Components of a "dynein regulatory complex" are located at the junction between the radial spokes and the dynein arms in Chlamydomonas flagella

Chlamydomonas as a model system to study cilia and flagella using genetics, biochemistry, and microscopy

The unicellular green alga, Chlamydomonas reinhardtii, has played a central role in discovering much of what is currently known about the composition, assembly, and function of cilia and flagella. Chlamydomonas combines excellent genetics, such as the ability to grow cells as haploids or diploids and to perform tetrad analysis, with an unparalleled ability to detach and isolate flagella in a single step without cell lysis. The combination of genetics and biochemistry that is possible in Chlamydomonas has allowed many of the key components of the cilium to be identified by looking for proteins that are missing in a defined mutant. Few if any other model organisms allow such a seamless combination of genetic and biochemical approaches. Other major advantages of Chlamydomonas compared to other systems include the ability to induce flagella to regenerate in a highly synchronous manner, allowing the kinetics of flagellar growth to be measured, and the ability of Chlamydomonas flagella to adhere to glass coverslips allowing Intraflagellar Transport to be easily imaged inside the flagella of living cells, with quantitative precision and single-molecule resolution. These advantages continue to work in favor of Chlamydomonas as a model system going forward, and are now augmented by extensive genomic resources, a knockout strain collection, and efficient CRISPR gene editing. While Chlamydomonas has obvious limitations for studying ciliary functions related to animal development or organ physiology, when it comes to studying the fundamental biology of cilia and flagella, Chlamydomonas is simply unmatched in terms of speed, efficiency, cost, and the variety of approaches that can be brought to bear on a question.

The MBO2/FAP58 heterodimer stabilizes assembly of inner arm dynein b and reveals axoneme asymmetries involved in ciliary waveform

Cilia generate three-dimensional waveforms required for cell motility and transport of fluid, mucus, and particles over the cell surface. This movement is driven by multiple dynein motors attached to nine outer doublet microtubules that form the axoneme. The outer and inner arm dyneins are organized into 96-nm repeats tandemly arrayed along the length of the doublets. Motility is regulated in part by projections from the two central pair microtubules that contact radial spokes located near the base of the inner dynein arms in each repeat. Although much is known about the structures and protein complexes within the axoneme, many questions remain about the regulatory mechanisms that allow the cilia to modify their waveforms in response to internal or external stimuli. Here, we used Chlamydomonas mbo (move backwards only) mutants with altered waveforms to identify at least two conserved proteins, MBO2/CCDC146 and FAP58/CCDC147, that form part of a L-shaped structure that varies between doublet microtubules. Comparative proteomics identified additional missing proteins that are altered in other motility mutants, revealing overlapping protein defects. Cryo-electron tomography and epitope tagging revealed that the L-shaped, MBO2/FAP58 structure interconnects inner dynein arms with multiple regulatory complexes, consistent with its function in modifying the ciliary waveform.

In vivo imaging reveals independent intraflagellar transport of the nexin-dynein regulatory complex subunits DRC2 and DRC4

Many axonemal proteins enter cilia and flagella on intraflagellar transport (IFT) trains, which move bidirectionally along the axonemal microtubules. Certain axonemal substructures including the radial spokes and outer dynein arms are preassembled in the cell body and transported as multisubunit complexes into flagella by IFT. Here, we used in vivo imaging to analyze the transport and assembly of DRC2 and DRC4, two core subunits of the nexin-dynein regulatory complex (N-DRC). Tagged DRC2 moved by IFT in mutants lacking DRC4 and vice versa, showing that they do not depend on each other for IFT. Simultaneous imaging of tagged DRC2 and DRC4, expressed from transgenes that rescue a corresponding double mutant, mostly showed transport on separate IFT trains, but occasional cotransports were also observed. The results demonstrate that DRC2 and DRC4 are transported largely independently of each other into flagella. These studies suggest that the N-DRC assembles onto the axoneme by the stepwise addition of subunits.

Morphological and Molecular Bases of Male Infertility: A Closer Look at Sperm Flagellum

Infertility is a major health problem worldwide without an effective therapy or cure. It is estimated to affect 8-12% of couples in the reproductive age group, equally affecting both genders. There is no single cause of infertility, and its knowledge is still far from complete, with about 30% of infertile couples having no cause identified (named idiopathic infertility). Among male causes of infertility, asthenozoospermia (i.e., reduced sperm motility) is one of the most observed, being estimated that more than 20% of infertile men have this condition. In recent years, many researchers have focused on possible factors leading to asthenozoospermia, revealing the existence of many cellular and molecular players. So far, more than 4000 genes are thought to be involved in sperm production and as regulators of different aspects of sperm development, maturation, and function, and all can potentially cause male infertility if mutated. In this review, we aim to give a brief overview of the typical sperm flagellum morphology and compile some of the most relevant information regarding the genetic factors involved in male infertility, with a focus on sperm immotility and on genes related to sperm flagellum development, structure, or function.

Bi-allelic variants in human TCTE1/DRC5 cause asthenospermia and male infertility

Asthenozoospermia (AZS) is a common male infertility phenotype, accounting for 18% of infertile patients. The N-DRC (Nexin-dynein Regulatory Complex) complex is the motor regulating device in the flagellum, which is found in most eukaryotic organisms with flagellum. The deletion of TCTE1 (T-Complex-Associated Testis-Expressed 1), a component of the N-DRC complex also known as DRC5 (Dynein regulatory complex subunit 5), has been shown to cause asthenospermia in mice. This study mainly introduces a clinical case of male infertility with normal sperm count, normal morphological structure, but low motility and weak forward movement. By whole-exome sequencing, we found that TCTE1 became a frameshift mutant, ENST00000371505.5: c.396_397insTC (p.Arg133Serfs*33), resulting in the rapid degradation of TCTE1 protein and male infertility. This phenotype is similar to the Tcte1^-/- (Tcte1 knockout) mice, which showed structural integrity but reduced motility. Further, different from mice, in vitro Fertilization (IVF) could successfully solve the patient's problem of infertility. Our data provides a better understanding of the biological functions of TCTE1 in human flagellum assembly and male fertility.

Centriole is the pivot coordinating dynamic signaling for cell proliferation and organization during early development in the vertebrates

Vertebrates have an elaborate and functionally segmented body. It evolves from a single cell by systematic cell proliferation but attains a complex body structure with exquisite precision. This development requires two cellular events: cell cycle and ciliogenesis. For these events, the dynamic molecular signaling is converged at the centriole. The cell cycle helps in cell proliferation and growth of the body and is a highly regulated and integrated process. Its errors cause malignancies and developmental disorders. The cells newly proliferated are organized during organogenesis. For a cellular organization, dedicated signaling hubs are developed in the cells, and most often cilia are utilized. The cilium is generated from one of the centrioles involved in cell proliferation. The developmental signaling pathways hosted in cilia are essential for the elaboration of the body plan. The cilium's compartmental seclusion is ideal for noise-free molecular signaling and is essential for the precision of the body layout. The dysfunctional centrioles and primary cilia distort the development of body layout that manifest as serious developmental disorders. Thus, centriole has a dual role in the growth and cellular organization. It organizes dynamically expressed molecules of cell cycle and ciliogenesis and plays a balancing act to generate new cells and organize them during development. A putative master molecule may regulate and coordinate the dynamic gene expression at the centrioles. The convergence of many critical signaling components at the centriole reiterates the idea that centriole is a major molecular workstation involved in elaborating the structural design and complexity in vertebrates. This article is protected by copyright. All rights reserved.

Motile ciliopathies

Motile cilia are highly complex hair-like organelles of epithelial cells lining the surface of various organ systems. Genetic mutations (usually with autosomal recessive inheritance) that impair ciliary beating cause a variety of motile ciliopathies, a heterogeneous group of rare disorders. The pathogenetic mechanisms, clinical symptoms and severity of the disease depend on the specific affected genes and the tissues in which they are expressed. Defects in the ependymal cilia can result in hydrocephalus, defects in the cilia in the fallopian tubes or in sperm flagella can cause female and male subfertility, respectively, and malfunctional motile monocilia of the left-right organizer during early embryonic development can lead to laterality defects such as situs inversus and heterotaxy. If mucociliary clearance in the respiratory epithelium is severely impaired, the disorder is referred to as primary ciliary dyskinesia, the most common motile ciliopathy. No single test can confirm a diagnosis of motile ciliopathy, which is based on a combination of tests including nasal nitric oxide measurement, transmission electron microscopy, immunofluorescence and genetic analyses, and high-speed video microscopy. With the exception of azithromycin, there is no evidence-based treatment for primary ciliary dyskinesia; therapies aim at relieving symptoms and reducing the effects of reduced ciliary motility.

Nexin-Dynein regulatory complex component DRC7 but not FBXL13 is required for sperm flagellum formation and male fertility in mice

Flagella and cilia are evolutionarily conserved cellular organelles. Abnormal formation or motility of these organelles in humans causes several syndromic diseases termed ciliopathies. The central component of flagella and cilia is the axoneme that is composed of the ‘9+2’ microtubule arrangement, dynein arms, radial spokes, and the Nexin-Dynein Regulatory Complex (N-DRC). The N-DRC is localized between doublet microtubules and has been extensively studied in the unicellular flagellate Chlamydomonas. Recently, it has been reported that TCTE1 (DRC5), a component of the N-DRC, is essential for proper sperm motility and male fertility in mice. Further, TCTE1 has been shown to interact with FBXL13 (DRC6) and DRC7; however, functional roles of FBXL13 and DRC7 in mammals have not been elucidated. Here we show that Fbxl13 and Drc7 expression are testes-enriched in mice. Although Fbxl13 knockout (KO) mice did not show any obvious phenotypes, Drc7 KO male mice were infertile due to their short immotile spermatozoa. In Drc7 KO spermatids, the axoneme is disorganized and the ‘9+2’ microtubule arrangement was difficult to detect. Further, other N-DRC components fail to incorporate into the flagellum without DRC7. These results indicate that Drc7, but not Fbxl13, is essential for the correct assembly of the N-DRC and flagella.

In recent years, almost one in six couples face infertility, and nearly 50% of cases are attributed to male factors. It has been shown that approximately 15% of male infertility is caused by genetic factors. The conditions of male infertility mainly include spermatozoa with abnormal morphology (teratozoospermia), reduced sperm motility (asthenozoospermia), and no or low sperm count (azoospermia). Multiple morphological abnormalities of the sperm flagella (MMAF) are characterized as asthenoteratozoospermia, which is a condition with abnormal sperm tail morphology, including absent, coiled, bent, or short tails. Sperm tails are formed during spermiogenesis; however, the mechanism that govern tail formation remains unclear. Here we mutated Fbxl13 and Drc7, two genes with strong expression in mouse testis and which have been shown to be important for flagellum formation and regulation in other systems. Deletion of Drc7 leads to aberrant tail formation in mouse spermatozoa that phenocopies patients with MMAF, while deletion of Fbxl13 has no observable effect on sperm function. Our results identified DRC7 as an important factor for sperm flagellum formation.

FAP57/WDR65 targets assembly of a subset of inner arm dyneins and connects to regulatory hubs in cilia

Ciliary motility depends on both the precise spatial organization of multiple dynein motors within the 96 nm axonemal repeat and the highly coordinated interactions between different dyneins and regulatory complexes located at the base of the radial spokes. Mutations in genes encoding cytoplasmic assembly factors, intraflagellar transport factors, docking proteins, dynein subunits, and associated regulatory proteins can all lead to defects in dynein assembly and ciliary motility. Significant progress has been made in the identification of dynein subunits and extrinsic factors required for preassembly of dynein complexes in the cytoplasm, but less is known about the docking factors that specify the unique binding sites for the different dynein isoforms on the surface of the doublet microtubules. We have used insertional mutagenesis to identify a new locus, IDA8/BOP2, required for targeting the assembly of a subset of inner dynein arms (IDAs) to a specific location in the 96 nm repeat. IDA8 encodes flagellar-associated polypeptide (FAP)57/WDR65, a highly conserved WD repeat, coiled coil domain protein. Using high resolution proteomic and structural approaches, we find that FAP57 forms a discrete complex. Cryo-electron tomography coupled with epitope tagging and gold labeling reveal that FAP57 forms an extended structure that interconnects multiple IDAs and regulatory complexes.

Chlamydomonas as a tool to study tubulin polyglutamylation

The diversity of α- and β-tubulin is facilitated by various post-translational modifications (PTMs), such as acetylation, tyrosination, glycylation, glutamylation, phosphorylation and methylation. These PTMs affect the stability and structure of microtubules as well as the interaction between microtubules and microtubule-associated proteins, including molecular motors. Therefore, it is extremely important to investigate the roles of tubulin PTMs for understanding the cell cycle, cell motility and intracellular trafficking. Tubulin PTMs were first studied in the 1980s, and considerable progress has been made since then; it is likely that additional mechanisms remain yet to be elucidated. Here, we discuss one such modification, tubulin glutamylation, and introduce our research on the eukaryotic flagellum of the unicellular green alga Chlamydomonas reinhardtii.

Turning dyneins off bends cilia

Ciliary and flagellar motility is caused by the ensemble action of inner and outer dynein arm motors acting on axonemal doublet microtubules. The switch point or switching hypothesis, for which much experimental and computational evidence exists, requires that dyneins on only one side of the axoneme are actively working during bending, and that this active motor region propagate along the axonemal length. Generation of a reverse bend results from switching active sliding to the opposite side of the axoneme. However, the mechanochemical states of individual dynein arms within both straight and curved regions and how these change during beating has until now eluded experimental observation. Recently, Lin and Nicastro used high-resolution cryo-electron tomography to determine the power stroke state of dyneins along flagella of sea urchin sperm that were rapidly frozen while actively beating. The results reveal that axonemal dyneins are generally in a pre-power stroke conformation that is thought to yield a force-balanced state in straight regions; inhibition of this conformational state and microtubule release on specific doublets may then lead to a force imbalance across the axoneme allowing for microtubule sliding and consequently the initiation and formation of a ciliary bend. Propagation of this inhibitory signal from base-to-tip and switching the microtubule doublet subsets that are inhibited is proposed to result in oscillatory motion.

Physcomitrella MADS-box genes regulate water supply and sperm movement for fertilization

MIKC classic (MIKC^C)-type MADS-box genes encode transcription factors that function in various developmental processes, including angiosperm floral organ identity. Phylogenetic analyses of the MIKC^C-type MADS-box family, including genes from non-flowering plants, suggest that the increased numbers of these genes in flowering plants is related to their functional divergence; however, their precise functions in non-flowering plants and their evolution throughout land plant diversification are unknown. Here, we show that MIKC^C-type MADS-box genes in the moss Physcomitrella patens function in two ways to enable fertilization. Analyses of protein localization, deletion mutants and overexpression lines of all six genes indicate that three MIKC^C-type MADS-box genes redundantly regulate cell division and growth in the stems for appropriate external water conduction, as well as the formation of sperm with motile flagella. The former function appears to be maintained in the flowering plant lineage, while the latter was lost in accordance with the loss of sperm.

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.

Three-dimensional structural labeling microscopy of cilia and flagella

Locating a molecule within a cell using protein-tagging and immunofluorescence is a fundamental technique in cell biology, whereas in three-dimensional electron microscopy, locating a subunit within a macromolecular complex remains challenging. Recently, we developed a new structural labeling method for cryo-electron tomography by taking advantage of the biotin-streptavidin system, and have intensively used this method to locate a number of proteins and protein domains in cilia and flagella. In this review, we summarize our findings on the three-dimensional architecture of the axoneme, especially the importance of coiled-coil proteins. In addition, we provide an overview of the technical aspects of our structural labeling method.

Ciliary Motility: Regulation of Axonemal Dynein Motors

Ciliary motility is crucial for the development and health of many organisms. Motility depends on the coordinated activity of multiple dynein motors arranged in a precise pattern on the outer doublet microtubules. Although significant progress has been made in elucidating the composition and organization of the dyneins, a comprehensive understanding of dynein regulation is lacking. Here, we focus on two conserved signaling complexes located at the base of the radial spokes. These include the I1/f inner dynein arm associated with radial spoke 1 and the calmodulin- and spoke-associated complex and the nexin-dynein regulatory complex associated with radial spoke 2. Current research is focused on understanding how these two axonemal hubs coordinate and regulate the dynein motors and ciliary motility.

Electrostatic interaction between polyglutamylated tubulin and the nexin-dynein regulatory complex regulates flagellar motility

The 3D localization of polyglutamylated tubulin in eukaryotic flagella is identified. Polyglutamylated tubulins are located on the interface between the microtubule and its cross-bridging structure, the N-DRC. Flagellar motility is regulated by electrostatic interaction between negatively charged tubulins and positively charged N-DRC.

Tubulins undergo various posttranslational modifications. Among them, polyglutamylation is involved in the motility of eukaryotic flagella and the stability of the axonemal microtubules. However, it remains unclear where polyglutamylated tubulin localizes precisely within the axoneme and how tubulin polyglutamylation affects flagellar motility. In this study, we identified the three-dimensional localization of the polyglutamylated tubulin in Chlamydomonas flagella using antibody labeling and cryo–electron tomography. Polyglutamylated tubulins specifically located in close proximity to a microtubule-cross-bridging structure called the nexin–dynein regulatory complex (N-DRC). Because N-DRC is positively charged, we hypothesized that there is an electrostatic interaction between the polyglutamylated tubulin and the N-DRC, and therefore we mutated the amino acid sequences of DRC4 to modify the charge of the N-DRC. We found that both augmentation and reduction of the positive charge on DRC4 resulted in reduced flagellar motility. Moreover, reduced motility in a mutant with a structurally defective N-DRC was partially restored by increasing the positive charge on DRC4. These results clearly indicate that beating motion of flagella is maintained by the electrostatic cross-bridge formed between the negatively charged polyglutamylated tubulins and the positively charged N-DRC.

Radial Spokes-A Snapshot of the Motility Regulation, Assembly, and Evolution of Cilia and Flagella

Propulsive forces generated by cilia and flagella are used in events that are critical for the thriving of diverse eukaryotic organisms in their environments. Despite distinctive strokes and regulations, the majority of them adopt the 9+2 axoneme that is believed to exist in the last eukaryotic common ancestor. Only a few outliers have opted for a simpler format that forsakes the signature radial spokes and the central pair apparatus, although both are unnecessary for force generation or rhythmicity. Extensive evidence has shown that they operate as an integral system for motility control. Recent studies have made remarkable progress on the radial spoke. This review will trace how the new structural, compositional, and evolutional insights pose significant implications on flagella biology and, conversely, ciliopathy.

Major regulatory mechanisms involved in sperm motility

The genetic bases and molecular mechanisms involved in the assembly and function of the flagellum components as well as in the regulation of the flagellar movement are not fully understood, especially in humans. There are several causes for sperm immotility, of which some can be avoided and corrected, whereas other are related to genetic defects and deserve full investigation to give a diagnosis to patients. This review was performed after an extensive literature search on the online databases PubMed, ScienceDirect, and Web of Science. Here, we review the involvement of regulatory pathways responsible for sperm motility, indicating possible causes for sperm immotility. These included the calcium pathway, the cAMP-dependent protein kinase pathway, the importance of kinases and phosphatases, the function of reactive oxygen species, and how the regulation of cell volume and osmolarity are also fundamental components. We then discuss main gene defects associated with specific morphological abnormalities. Finally, we slightly discuss some preventive and treatments approaches to avoid development of conditions that are associated with unspecified sperm immotility. We believe that in the near future, with the development of more powerful techniques, the genetic causes of sperm immotility and the regulatory mechanisms of sperm motility will be better understand, thus enabling to perform a full diagnosis and uncover new therapies.

Axoneme Structure from Motile Cilia

The axoneme is the main extracellular part of cilia and flagella in eukaryotes. It consists of a microtubule cytoskeleton, which normally comprises nine doublets. In motile cilia, dynein ATPase motor proteins generate sliding motions between adjacent microtubules, which are integrated into a well-orchestrated beating or rotational motion. In primary cilia, there are a number of sensory proteins functioning on membranes surrounding the axoneme. In both cases, as the study of proteomics has elucidated, hundreds of proteins exist in this compartmentalized biomolecular system. In this article, we review the recent progress of structural studies of the axoneme and its components using electron microscopy and X-ray crystallography, mainly focusing on motile cilia. Structural biology presents snapshots (but not live imaging) of dynamic structural change and gives insights into the force generation mechanism of dynein, ciliary bending mechanism, ciliogenesis, and evolution of the axoneme.

Mutation of Growth Arrest Specific 8 Reveals a Role in Motile Cilia Function and Human Disease

Ciliopathies are genetic disorders arising from dysfunction of microtubule-based cellular appendages called cilia. Different cilia types possess distinct stereotypic microtubule doublet arrangements with non-motile or 'primary' cilia having a 9+0 and motile cilia have a 9+2 array of microtubule doublets. Primary cilia are critical sensory and signaling centers needed for normal mammalian development. Defects in their structure/function result in a spectrum of clinical and developmental pathologies including abnormal neural tube and limb patterning. Altered patterning phenotypes in the limb and neural tube are due to perturbations in the hedgehog (Hh) signaling pathway. Motile cilia are important in fluid movement and defects in motility result in chronic respiratory infections, altered left-right asymmetry, and infertility. These features are the hallmarks of Primary Ciliary Dyskinesia (PCD, OMIM 244400). While mutations in several genes are associated with PCD in patients and animal models, the genetic lesion in many cases is unknown. We assessed the in vivo functions of Growth Arrest Specific 8 (GAS8). GAS8 shares strong sequence similarity with the Chlamydomonas Nexin-Dynein Regulatory Complex (NDRC) protein 4 (DRC4) where it is needed for proper flagella motility. In mammalian cells, the GAS8 protein localizes not only to the microtubule axoneme of motile cilia, but also to the base of non-motile cilia. Gas8 was recently implicated in the Hh signaling pathway as a regulator of Smoothened trafficking into the cilium. Here, we generate the first mouse with a Gas8 mutation and show that it causes severe PCD phenotypes; however, there were no overt Hh pathway phenotypes. In addition, we identified two human patients with missense variants in Gas8. Rescue experiments in Chlamydomonas revealed a subtle defect in swim velocity compared to controls. Further experiments using CRISPR/Cas9 homology driven repair (HDR) to generate one of these human missense variants in mice demonstrated that this allele is likely pathogenic.

The nexin link and B-tubule glutamylation maintain the alignment of outer doublets in the ciliary axoneme

We developed quantitative assays to test the hypothesis that the N-DRC is required for integrity of the ciliary axoneme. We examined reactivated motility of demembranated drc cells, commonly termed "reactivated cell models." ATP-induced reactivation of wild-type cells resulted in the forward swimming of ∼90% of cell models. ATP-induced reactivation failed in a subset of drc cell models, despite forward motility in live drc cells. Dark-field light microscopic observations of drc cell models revealed various degrees of axonemal splaying. In contrast, >98% of axonemes from wild-type reactivated cell models remained intact. The sup-pf4 and drc3 mutants, unlike other drc mutants, retain most of the N-DRC linker that interconnects outer doublet microtubules. Reactivated sup-pf4 and drc3 cell models displayed nearly wild-type levels of forward motility. Thus, the N-DRC linker is required for axonemal integrity. We also examined reactivated motility and axoneme integrity in mutants defective in tubulin polyglutamylation. ATP-induced reactivation resulted in forward swimming of >75% of tpg cell models. Analysis of double mutants defective in tubulin polyglutamylation and different regions of the N-DRC indicate B-tubule polyglutamylation and the distal lobe of the linker region are both important for axonemal integrity and normal N-DRC function. © 2016 Wiley Periodicals, Inc.

Mutations in GAS8, a Gene Encoding a Nexin-Dynein Regulatory Complex Subunit, Cause Primary Ciliary Dyskinesia with Axonemal Disorganization

Primary ciliary dyskinesia (PCD) is an autosomal recessive disease characterized by chronic respiratory infections of the upper and lower airways, hypofertility, and, in approximately half of the cases, situs inversus. This complex phenotype results from defects in motile cilia and sperm flagella. Among the numerous genes involved in PCD, very few-including CCDC39 and CCDC40-carry mutations that lead to a disorganization of ciliary axonemes with microtubule misalignment. Focusing on this particular phenotype, we identified bi-allelic loss-of-function mutations in GAS8, a gene that encodes a subunit of the nexin-dynein regulatory complex (N-DRC) orthologous to DRC4 of the flagellated alga Chlamydomonas reinhardtii. Unlike the majority of PCD patients, individuals with GAS8 mutations have motile cilia, which, as documented by high-speed videomicroscopy, display a subtle beating pattern defect characterized by slightly reduced bending amplitude. Immunofluorescence studies performed on patients' respiratory cilia revealed that GAS8 is not required for the proper expression of CCDC39 and CCDC40. Rather, mutations in GAS8 affect the subcellular localization of another N-DRC subunit called DRC3. Overall, this study, which identifies GAS8 as a PCD gene, unveils the key importance of the corresponding protein in N-DRC integrity and in the proper alignment of axonemal microtubules in humans.

Independent Control of the Static and Dynamic Components of the Chlamydomonas Flagellar Beat

When the green alga Chlamydomonas reinhardtii swims, it uses the breaststroke beat of its two flagella to pull itself forward [1]. The flagellar waveform can be decomposed into a static component, corresponding to an asymmetric time-averaged shape, and a dynamic component, corresponding to the time-varying wave [2]. Extreme lightening conditions photoshock the cell, converting the breaststroke beat into a symmetric sperm-like beat, which causes a reversal of the direction of swimming [3]. Waveform conversion is achieved by a reduction in magnitude of the static component, whereas the dynamic component remains unchanged [2]. The coupling between static and dynamic components, however, is poorly understood, and it is not known whether the static component requires the dynamic component or whether it can exist independently. We used isolated and reactivated axonemes [4] to investigate the relation between the two beat components. We discovered that, when reactivated in the presence of low ATP concentrations, axonemes displayed the static beat component in absence of the dynamic component. Furthermore, we found that the amplitudes of the two components depend on ATP in qualitatively different ways. These results show that the decomposition into static and dynamic components is not just a mathematical concept but that the two components can independently control different aspects of cell motility: the static component controls swimming direction, whereas the dynamic component provides propulsion.

Absence of Radial Spokes in Mouse Node Cilia Is Required for Rotational Movement but Confers Ultrastructural Instability as a Trade-Off

Determination of left-right asymmetry in mouse embryos is established by a leftward fluid flow that is generated by clockwise rotation of node cilia. How node cilia achieve stable unidirectional rotation has remained unknown, however. Here we show that brief exposure to the microtubule-stabilizing drug paclitaxel (Taxol) induces randomly directed rotation and changes the ultrastructure of node cilia. In vivo observations and a computer simulation revealed that a regular 9+0 arrangement of doublet microtubules is essential for stable unidirectional rotation of node cilia. The 9+2 motile cilia of the airway, which manifest planar beating, are resistant to Taxol treatment. However, the airway cilia of mice lacking the radial spoke head protein Rsph4a undergo rotational movement instead of planar beating, are prone to microtubule rearrangement, and are sensitive to Taxol. Our results suggest that the absence of radial spokes allows node cilia to rotate unidirectionally but, as a trade-off, renders them ultrastructurally fragile.

Sperm dysfunction and ciliopathy

Sperm motility is driven by motile cytoskeletal elements in the tail, called axonemes. The structure of axonemes consists of 9 + 2 microtubules, molecular motors (dyneins), and their regulatory structures. Axonemes are well conserved in motile cilia and flagella through eukaryotic evolution. Deficiency in the axonemal structure causes defects in sperm motility, and often leads to male infertility. It has been known since the 1970s that, in some cases, male infertility is linked with other symptoms or diseases such as Kartagener syndrome. Given that these links are mostly caused by deficiencies in the common components of cilia and flagella, they are called "immotile cilia syndrome" or "primary ciliary dyskinesia," or more recently, "ciliopathy," which includes deficiencies in primary and sensory cilia. Here, we review the structure of the sperm flagellum and epithelial cilia in the human body, and discuss how male fertility is linked to ciliopathy.

DRC3 connects the N-DRC to dynein g to regulate flagellar waveform

The nexin-dynein regulatory complex (N-DRC), which is a major hub for the control of flagellar motility, contains at least 11 different subunits. A major challenge is to determine the location and function of each of these subunits within the N-DRC. We characterized a Chlamydomonas mutant defective in the N-DRC subunit DRC3. Of the known N-DRC subunits, the drc3 mutant is missing only DRC3. Like other N-DRC mutants, the drc3 mutant has a defect in flagellar motility. However, in contrast to other mutations affecting the N-DRC, drc3 does not suppress flagellar paralysis caused by loss of radial spokes. Cryo-electron tomography revealed that the drc3 mutant lacks a portion of the N-DRC linker domain, including the L1 protrusion, part of the distal lobe, and the connection between these two structures, thus localizing DRC3 to this part of the N-DRC. This and additional considerations enable us to assign DRC3 to the L1 protrusion. Because the L1 protrusion is the only non-dynein structure in contact with the dynein g motor domain in wild-type axonemes and this is the only N-DRC-dynein connection missing in the drc3 mutant, we conclude that DRC3 interacts with dynein g to regulate flagellar waveform.

The mouse radial spoke protein 3 is a nucleocytoplasmic shuttling protein that promotes neurogenesis

Radial spoke protein 3 (RSP3) was first identified in Chlamydomonas as a component of radial spoke, which is important for flagellar motility. The mammalian homolog of the Chlamydomonas RSP3 protein is found to be a mammalian protein kinase A-anchoring protein that binds ERK1/2. Here we show that mouse RSP3 is a nucleocytoplasmic shuttling protein. The full-length RSP3-EGFP fusion protein is mainly located in the cytoplasm of Chinese hamster ovary cells. However, by using deletion mutants of RSP3, we identified two nuclear localization signals and a nuclear export signal in RSP3. Moreover, using in utero electroporation, we found that overexpression of RSP3 in the developing cerebral cortex promotes neurogenesis. The layer II/III of the neocortex was much thicker in the RSP3-transfected region than that of the untransfected region in the neocortex. We also show that RSP3 is specifically located in the primary cilia of the radial glial cells, where it acts as a signaling mediator that regulates neurogenesis. Thus, our results suggest that RSP3 is a nucleocytoplasmic shuttling protein and plays an essential role in neurogenesis.

In situ localization of N and C termini of subunits of the flagellar nexin-dynein regulatory complex (N-DRC) using SNAP tag and cryo-electron tomography

Cryo-electron tomography (cryo-ET) has reached nanoscale resolution for in situ three-dimensional imaging of macromolecular complexes and organelles. Yet its current resolution is not sufficient to precisely localize or identify most proteins in situ; for example, the location and arrangement of components of the nexin-dynein regulatory complex (N-DRC), a key regulator of ciliary/flagellar motility that is conserved from algae to humans, have remained elusive despite many cryo-ET studies of cilia and flagella. Here, we developed an in situ localization method that combines cryo-ET/subtomogram averaging with the clonable SNAP tag, a widely used cell biological probe to visualize fusion proteins by fluorescence microscopy. Using this hybrid approach, we precisely determined the locations of the N and C termini of DRC3 and the C terminus of DRC4 within the three-dimensional structure of the N-DRC in Chlamydomonas flagella. Our data demonstrate that fusion of SNAP with target proteins allowed for protein localization with high efficiency and fidelity using SNAP-linked gold nanoparticles, without disrupting the native assembly, structure, or function of the flagella. After cryo-ET and subtomogram averaging, we localized DRC3 to the L1 projection of the nexin linker, which interacts directly with a dynein motor, whereas DRC4 was observed to stretch along the N-DRC base plate to the nexin linker. Application of the technique developed here to the N-DRC revealed new insights into the organization and regulatory mechanism of this complex, and provides a valuable tool for the structural dissection of macromolecular complexes in situ.

Detailed structural and biochemical characterization of the nexin-dynein regulatory complex

The nexin-dynein regulatory complex (N-DRC) is a microtubule-cross-bridging structure in cilia/flagella. The precise 3D positions of N-DRC subunits are identified using cryo–electron tomography and structural labeling. The N-DRC is purified and its composition and microtubule-binding properties were characterized.

The nexin-dynein regulatory complex (N-DRC) forms a cross-bridge between the outer doublet microtubules of the axoneme and regulates dynein motor activity in cilia/flagella. Although the molecular composition and the three-dimensional structure of N-DRC have been studied using mutant strains lacking N-DRC subunits, more accurate approaches are necessary to characterize the structure and function of N-DRC. In this study, we precisely localized DRC1, DRC2, and DRC4 using cryo–electron tomography and structural labeling. All three N-DRC subunits had elongated conformations and spanned the length of N-DRC. Furthermore, we purified N-DRC and characterized its microtubule-binding properties. Purified N-DRC bound to the microtubule and partially inhibited microtubule sliding driven by the outer dynein arms (ODAs). Of interest, microtubule sliding was observed even in the presence of fourfold molar excess of N-DRC relative to ODA. These results provide insights into the role of N-DRC in generating the beating motions of cilia/flagella.

Motility and more: the flagellum of Trypanosoma brucei

Trypanosoma brucei is a pathogenic unicellular eukaryote that infects humans and other mammals in sub-Saharan Africa. A central feature of trypanosome biology is the single flagellum of the parasite, which is an essential and multifunctional organelle that facilitates cell propulsion, controls cell morphogenesis and directs cytokinesis. Moreover, the flagellar membrane is a specialized subdomain of the cell surface that mediates attachment to host tissues and harbours multiple virulence factors. In this Review, we discuss the structure, assembly and function of the trypanosome flagellum, including canonical roles in cell motility as well as novel and emerging roles in cell morphogenesis and host-parasite interactions.

Motility of fish spermatozoa: from external signaling to flagella response

For successful fertilization, spermatozoa must access, bind, and penetrate an egg, processes for which activation of spermatozoa motility is a prerequisite. Fish spermatozoa are stored in seminal plasma where they are immotile during transit through the genital tract of most externally fertilizing teleosts and chondrosteans. Under natural conditions, motility is induced immediately following release of spermatozoa from the male genital tract into the aqueous environment. The nature of an external trigger for the initiation of motility is highly dependent on the aquatic environment (fresh or salt water) and the species' reproductive behavior. Triggering signals include osmotic pressure, ionic and gaseous components of external media and, in some cases, egg-derived substances. Extensive study of environmental factors influencing fish spermatozoa motility has led to the proposal of several mechanisms of activation in freshwater and marine fish. However, the signal transduction pathways initiated by these mechanisms remain clear. This review presents the current knowledge with respect to (1) membrane reception of the activation signal and its transduction through the spermatozoa plasma membrane via the external membrane components, ion channels, and aquaporins; (2) cytoplasmic trafficking of the activation signal; (3) final steps of the signaling, including signal transduction to the axonemal machinery, and activation of axonemal dyneins and regulation of their activity; and (4) pathways supplying energy for flagellar motility.

The MIA complex is a conserved and novel dynein regulator essential for normal ciliary motility

The MIA complex, composed of FAP100 and FAP73, interacts with I1 dynein components and is required for normal ciliary beat frequency.

Axonemal dyneins must be precisely regulated and coordinated to produce ordered ciliary/flagellar motility, but how this is achieved is not understood. We analyzed two Chlamydomonas reinhardtii mutants, mia1 and mia2, which display slow swimming and low flagellar beat frequency. We found that the MIA1 and MIA2 genes encode conserved coiled-coil proteins, FAP100 and FAP73, respectively, which form the modifier of inner arms (MIA) complex in flagella. Cryo–electron tomography of mia mutant axonemes revealed that the MIA complex was located immediately distal to the intermediate/light chain complex of I1 dynein and structurally appeared to connect with the nexin–dynein regulatory complex. In axonemes from mutants that lack both the outer dynein arms and the MIA complex, I1 dynein failed to assemble, suggesting physical interactions between these three axonemal complexes and a role for the MIA complex in the stable assembly of I1 dynein. The MIA complex appears to regulate I1 dynein and possibly outer arm dyneins, which are both essential for normal motility.

The N-DRC forms a conserved biochemical complex that maintains outer doublet alignment and limits microtubule sliding in motile axonemes

The nexin–dynein regulatory complex (N-DRC) is implicated in the control of dynein activity as a structural component of the nexin link. This study identifies several new subunits of the N-DRC and demonstrates for the first time that it forms a discrete biochemical complex that maintains outer doublet integrity and regulates microtubule sliding.

The nexin–dynein regulatory complex (N-DRC) is proposed to coordinate dynein arm activity and interconnect doublet microtubules. Here we identify a conserved region in DRC4 critical for assembly of the N-DRC into the axoneme. At least 10 subunits associate with DRC4 to form a discrete complex distinct from other axonemal substructures. Transformation of drc4 mutants with epitope-tagged DRC4 rescues the motility defects and restores assembly of missing DRC subunits and associated inner-arm dyneins. Four new DRC subunits contain calcium-signaling motifs and/or AAA domains and are nearly ubiquitous in species with motile cilia. However, drc mutants are motile and maintain the 9 + 2 organization of the axoneme. To evaluate the function of the N-DRC, we analyzed ATP-induced reactivation of isolated axonemes. Rather than the reactivated bending observed with wild-type axonemes, ATP addition to drc-mutant axonemes resulted in splaying of doublets in the distal region, followed by oscillatory bending between pairs of doublets. Thus the N-DRC provides some but not all of the resistance to microtubule sliding and helps to maintain optimal alignment of doublets for productive flagellar motility. These findings provide new insights into the mechanisms that regulate motility and further highlight the importance of the proximal region of the axoneme in generating flagellar bending.

Polarity and asymmetry in the arrangement of dynein and related structures in the Chlamydomonas axoneme

Cryoelectron tomography and subtomogram averaging reveal a high degree of structural asymmetry and polarization in dynein localization in the Chlamydomonas flagella.

Understanding the molecular architecture of the flagellum is crucial to elucidate the bending mechanism produced by this complex organelle. The current known structure of the flagellum has not yet been fully correlated with the complex composition and localization of flagellar components. Using cryoelectron tomography and subtomogram averaging while distinguishing each one of the nine outer doublet microtubules, we systematically collected and reconstructed the three-dimensional structures in different regions of the Chlamydomonas flagellum. We visualized the radial and longitudinal differences in the flagellum. One doublet showed a distinct structure, whereas the other eight were similar but not identical to each other. In the proximal region, some dyneins were missing or replaced by minor dyneins, and outer–inner arm dynein links were variable among different microtubule doublets. These findings shed light on the intricate organization of Chlamydomonas flagella, provide clues to the mechanism that produces asymmetric flagellar beating, and pose a new challenge for the functional study of the flagella.

The nexin-dynein regulatory complex subunit DRC1 is essential for motile cilia function in algae and humans

Primary ciliary dyskinesia (PCD) is characterized by dysfunction of respiratory cilia and sperm flagella and random determination of visceral asymmetry. Here, we identify the DRC1 subunit of the nexin-dynein regulatory complex (N-DRC), an axonemal structure critical for the regulation of dynein motors, and show that mutations in the gene encoding DRC1, CCDC164, are involved in PCD pathogenesis. Loss-of-function mutations disrupting DRC1 result in severe defects in assembly of the N-DRC structure and defective ciliary movement in Chlamydomonas reinhardtii and humans. Our results highlight a role for N-DRC integrity in regulating ciliary beating and provide the first direct evidence that mutations in DRC genes cause human disease.

CDKL5 regulates flagellar length and localizes to the base of the flagella in Chlamydomonas

Two mutations in LF5, which encodes a protein kinase orthologous to human CDKL5, cause abnormally long flagella in Chlamydomonas. The localization of LF5p to the very proximal region of flagella in WT cells is regulated by three other LF gene products, which make up the cytoplasmic length regulatory complex.

The length of Chlamydomonas flagella is tightly regulated. Mutations in four genes—LF1, LF2, LF3, and LF4—cause cells to assemble flagella up to three times wild-type length. LF2 and LF4 encode protein kinases. Here we describe a new gene, LF5, in which null mutations cause cells to assemble flagella of excess length. The LF5 gene encodes a protein kinase very similar in sequence to the protein kinase CDKL5. In humans, mutations in this kinase cause a severe form of juvenile epilepsy. The LF5 protein localizes to a unique location: the proximal 1 μm of the flagella. The proximal localization of the LF5 protein is lost when genes that make up the proteins in the cytoplasmic length regulatory complex (LRC)—LF1, LF2, and LF3—are mutated. In these mutants LF5p becomes localized either at the distal tip of the flagella or along the flagellar length, indicating that length regulation involves, at least in part, control of LF5p localization by the LRC.

Tubulin polyglutamylation regulates flagellar motility by controlling a specific inner-arm dynein that interacts with the dynein regulatory complex

The tpg1 mutant of Chlamydomonas lacks the tubulin polyglutamylase TTLL9 and is deficient in flagellar tubulin polyglutamylation. It exhibits slow swimming, whereas the double mutant with oda2 (a slow-swimming mutant that lacks outer-arm dynein) is completely nonmotile. Thus, tubulin polyglutamylation must be important for the functioning of inner-arm dynein(s). In this study, we show that the tpg1 mutation only slightly affects the motility of mutants that lack dynein "e," one of the seven species of major inner-arm dyneins, whereas it greatly reduces the motility of mutants lacking other inner-arm dynein species. This suggests that dynein e is the main target of motility regulation by tubulin polyglutamylation. Furthermore, the motility of various mutants in the background of the tpg1 mutation raises the possibility that tubulin polyglutamylation also affects the dynein regulatory complex, a dynein e-associated key regulator of flagellar motility, which possibly constitutes the interdoublet (nexin) link. Tubulin polyglutamylation thus may play a central role in the regulation of ciliary and flagellar motility. © 2012 Wiley Periodicals, Inc.

Structural studies of ciliary components

Cilia are organelles found on most eukaryotic cells, where they serve important functions in motility, sensory reception, and signaling. Recent advances in electron tomography have facilitated a number of ultrastructural studies of ciliary components that have significantly improved our knowledge of cilium architecture. These studies have produced nanometer-resolution structures of axonemal dynein complexes, microtubule doublets and triplets, basal bodies, radial spokes, and nexin complexes. In addition to these electron tomography studies, several recently published crystal structures provide insights into the architecture and mechanism of dynein as well as the centriolar protein SAS-6, important for establishing the 9-fold symmetry of centrioles. Ciliary assembly requires intraflagellar transport (IFT), a process that moves macromolecules between the tip of the cilium and the cell body. IFT relies on a large 20-subunit protein complex that is thought to mediate the contacts between ciliary motor and cargo proteins. Structural investigations of IFT complexes are starting to emerge, including the first three-dimensional models of IFT material in situ, revealing how IFT particles organize into larger train-like arrays, and the high-resolution structure of the IFT25/27 subcomplex. In this review, we cover recent advances in the structural and mechanistic understanding of ciliary components and IFT complexes.

Cryoelectron tomography reveals doublet-specific structures and unique interactions in the I1 dynein

Cilia and flagella are highly conserved motile and sensory organelles in eukaryotes, and defects in ciliary assembly and motility cause many ciliopathies. The two-headed I1 inner arm dynein is a critical regulator of ciliary and flagellar beating. To understand I1 architecture and function better, we analyzed the 3D structure and composition of the I1 dynein in Chlamydomonas axonemes by cryoelectron tomography and subtomogram averaging. Our data revealed several connections from the I1 dynein to neighboring structures that are likely to be important for assembly and/or regulation, including a tether linking one I1 motor domain to the doublet microtubule and doublet-specific differences potentially contributing to the asymmetrical distribution of dynein activity required for ciliary beating. We also imaged three I1 mutants and analyzed their polypeptide composition using 2D gel-based proteomics. Structural and biochemical comparisons revealed the likely location of the regulatory IC138 phosphoprotein and its associated subcomplex. Overall, our studies demonstrate that I1 dynein is connected to multiple structures within the axoneme, and therefore ideally positioned to integrate signals that regulate ciliary motility.

Computer-assisted image analysis of human cilia and Chlamydomonas flagella reveals both similarities and differences in axoneme structure

In the past decade, investigations from several different fields have revealed the critical role of cilia in human health and disease. Because of the highly conserved nature of the basic axonemal structure, many different model systems have proven useful for the study of ciliopathies, especially the unicellular, biflagellate green alga Chlamydomonas reinhardtii. Although the basic axonemal structure of cilia and flagella is highly conserved, these organelles often perform specialized functions unique to the cell or tissue in which they are found. These differences in function are likely reflected in differences in structural organization. In this work, we directly compare the structure of isolated axonemes from human cilia and Chlamydomonas flagella to identify similarities and differences that potentially play key roles in determining their functionality. Using transmission electron microscopy and 2D image averaging techniques, our analysis has confirmed the overall structural similarity between these two species, but also revealed clear differences in the structure of the outer dynein arms, the central pair projections, and the radial spokes. We also show how the application of 2D image averaging can clarify the underlying structural defects associated with primary ciliary dyskinesia (PCD). Overall, our results document the remarkable similarity between these two structures separated evolutionarily by over a billion years, while highlighting several significant differences, and demonstrate the potential of 2D image averaging to improve the diagnosis and understanding of PCD.

Tubulin-dynein system in flagellar and ciliary movement

Eukaryotic flagella and cilia have attracted the attention of many researchers over the last century, since they are highly arranged organelles and show sophisticated bending movements. Two important cytoskeletal and motor proteins, tubulin and dynein, were first found and described in flagella and cilia. Half a century has passed since the discovery of these two proteins, and much information has been accumulated on their molecular structures and their roles in the mechanism of microtubule sliding, as well as on the architecture, the mechanism of bending movement and the regulation and signal transduction in flagella and cilia. Historical background and the recent advance in this field are described.

Ciliary and flagellar structure and function--their regulations by posttranslational modifications of axonemal tubulin

Eukaryotic cilia and flagella are evolutionarily conserved microtubule-based organelles protruding from the cell surface. They perform dynein-driven beating which contributes to cell locomotion or flow generation. They also play important roles in sensing as cellular antennae, which allows cells to respond to various external stimuli. The main components of cilia and flagella, α- and β-tubulins, are known to undergo various posttranslational modifications (PTMs), including phosphorylation, palmitoylation, tyrosination/detyrosination, Δ2 modification, acetylation, glutamylation, and glycylation. Recent identification of tubulin-modifying enzymes, especially tubulin tyrosine ligase-like proteins which perform tubulin glutamylation and glycylation, has demonstrated the importance of tubulin modifications for the assembly and functions of cilia and flagella. In this chapter, we review recent work on PTMs of ciliary and flagellar tubulins in conjunction with discussing the basic knowledge.

The Pcdp1 complex coordinates the activity of dynein isoforms to produce wild-type ciliary motility

Generating the complex waveforms characteristic of beating cilia requires the coordinated activity of multiple dynein isoforms anchored to the axoneme. We previously identified a complex associated with the C1d projection of the central apparatus that includes primary ciliary dyskinesia protein 1 (Pcdp1). Reduced expression of complex members results in severe motility defects, indicating that C1d is essential for wild-type ciliary beating. To define a mechanism for Pcdp1/C1d regulation of motility, we took a functional and structural approach combined with mutants lacking C1d and distinct subsets of dynein arms. Unlike mutants completely lacking the central apparatus, dynein-driven microtubule sliding velocities are wild type in C1d- defective mutants. However, coordination of dynein activity among microtubule doublets is severely disrupted. Remarkably, mutations in either outer or inner dynein arm restore motility to mutants lacking C1d, although waveforms and beat frequency differ depending on which isoform is mutated. These results define a unique role for C1d in coordinating the activity of specific dynein isoforms to control ciliary motility.

Cryo-electron tomography reveals conserved features of doublet microtubules in flagella

The axoneme forms the essential and conserved core of cilia and flagella. We have used cryo-electron tomography of Chlamydomonas and sea urchin flagella to answer long-standing questions and to provide information about the structure of axonemal doublet microtubules (DMTs). Solving an ongoing controversy, we show that B-tubules of DMTs contain exactly 10 protofilaments (PFs) and that the inner junction (IJ) and outer junction between the A- and B-tubules are fundamentally different. The outer junction, crucial for the initiation of doublet formation, appears to be formed by close interactions between the tubulin subunits of three PFs with unusual tubulin interfaces; other investigators have reported that this junction is weakened by mutations affecting posttranslational modifications of tubulin. The IJ consists of an axially periodic ladder-like structure connecting tubulin PFs of the A- and B-tubules. The recently discovered microtubule inner proteins (MIPs) on the inside of the A- and B-tubules are more complex than previously thought. They are composed of alternating small and large subunits with periodicities of 16 and/or 48 nm. MIP3 forms arches connecting B-tubule PFs, contrary to an earlier report that MIP3 forms the IJ. Finally, the "beak" structures within the B-tubules of Chlamydomonas DMT1, DMT5, and DMT6 are clearly composed of a longitudinal band of proteins repeating with a periodicity of 16 nm. These findings, discussed in relation to genetic and biochemical data, provide a critical foundation for future work on the molecular assembly and stability of the axoneme, as well as its function in motility and sensory transduction.

Building blocks of the nexin-dynein regulatory complex in Chlamydomonas flagella

The directional flow generated by motile cilia and flagella is critical for many processes, including human development and organ function. Normal beating requires the control and coordination of thousands of dynein motors, and the nexin-dynein regulatory complex (N-DRC) has been identified as an important regulatory node for orchestrating dynein activity. The nexin link appears to be critical for the transformation of dynein-driven, linear microtubule sliding to flagellar bending, yet the molecular composition and mechanism of the N-DRC remain largely unknown. Here, we used proteomics with special attention to protein phosphorylation to analyze the composition of the N-DRC and to determine which subunits may be important for signal transduction. Two-dimensional electrophoresis and MALDI-TOF mass spectrometry of WT and mutant flagellar axonemes from Chlamydomonas identified 12 N-DRC-associated proteins, including all seven previously observed N-DRC components. Sequence and PCR analyses identified the mutation responsible for the phenotype of the sup-pf-4 strain, and biochemical comparison with a radial spoke mutant revealed two components that may link the N-DRC and the radial spokes. Phosphoproteomics revealed eight proteins with phosphorylated isoforms for which the isoform patterns changed with the genotype as well as two components that may play pivotal roles in N-DRC function through their phosphorylation status. These data were assembled into a model of the N-DRC that explains aspects of its regulatory function.

Sperm flagella: comparative and phylogenetic perspectives of protein components

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