgehog signaling decreases Smo ubiquitination and ciliary removal, resulting in its accumulation. Blocking ubiquitination of Smo by an E1 ligase inhibitor or by mutating two lysine residues in intracellular loop three causes Smo to aberrantly accumulate in cilia without pathway activation. These data provide a mechanism to control Smo's ciliary level during Hedgehog signaling by regulating the ubiquitination state of the receptor.

SUMOylation of α-tubulin is a novel modification regulating microtubule dynamics

Microtubules (MTs) are regulated by a number of known posttranslational modifications (PTMs) on α/β-tubulin to fulfill diverse cellular functions. Here, we showed that SUMOylation is a novel PTM on α-tubulin in vivo and in vitro. The SUMOylation on α-tubulin mainly occurred at Lys 96 (K96), K166, and K304 of soluble α-tubulin and could be removed by small ubiquitin-related modifier (SUMO)-specific peptidase 1. In vitro experiments showed that tubulin SUMOylation could reduce interprotofilament interaction, promote MT catastrophe, and impede MT polymerization. In cells, mutation of the SUMOylation sites on α-tubulin reduced catastrophe frequency and increased the proportion of polymerized α-tubulin, while upregulation of SUMOylation with fusion of SUMO1 reduced α-tubulin assembly into MTs. Additionally, overexpression of SUMOylation-deficient α-tubulin attenuated the neurite extension in Neuro-2a cells. Thus, SUMOylation on α-tubulin represents a new player in the regulation of MT properties.

Ubiquitin chains earmark GPCRs for BBSome-mediated removal from cilia

Regulated trafficking of G protein-coupled receptors (GPCRs) controls cilium-based signaling pathways. β-Arrestin, a molecular sensor of activated GPCRs, and the BBSome, a complex of Bardet-Biedl syndrome (BBS) proteins, are required for the signal-dependent exit of ciliary GPCRs, but the functional interplay between β-arrestin and the BBSome remains elusive. Here we find that, upon activation, ciliary GPCRs become tagged with ubiquitin chains comprising K63 linkages (UbK63) in a β-arrestin-dependent manner before BBSome-mediated exit. Removal of ubiquitin acceptor residues from the somatostatin receptor 3 (SSTR3) and from the orphan GPCR GPR161 demonstrates that ubiquitination of ciliary GPCRs is required for their regulated exit from cilia. Furthermore, targeting a UbK63-specific deubiquitinase to cilia blocks the exit of GPR161, SSTR3, and Smoothened (SMO) from cilia. Finally, ubiquitinated proteins accumulate in cilia of mammalian photoreceptors and Chlamydomonas cells when BBSome function is compromised. We conclude that Ub chains mark GPCRs and other unwanted ciliary proteins for recognition by the ciliary exit machinery.

Emerging Functions of Actins and Actin Binding Proteins in Trypanosomatids

Actin is the major protein constituent of the cytoskeleton that performs wide range of cellular functions. It exists in monomeric and filamentous forms, dynamics of which is regulated by a large repertoire of actin binding proteins. However, not much was known about existence of these proteins in trypanosomatids, till the genome sequence data of three important organisms of this class, viz. Trypanosoma brucei, Trypanosoma cruzi and Leishmania major, became available. Here, we have reviewed most of the findings reported to date on the intracellular distribution, structure and functions of these proteins and based on them, we have hypothesized some of their functions. The major findings are as follows: (1) All the three organisms encode at least a set of ten actin binding proteins (profilin, twinfilin, ADF/cofilin, CAP/srv2, CAPz, coronin, two myosins, two formins) and one isoform of actin, except that T. cruzi encodes for three formins and several myosins along with four actins. (2) Actin 1 and a few actin binding proteins (ADF/cofilin, profilin, twinfilin, coronin and myosin13 in L. donovani; ADF/cofilin, profilin and myosin1 in T. brucei; profilin and myosin-F in T.cruzi) have been identified and characterized. (3) In all the three organisms, actin cytoskeleton has been shown to regulate endocytosis and intracellular trafficking. (4) Leishmania actin1 has been the most characterized protein among trypanosomatid actins. (5) This protein is localized to the cytoplasm as well as in the flagellum, nucleus and kinetoplast, and in vitro, it binds to DNA and displays scDNA relaxing and kDNA nicking activities. (6) The pure protein prefers to form bundles instead of thin filaments, and does not bind DNase1 or phalloidin. (7) Myosin13, myosin1 and myosin-F regulate endocytosis and intracellular trafficking, respectively, in Leishmania, T. brucei and T. cruzi. (8) Actin-dependent myosin13 motor is involved in dynamics and assembly of Leishmania flagellum. (9) Leishmania twinfilin localizes mostly to the nucleolus and coordinates karyokinesis by effecting splindle elongation and DNA synthesis. (10) Leishmania coronin binds and promotes actin filament formation and exists in tetrameric form rather than trimeric form, like other coronins. (11) Trypanosomatid profilins are essential for survival of all the three parasites.

Cilium axoneme internalization and degradation in chytrid fungi

Loss of the cilium is important for cell cycle progression and certain developmental transitions. Chytrid fungi are a group of basal fungi that have retained centrioles and cilia, and they can disassemble their cilia via axoneme internalization as part of the transition from free-swimming spores to sessile sporangia. While this type of cilium disassembly has been observed in many single-celled eukaryotes, it has not been well characterized because it is not observed in common model organisms. To better characterize cilium disassembly via axoneme internalization, we focused on chytrids Rhizoclosmatium globosum and Spizellomyces punctatus to represent two lineages of chytrids with different motility characteristics. Our results show that each chytrid species can reel in its axoneme into the cell body along its cortex on the order of minutes, while S. punctatus has additional faster ciliary compartment loss and lash-around mechanisms. S. punctatus retraction can also occur away from the cell cortex and is partially actin dependent. Post-internalization, the tubulin of the axoneme is degraded in both chytrids over the course of about 2 hr. Axoneme disassembly and axonemal tubulin degradation are both partially proteasome dependent. Overall, chytrid cilium disassembly is a fast process that separates axoneme internalization and degradation.

Structural insights into the architecture and assembly of eukaryotic flagella

Cilia and flagella are slender projections found on most eukaryotic cells including unicellular organisms such as Chlamydomonas, Trypanosoma and Tetrahymena, where they serve motility and signaling functions. The cilium is a large molecular machine consisting of hundreds of different proteins that are trafficked into the organelle to organize a repetitive microtubule-based axoneme. Several recent studies took advantage of improved cryo-EM methodology to unravel the high-resolution structures of ciliary complexes. These include the recently reported purification and structure determination of axonemal doublet microtubules from the green algae Chlamydomonas reinhardtii, which allows for the modeling of more than 30 associated protein factors to provide deep molecular insight into the architecture and repetitive nature of doublet microtubules. In addition, we will review several recent contributions that dissect the structure and function of ciliary trafficking complexes that ferry structural and signaling components between the cell body and the cilium organelle.

Distinctive roles of Abi1 in regulating actin-associated proteins during human smooth muscle cell migration

Smooth muscle cell migration is essential for many diverse biological processes such as pulmonary/cardiovascular development and homeostasis. Abi1 (Abelson interactor 1) is an adapter protein that has been implicated in nonmuscle cell migration. However, the role and mechanism of Abi1 in smooth muscle migration are largely unknown. Here, Abi1 knockdown by shRNA reduced human airway smooth muscle cell migration, which was restored by Abi1 rescue. Abi1 localized at the tip of lamellipodia and its protrusion coordinated with F-actin at the leading cell edge of live cells. In addition, we identified profilin-1 (Pfn-1), a G-actin transporter, as a new partner for Abi1. Abi1 knockdown reduced the recruitment of Pfn-1 to the leading cell edge. Moreover, Abi1 knockdown reduced the localization of the actin-regulatory proteins c-Abl (Abelson tyrosine kinase) and N-WASP (neuronal Wiskott–Aldrich Syndrome Protein) at the cell edge without affecting other migration-related proteins including pVASP (phosphorylated vasodilator stimulated phosphoprotein), cortactin and vinculin. Furthermore, we found that c-Abl and integrin β1 regulated the positioning of Abi1 at the leading edge. Taken together, the results suggest that Abi1 regulates cell migration by affecting Pfn-1 and N-WASP, but not pVASP, cortactin and focal adhesions. Integrin β1 and c-Abl are important for the recruitment of Abi1 to the leading edge.

Deciphering the role of UBA-like domains in intraflagellar distribution and functions of myosin XXI in Leishmania

Myosin XXI (Myo21) is a novel class of myosin present in all kinetoplastid parasites, such as Trypanosoma and Leishmania. This protein in Leishmania promastigotes is predominantly localized to the proximal region of the flagellum, and is involved in the flagellum assembly, cell motility and intracellular vesicle transport. As Myo21 contains two ubiquitin associated (UBA)-like domains (UBLD) in its amino acid sequence, we considered it of interest to analyze the role of these domains in the intracellular distribution and functions of this protein in Leishmania cells. In this context, we created green fluorescent protein (GFP)-conjugates of Myo21 constructs lacking one of the two UBLDs at a time or both the UBLDs as well as GFP-conjugates of only the two UBLDs and Myo21 tail lacking the two UBLDs and separately expressed them in the Leishmania cells. Our results show that unlike Myo21-GFP, Myo21-GFP constructs lacking either one or both the UBLDs failed to concentrate and co-distribute with actin in the proximal region of the flagellum. Nevertheless, the GFP conjugate of only the two UBLDs was found to predominantly localize to the flagellum base. Additionally, the cells that expressed only one or both the UBLDs-deleted Myo21-GFP constructs possessed shorter flagellum and displayed slower motility, compared to Myo21-GFP expressing cells. Further, the intracellular vesicle transport and cell growth were severely impaired in the cells that expressed both the UBLDs deleted Myo21-GFP construct, but in contrast, virtually no effect was observed on the intracellular vesicle transport and growth in the cells that expressed single UBLD deleted mutant proteins. Moreover, the observed slower growth of both the UBLDs-deleted Myo21-GFP expressing cells was primarily due to delayed G2/M phase caused by aberrant nuclear and daughter cell segregation during their cell division process. These results taken together clearly reveal that the presence of UBLDs in Myo21 are essentially required for its predominant localization to the flagellum base, and perhaps also in its involvement in the flagellum assembly and cell division. Possible role of UBLDs in involvement of Myo21 during Leishmania flagellum assembly and cell cycle is discussed.

The tubulin code and its role in controlling microtubule properties and functions

Microtubules are core components of the eukaryotic cytoskeleton with essential roles in cell division, shaping, motility and intracellular transport. Despite their functional heterogeneity, microtubules have a highly conserved structure made from almost identical molecular building blocks: the tubulin proteins. Alternative tubulin isotypes and a variety of post-translational modifications control the properties and functions of the microtubule cytoskeleton, a concept known as the 'tubulin code'. Here we review the current understanding of the molecular components of the tubulin code and how they impact microtubule properties and functions. We discuss how tubulin isotypes and post-translational modifications control microtubule behaviour at the molecular level and how this translates into physiological functions at the cellular and organism levels. We then go on to show how fine-tuning of microtubule function by some tubulin modifications can affect homeostasis and how perturbation of this fine-tuning can lead to a range of dysfunctions, many of which are linked to human disease.

Cell Cycle-Dependent Flagellar Disassembly in a Firebug Trypanosomatid Leptomonas pyrrhocoris

Current understanding of flagellum/cilium length regulation focuses on a few model organisms with flagella of uniform length. Leptomonas pyrrhocoris is a monoxenous trypanosomatid parasite of firebugs. When cultivated in vitro, L. pyrrhocoris duplicates every 4.2 ± 0.2 h, representing the shortest doubling time reported for trypanosomatids so far. Each L. pyrrhocoris cell starts its cell cycle with a single flagellum. A new flagellum is assembled de novo, while the old flagellum persists throughout the cell cycle. The flagella in an asynchronous L. pyrrhocoris population exhibited a vast length variation of ∼3 to 24 μm, casting doubt on the presence of a length regulation mechanism based on a single balance point between the assembly and disassembly rate in these cells. Through imaging of live L. pyrrhocoris cells, a rapid, partial disassembly of the existing, old flagellum is observed upon, if not prior to, the initial assembly of a new flagellum. Mathematical modeling demonstrated an inverse correlation between the flagellar growth rate and flagellar length and inferred the presence of distinct, cell cycle-dependent disassembly mechanisms with different rates. On the basis of these observations, we proposed a min-max model that could account for the vast flagellar length range observed for asynchronous L. pyrrhocoris. This model may also apply to other flagellated organisms with flagellar length variation.IMPORTANCE Current understanding of flagellum biogenesis during the cell cycle in trypanosomatids is limited to a few pathogenic species, including Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp. The most notable characteristics of trypanosomatid flagella studied so far are the extreme stability and lack of ciliary disassembly/absorption during the cell cycle. This is different from cilia in Chlamydomonas and mammalian cells, which undergo complete absorption prior to cell cycle initiation. In this study, we examined flagellum duplication during the cell cycle of Leptomonas pyrrhocoris With the shortest duplication time documented for all Trypanosomatidae and its amenability to culture on agarose gel with limited mobility, we were able to image these cells through the cell cycle. Rapid, cell cycle-specific flagellum disassembly different from turnover was observed for the first time in trypanosomatids. Given the observed length-dependent growth rate and the presence of different disassembly mechanisms, we proposed a min-max model that can account for the flagellar length variation observed in L. pyrrhocoris.

LF4/MOK and a CDK-related kinase regulate the number and length of cilia in Tetrahymena

The length of cilia is controlled by a poorly understood mechanism that involves members of the conserved RCK kinase group, and among them, the LF4/MOK kinases. The multiciliated protist model, Tetrahymena, carries two types of cilia (oral and locomotory) and the length of the locomotory cilia is dependent on their position with the cell. In Tetrahymena, loss of an LF4/MOK ortholog, LF4A, lengthened the locomotory cilia, but also reduced their number. Without LF4A, cilia assembled faster and showed signs of increased intraflagellar transport (IFT). Consistently, overproduced LF4A shortened cilia and downregulated IFT. GFP-tagged LF4A, expressed in the native locus and imaged by total internal reflection microscopy, was enriched at the basal bodies and distributed along the shafts of cilia. Within cilia, most LF4A-GFP particles were immobile and a few either diffused or moved by IFT. We suggest that the distribution of LF4/MOK along the cilium delivers a uniform dose of inhibition to IFT trains that travel from the base to the tip. In a longer cilium, the IFT machinery may experience a higher cumulative dose of inhibition by LF4/MOK. Thus, LF4/MOK activity could be a readout of cilium length that helps to balance the rate of IFT-driven assembly with the rate of disassembly at steady state. We used a forward genetic screen to identify a CDK-related kinase, CDKR1, whose loss-of-function suppressed the shortening of cilia caused by overexpression of LF4A, by reducing its kinase activity. Loss of CDKR1 alone lengthened both the locomotory and oral cilia. CDKR1 resembles other known ciliary CDK-related kinases: LF2 of Chlamydomonas, mammalian CCRK and DYF-18 of C. elegans, in lacking the cyclin-binding motif and acting upstream of RCKs. The new genetic tools we developed here for Tetrahymena have potential for further dissection of the principles of cilia length regulation in multiciliated cells.

First person – Qiyu Wang

First Person is a series of interviews with the first authors of a selection of papers published in Journal of Cell Science, helping early-career researchers promote themselves alongside their papers. Qiyu Wang is first author on ‘Poly-ubiquitylation of α-tubulin at K304 is required for flagellar disassembly in Chlamydomonas’, published in JCS. Qiyu is a PhD student in the lab of Kaiyao Huang in the Key Laboratory of Algal Biology, at the Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan, China, where he explores the role of post-translational modifications in cytoskeleton dynamics.