Outer and inner dynein arms of cilia and flagella

The mechanics of cilia and flagella: What we know and what we need to know

In this review, we provide a condensed overview of what is currently known about the mechanical functioning of the flagellar/ciliary axoneme. We also present a list of 10 specific areas where our current knowledge is incomplete and explain the benefits of further experimental investigation. Many of the physical parameters of the axoneme and its component parts have not been determined. This limits our ability to understand how the axoneme structure contributes to its functioning in several regards. It restricts our ability to understand how the mechanics of the structure contribute to the regulation of motor function. It also confines our ability to understand the three-dimensional workings of the axoneme and how various beating modes are accomplished. Lastly, it prevents accurate computational modeling of the axoneme in three-dimensions.

A Synthetic Minimal Beating Axoneme

Cilia and flagella are beating rod-like organelles that enable the directional movement of microorganisms in fluids and fluid transport along the surface of biological organisms or inside organs. The molecular motor axonemal dynein drives their beating by interacting with microtubules. Constructing synthetic beating systems with axonemal dynein capable of mimicking ciliary beating still represents a major challenge. Here, the bottom-up engineering of a sustained beating synthoneme consisting of a pair of microtubules connected by a series of periodic arrays of approximately eight axonemal dyneins is reported. A model leads to the understanding of the motion through the cooperative, cyclic association-dissociation of the molecular motor from the microtubules. The synthoneme represents a bottom-up self-organized bio-molecular machine at the nanoscale with cilia-like properties.

Structure of Motile Cilia

Cilia are tail-like organelles responsible for motility, transportation, and sensory functions in eukaryotic cells. Cilia research has been providing multifaceted questions, attracting biologists of various areas and inducing interdisciplinary studies. In this chapter, we mainly focus on efforts to elucidate the molecular mechanism of ciliary beating motion, a field of research that has a long history and is still ongoing. We also overview topics closely related to the motility mechanism, such as ciliogenesis, cilia-related diseases, and sensory cilia. Subnanometer-scale to submillimeter-scale 3D imaging of the axoneme and the basal body resulted in a wide variety of insights into these questions.

Structural organization of the intermediate and light chain complex of Chlamydomonas ciliary I1 dynein

Axonemal I1 dynein (dynein f) is the largest inner dynein arm in cilia and a key regulator of ciliary beating. It consists of two dynein heavy chains, and an intermediate chain/light chain (ICLC) complex. However, the structural organization of the nine ICLC subunits remains largely unknown. Here, we used biochemical and genetic approaches, and cryo-electron tomography imaging in Chlamydomonas to dissect the molecular architecture of the I1 dynein ICLC complex. Using a strain expressing SNAP-tagged IC140, tomography revealed the location of the IC140 N-terminus at the proximal apex of the ICLC structure. Mass spectrometry of a tctex2b mutant showed that TCTEX2B dynein light chain is required for the stable assembly of TCTEX1 and inner dynein arm interacting proteins IC97 and FAP120. The structural defects observed in tctex2b located these 4 subunits in the center and bottom regions of the ICLC structure, which overlaps with the location of the IC138 regulatory subcomplex, which contains IC138, IC97, FAP120, and LC7b. These results reveal the three-dimensional organization of the native ICLC complex and indicate potential protein-protein interactions that are involved in the pathway by which I1 regulates ciliary motility.

The tubulin code specializes neuronal cilia for extracellular vesicle release

Cilia are microtubule-based organelles that display diversity in morphology, ultrastructure, protein composition, and function. The ciliary microtubules of C. elegans sensory neurons exemplify this diversity and provide a paradigm to understand mechanisms driving ciliary specialization. Only a subset of ciliated neurons in C. elegans are specialized to make and release bioactive extracellular vesicles (EVs) into the environment. The cilia of extracellular vesicle releasing neurons have distinct axonemal features and specialized intraflagellar transport that are important for releasing EVs. In this review, we discuss the role of the tubulin code in the specialization of microtubules in cilia of EV releasing neurons.

Shulin packages axonemal outer dynein arms for ciliary targeting

The main force generators in eukaryotic cilia and flagella are axonemal outer dynein arms (ODAs). During ciliogenesis, these ~1.8-megadalton complexes are assembled in the cytoplasm and targeted to cilia by an unknown mechanism. Here, we used the ciliate Tetrahymena to identify two factors (Q22YU3 and Q22MS1) that bind ODAs in the cytoplasm and are required for ODA delivery to cilia. Q22YU3, which we named Shulin, locked the ODA motor domains into a closed conformation and inhibited motor activity. Cryo-electron microscopy revealed how Shulin stabilized this compact form of ODAs by binding to the dynein tails. Our findings provide a molecular explanation for how newly assembled dyneins are packaged for delivery to the cilia.

Force-Generating Mechanism of Axonemal Dynein in Solo and Ensemble

In eukaryotic cilia and flagella, various types of axonemal dyneins orchestrate their distinct functions to generate oscillatory bending of axonemes. The force-generating mechanism of dyneins has recently been well elucidated, mainly in cytoplasmic dyneins, thanks to progress in single-molecule measurements, X-ray crystallography, and advanced electron microscopy. These techniques have shed light on several important questions concerning what conformational changes accompany ATP hydrolysis and whether multiple motor domains are coordinated in the movements of dynein. However, due to the lack of a proper expression system for axonemal dyneins, no atomic coordinates of the entire motor domain of axonemal dynein have been reported. Therefore, a substantial amount of knowledge on the molecular architecture of axonemal dynein has been derived from electron microscopic observations on dynein arms in axonemes or on isolated axonemal dynein molecules. This review describes our current knowledge and perspectives of the force-generating mechanism of axonemal dyneins in solo and in ensemble.

The I1 dynein-associated tether and tether head complex is a conserved regulator of ciliary motility

Motile cilia are essential for propelling cells and moving fluids across tissues. The activity of axonemal dynein motors must be precisely coordinated to generate ciliary motility, but their regulatory mechanisms are not well understood. The tether and tether head (T/TH) complex was hypothesized to provide mechanical feedback during ciliary beating because it links the motor domains of the regulatory I1 dynein to the ciliary doublet microtubule. Combining genetic and biochemical approaches with cryoelectron tomography, we identified FAP44 and FAP43 (plus the algae-specific, FAP43-redundant FAP244) as T/TH components. WT-mutant comparisons revealed that the heterodimeric T/TH complex is required for the positional stability of the I1 dynein motor domains, stable anchoring of CK1 kinase, and proper phosphorylation of the regulatory IC138-subunit. T/TH also interacts with inner dynein arm d and radial spoke 3, another important motility regulator. The T/TH complex is a conserved regulator of I1 dynein and plays an important role in the signaling pathway that is critical for normal ciliary motility.

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.

X-Ray Fiber Diffraction Recordings from Oriented Demembranated Chlamydomonas Flagellar Axonemes

The high homology of its axonemal components with humans and a large repertoire of axonemal mutants make Chlamydomonas a useful model system for experiments on the structure and function of eukaryotic cilia and flagella. Using this organism, we explored the spatial arrangement of axonemal components under physiological conditions by small-angle x-ray fiber diffraction. Axonemes were oriented in physiological solution by continuous shear flow and exposed to intense and stable x rays generated in the synchrotron radiation facility SPring-8, BL45XU. We compared diffraction patterns from axonemes isolated from wild-type and mutant strains lacking the whole outer arm (oda1), radial spoke (pf14), central apparatus (pf18), or the α-chain of the outer arm dynein (oda11). Diffraction of the axonemes showed a series of well-defined meridional/layer-line and equatorial reflections. Diffraction patterns from mutant axonemes exhibited a systematic loss/attenuation of meridional/layer-line reflections, making it possible to determine the origin of various reflections. The 1/24 and 1/12 nm(-1) meridional reflections of oda1 and oda11 were much weaker than those of the wild-type, suggesting that the outer dynein arms are the main contributor to these reflections. The weaker 1/32 and 1/13.7 nm(-1) meridional reflections from pf14 compared with the wild-type suggest that these reflections come mainly from the radial spokes. The limited contribution of the central pair apparatus to the diffraction patterns was confirmed by the similarity between the patterns of the wild-type and pf18. The equatorial reflections were complex, but a comparison with electron micrograph-based models allowed the density of each axonemal component to be estimated. Addition of ATP to rigor-state axonemes also resulted in subtle changes in equatorial intensity profiles, which could report nucleotide-dependent structural changes of the dynein arms. The first detailed description of axonemal reflections presented here serves as a landmark for further x-ray diffraction studies to monitor the action of constituent proteins in functional axonemes.

ATPases, ion exchangers and human sperm motility

Human sperm has several mechanisms to control its ionic milieu, such as the Na,K-ATPase (NKA), the Ca-ATPase of the plasma membrane (PMCA), the Na+/Ca2+-exchanger (NCX) and the Na+/H+-exchanger (NHE). On the other hand, the dynein-ATPase is the intracellular motor for sperm motility. In this work, we evaluated NKA, PMCA, NHE, NCX and dynein-ATPase activities in human sperm and investigated their correlation with sperm motility. Sperm motility was measured by Computer Assisted Semen Analysis. It was found that the NKA activity is inhibited by ouabain with twoKi(7.9×10−9and 9.8×10−5 M), which is consistent with the presence of two isoforms of α subunit of the NKA in the sperm plasma membranes (α1 and α4), being α4 more sensitive to ouabain. The decrease in NKA activity is associated with a reduction in sperm motility. In addition, sperm motility was evaluated in the presence of known inhibitors of NHE, PMCA and NCX, such as amiloride, eosin, and KB-R7943, respectively, as well as in the presence of nigericin after incubation with ouabain. Amiloride, eosin and KB-R7943 significantly reduced sperm motility. Nigericin reversed the effect of ouabain and amiloride on sperm motility. Dynein-ATPase activity was inhibited by acidic pH and micromolar concentrations of Ca2+. We explain our results in terms of inhibition of the dynein-ATPase in the presence of higher cytosolic H+and Ca2+, and therefore inhibition of sperm motility.

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.

The ciliary inner dynein arm, I1 dynein, is assembled in the cytoplasm and transported by IFT before axonemal docking

To determine mechanisms of assembly of ciliary dyneins, we focused on the Chlamydomonas inner dynein arm, I1 dynein, also known as dynein f. I1 dynein assembles in the cytoplasm as a 20S complex similar to the 20S I1 dynein complex isolated from the axoneme. The intermediate chain subunit, IC140 (IDA7), and heavy chains (IDA1, IDA2) are required for 20S I1 dynein preassembly in the cytoplasm. Unlike I1 dynein derived from the axoneme, the cytoplasmic 20S I1 complex will not rebind I1-deficient axonemes in vitro. To test the hypothesis that I1 dynein is transported to the distal tip of the cilia for assembly in the axoneme, we performed cytoplasmic complementation in dikaryons formed between wild-type and I1 dynein mutant cells. Rescue of I1 dynein assembly in mutant cilia occurred first at the distal tip and then proceeded toward the proximal axoneme. Notably, in contrast to other combinations, I1 dynein assembly was significantly delayed in dikaryons formed between ida7 and ida3. Furthermore, rescue of I1 dynein assembly required new protein synthesis in the ida7 × ida3 dikaryons. On the basis of the additional observations, we postulate that IDA3 is required for 20S I1 dynein transport. Cytoplasmic complementation in dikaryons using the conditional kinesin-2 mutant, fla10-1 revealed that transport of I1 dynein is dependent on kinesin-2 activity. Thus, I1 dynein complex assembly depends upon IFT for transport to the ciliary distal tip prior to docking in the axoneme.

3D structure of eukaryotic flagella/cilia by cryo-electron tomography

Flagella/cilia are motile organelles with more than 400 proteins. To understand the mechanism of such complex systems, we need methods to describe molecular arrange-ments and conformations three-dimensionally in vivo. Cryo-electron tomography enabled us such a 3D structural analysis. Our group has been working on 3D structure of flagella/cilia using this method and revealed highly ordered and beautifully organized molecular arrangement. 3D structure gave us insights into the mechanism to gener-ate bending motion with well defined waveforms. In this review, I summarize our recent structural studies on fla-gella/cilia by cryo-electron tomography, mainly focusing on dynein microtubule-based ATPase motor proteins and the radial spoke, a regulatory protein complex.

ATP-driven remodeling of the linker domain in the dynein motor

Dynein ATPases are the largest known cytoskeletal motors and perform critical functions in cells: carrying cargo along microtubules in the cytoplasm and powering flagellar beating. Dyneins are members of the AAA+ superfamily of ring-shaped enzymes, but how they harness this architecture to produce movement is poorly understood. Here, we have used cryo-EM to determine 3D maps of native flagellar dynein-c and a cytoplasmic dynein motor domain in different nucleotide states. The structures show key sites of conformational change within the AAA+ ring and a large rearrangement of the “linker” domain, involving a hinge near its middle. Analysis of a mutant in which the linker “undocks” from the ring indicates that linker remodeling requires energy that is supplied by interactions with the AAA+ modules. Fitting the dynein-c structures into flagellar tomograms suggests how this mechanism could drive sliding between microtubules, and also has implications for cytoplasmic cargo transport.

The dynamic cytoskeleton of the developing male germ cell

Mammalian spermatogenesis is characterised by dramatic cellular change to transform the non-polar spermatogonium into a highly polarised and functional spermatozoon. The acquisition of cell polarity is a requisite step for formation of viable sperm. The polarity of the spermatozoon is clearly demonstrated by the acrosome at the apical pole of the cell and the flagellum at the opposite end. Spermatogenesis consists of three basic phases: mitosis, meiosis and spermiogenesis. The final phase represents the period of greatest cellular change where cell-type specific organelles such as the acrosome and the flagellum form, the nucleus migrates to the plasma membrane and elongates, chromatin condenses and residual cytoplasm is removed. An important feature of spermatogenesis is the change in the cytoskeleton that occurs throughout this pathway. In this review, the author will provide an overview of these transformations and provide insight into possible modes of regulation of these rearrangements during spermatogenesis. Although primary focus will be given to the microtubule cytoskeleton, the importance of actin filaments to the cellular transformation of the male germ cell will also be discussed.

Ciliary motility: the components and cytoplasmic preassembly mechanisms of the axonemal dyneins

Motile cilia and flagella are organelles, which function in cell motility and in the transport of fluids over the surface of cells. Motility defects often result in a rare human disease, primary ciliary dyskinesia (PCD). Cell motility depends on axonemal dynein, a molecular motor that drives the beating of cilia and flagella. The dyneins are composed of multiple subunits, which are thought to be preassembled in the cytoplasm before they are transported into cilia and flagella. Axonemal dyneins have been extensively studied in Chlamydomonas. In addition, analyses of human PCDs over the past decade, together with studies in other model animals, have identified the conserved components required for dynein assembly. Recently also, the first cytoplasmic component of dynein assembly, kintoun (ktu), was elucidated through the analysis of a medaka mutant in combination with human genetics and cell biology and biochemical studies of Chlamydomonas. The components of dynein and the proteins involved in its cytoplasmic assembly process are discussed.

bop5 Mutations reveal new roles for the IC138 phosphoprotein in the regulation of flagellar motility and asymmetric waveforms

Mutations in the IC138 regulatory subunit of I1 dynein alter dynein motor activity and the flagellar waveform but do not affect phototaxis.

I1 dynein, or dynein f, is a highly conserved inner arm isoform that plays a key role in the regulation of flagellar motility. To understand how the IC138 IC/LC subcomplex modulates I1 activity, we characterized the molecular lesions and motility phenotypes of several bop5 alleles. bop5-3, bop5-4, and bop5-5 are null alleles, whereas bop5-6 is an intron mutation that reduces IC138 expression. I1 dynein assembles into the axoneme, but the IC138 IC/LC subcomplex is missing. bop5 strains, like other I1 mutants, swim forward with reduced swimming velocities and display an impaired reversal response during photoshock. Unlike mutants lacking the entire I1 dynein, however, bop5 strains exhibit normal phototaxis. bop5 defects are rescued by transformation with the wild-type IC138 gene. Analysis of flagellar waveforms reveals that loss of the IC138 subcomplex reduces shear amplitude, sliding velocities, and the speed of bend propagation in vivo, consistent with the reduction in microtubule sliding velocities observed in vitro. The results indicate that the IC138 IC/LC subcomplex is necessary to generate an efficient waveform for optimal motility, but it is not essential for phototaxis. These findings have significant implications for the mechanisms by which IC/LC complexes regulate dynein motor activity independent of effects on cargo binding or complex stability.

The origins and evolution of freeze-etch electron microscopy

The introduction of the Balzers freeze-fracture machine by Moor in 1961 had a much greater impact on the advancement of electron microscopy than he could have imagined. Devised originally to circumvent the dangers of classical thin-section techniques, as well as to provide unique en face views of cell membranes, freeze-fracturing proved to be crucial for developing modern concepts of how biological membranes are organized and proved that membranes are bilayers of lipids within which proteins float and self-assemble. Later, when freeze-fracturing was combined with methods for freezing cells that avoided the fixation and cryoprotection steps that Moor still had to use to prepare the samples for his original invention, it became a means for capturing membrane dynamics on the millisecond time-scale, thus allowing a deeper understanding of the functions of biological membranes in living cells as well as their static ultrastructure. Finally, the realization that unfixed, non-cryoprotected samples could be deeply vacuum-etched or even freeze-dried after freeze-fracturing opened up a whole new way to image all the other molecular components of cells besides their membranes and also provided a powerful means to image the interactions of all the cytoplasmic components with the various membranes of the cell. The purpose of this review is to outline the history of these technical developments, to describe how they are being used in electron microscopy today and to suggest how they can be improved in order to further their utility for biological electron microscopy in the future.

Clinical and genetic aspects of primary ciliary dyskinesia/Kartagener syndrome

Primary ciliary dyskinesia is a genetically heterogeneous disorder of motile cilia. Most of the disease-causing mutations identified to date involve the heavy (dynein axonemal heavy chain 5) or intermediate(dynein axonemal intermediate chain 1) chain dynein genes in ciliary outer dynein arms, although a few mutations have been noted in other genes. Clinical molecular genetic testing for primary ciliary dyskinesia is available for the most common mutations. The respiratory manifestations of primary ciliary dyskinesia (chronic bronchitis leading to bronchiectasis, chronic rhino-sinusitis, and chronic otitis media)reflect impaired mucociliary clearance owing to defective axonemal structure. Ciliary ultrastructural analysis in most patients (>80%) reveals defective dynein arms, although defects in other axonemal components have also been observed. Approximately 50% of patients with primary ciliary dyskinesia have laterality defects (including situs inversus totalis and, less commonly, heterotaxy, and congenital heart disease),reflecting dysfunction of embryological nodal cilia. Male infertility is common and reflects defects in sperm tail axonemes. Most patients with primary ciliary dyskinesia have a history of neonatal respiratory distress, suggesting that motile cilia play a role in fluid clearance during the transition from a fetal to neonatal lung. Ciliopathies involving sensory cilia, including autosomal dominant or recessive polycystic kidney disease, Bardet-Biedl syndrome, and Alstrom syndrome, may have chronic respiratory symptoms and even bronchiectasis suggesting clinical overlap with primary ciliary dyskinesia.

IC138 defines a subdomain at the base of the I1 dynein that regulates microtubule sliding and flagellar motility

To understand the mechanisms that regulate the assembly and activity of flagellar dyneins, we focused on the I1 inner arm dynein (dynein f) and a null allele, bop5-2, defective in the gene encoding the IC138 phosphoprotein subunit. I1 dynein assembles in bop5-2 axonemes but lacks at least four subunits: IC138, IC97, LC7b, and flagellar-associated protein (FAP) 120--defining a new I1 subcomplex. Electron microscopy and image averaging revealed a defect at the base of the I1 dynein, in between radial spoke 1 and the outer dynein arms. Microtubule sliding velocities also are reduced. Transformation with wild-type IC138 restores assembly of the IC138 subcomplex and rescues microtubule sliding. These observations suggest that the IC138 subcomplex is required to coordinate I1 motor activity. To further test this hypothesis, we analyzed microtubule sliding in radial spoke and double mutant strains. The results reveal an essential role for the IC138 subcomplex in the regulation of I1 activity by the radial spoke/phosphorylation pathway.

Three-dimensional structures of the flagellar dynein-microtubule complex by cryoelectron microscopy

The outer dynein arms (ODAs) of the flagellar axoneme generate forces needed for flagellar beating. Elucidation of the mechanisms underlying the chemomechanical energy conversion by the dynein arms and their orchestrated movement in cilia/flagella is of great importance, but the nucleotide-dependent three-dimensional (3D) movement of dynein has not yet been observed. In this study, we establish a new method for reconstructing the 3D structure of the in vitro reconstituted ODA–microtubule complex and visualize nucleotide-dependent conformational changes using cryoelectron microscopy and image analysis. As the complex went from the rigor state to the relaxed state, the head domain of the β heavy chain shifted by 3.7 nm toward the B tubule and inclined 44° inwards. These observations suggest that there is a mechanism that converts head movement into the axonemal sliding motion.

Keeping an eye on I1: I1 dynein as a model for flagellar dynein assembly and regulation

Among the major challenges in understanding ciliary and flagellar motility is to determine how the dynein motors are assembled and localized and how dynein-driven outer doublet microtubule sliding is controlled. Diverse studies, particularly in Chlamydomonas, have determined that the inner arm dynein I1 is targeted to a unique structural position and is critical for regulating the microtubule sliding required for normal ciliary/flagellar bending. As described in this review, I1 dynein offers additional opportunities to determine the principles of assembly and targeting of dyneins to cellular locations and for studying the mechanisms that regulate dynein activity and control of motility by phosphorylation.

Is the dynein motor a winch?

Dyneins are the largest and most complex of the three classes of linear motor proteins in eukaryotic cells. The mass of the dynein motor domain is about ten times that of the other microtubule motor, kinesin. Dynein's homology with the AAA+ superfamily of mechanoenzymes distinguishes it from both kinesin and myosin, which share a common fold and ancestry as members of the G-protein superfamily. In contrast to the other motor proteins, little is known about the mechanism of dynein; its three-dimensional structure is unknown even at low resolution. Recent two-dimensional images from electron microscopy have revealed new details of its structure and how this changes to produce movement. These and the recently solved crystal structure of another AAA+ protein, ClpB, offer tantalising hints about dynein's mechanism, suggesting it may act like a molecular winch.

Patterns of expression of sperm flagellar genes: early expression of genes encoding axonemal proteins during the spermatogenic cycle and shared features of promoters of genes encoding central apparatus proteins*

Sperm are motile cells. Thus, a significant component of the spermatogenic cycle is devoted to the formation of flagellum, a process that must be coordinated to insure proper construction. To document the temporal pattern of flagellar gene expression, we employed real-time PCR to assess changes in accumulation of a cohort of genes encoding axoneme, outer dense fibre (ODF) and fibrous sheath (FS) proteins during the first wave of spermatogenesis in the mouse. Axoneme genes were expressed first at the pachytene spermatocyte stage, followed by expression of transcripts encoding ODF and FS components. However, there were differences among these families with respect to the time of initial expression and the rate of mRNA accumulation. To gain understanding of factors that determine these patterns of expression, we cloned the promoters of three axoneme central apparatus genes (Pf6, Spag6 and Pf20). These promoters shared common features including the absence of a TATA box, and putative binding sites for several factors implicated in spermatogenesis (CREB/CREM, SOX17 and SPZ1) as well as ciliogenesis (FOXJ1). Collectively, our findings demonstrate a sequential pattern of expression of flagellar component genes, differential times of expression or rates of transcript accumulation within each class and shared promoter features within a class.

Ten-year experience using a plastic, disposable curette for the diagnosis of primary ciliary dyskinesia

BACKGROUND

Primary ciliary dyskinesia (PCD) results in impaired mucociliary clearance. Patients with this disorder develop chronic sinopulmonary disease with recurrent sinusitis, otitis media, nasal polyposis, pneumonia, and, ultimately, bronchiectasis. Other associated findings of dysfunctional ciliary activity include situs inversus, dextrocardia, and infertility.

OBJECTIVE

To describe our 10-year experience using a small, plastic, disposable curette to perform a screening procedure for cilia function and to collect samples for electron microscopy.

METHODS

In the past 10 years, we screened infants and children with severe chronic sinusitis and other chronic recurrent upper respiratory tract problems for PCD by using a plastic, disposable curette to collect tissue samples from the nasal mucosa. Samples were placed in sterile saline and examined under light microscopy for the presence of cilia. Failure to note ciliary movement prompted another examination 1 month later. If no functional cilia were noted at the follow-up examination, a specimen was obtained and sent for electron microscopy.

RESULTS

We identified 7 patients with PCD; 2 had situs inversus totalis. Average age at diagnosis was 3 years. The most common symptom at presentation was frequent upper respiratory tract infections with severe otitis media (7 patients) and sinusitis (5 patients). Recurrent pneumonia was present in 6 patients. Dynein arm deficiency was the most common electron microscopic diagnosis.

CONCLUSIONS

Evaluating children for PCD by using a plastic, disposable curette is a relatively simple procedure that could be used by allergists in practice. Primary ciliary dyskinesia occurs frequently enough that physicians should consider it as part of the differential diagnosis in evaluating children with recurrent, severe sinopulmonary infections.

The structure of dynein-c by negative stain electron microscopy

Dynein ATPases contain six concatenated AAA modules within the motor region of their heavy chains. Additional regions of sequence are required to form a functional ATPase, which a previous study suggested forms seven or eight subdomains arranged in either a ring or hollow sphere. A more recent homology model of the six AAA modules suggests that these form a ring. Therefore both the number and arrangement of subdomains remain uncertain. We show two-dimensional projection images of dynein-c in negative stain which reveal new details of its structure. Initial electron cryomicroscopy shows a similar overall morphology. The molecule consists of three domains: stem, head, and stalk. In the absence of nucleotide the head has seven lobes of density forming an asymmetric ring. An eighth lobe protrudes from one side of this heptameric ring and appears to join the elongated cargo-binding stem. The proximal stem is flexible, as is the stalk, suggesting that they act as compliant elements within the motor. A new analysis of pre- and post-power stroke conformations shows the combined effect of their flexibility on the spatial distribution of the microtubule-binding domain and therefore the potential range of power stroke sizes. We present and compare two alternative models of the structure of dynein.

Structural and molecular characterization of dynein in a gall-midge insect having motile sperm with only the outer arm

The dipteran Monarthropalpus flavus possesses a peculiar sperm axoneme, characterized by multiple rows of microtubular doublets linked by the outer dynein arms only, lacking any equivalent of the central pair/radial spoke complex. The structure of these dynein molecules was studied by electron microscopy (EM). Using the quick-freeze, deep-etch method of EM, they were found to be similar to outer dynein arms described previously. Two globular "heads," each subdivided by a cleft, are clearly discernible. "Stalks" extend from proximal head to contact the B-tubule of the adjacent doublet. Unlike the situation in vertebrate sperm, the stalks sometimes branch into two thinner strands that contact the B-tubule at different sites. Treatment of demembranated sperm cells with ATP and vanadate induces conformational changes in the dynein outer arms. These are interpreted as the result of rotation of the dynein head with respect to what is observed in axonemes in rigor condition (after ATP depletion). SDS-PAGE indicates that the high-molecular-weight complement of this molecule comprises a single heavy chain. Specific dynein heavy chain-related DNA sequences corresponding to the catalytic-phosphate binding region were amplified by RT-PCR. Only one axonemal dynein sequence was identified among all amplified fragments. Southern blot analysis performed on genomic DNA using this sequence as a probe identified two hybridizing genes, only one of which is able to encode a functional product. Thus, genetic analysis indicates that this axonemal outer arm dynein is a homodymer of a single heavy chain subunit. In vivo, spermatozoa of this species are stored in a rolled configuration in female spermatheca, where they move rapidly with a wave-like motion. This movement could not be reproduced in vitro, except when spermatozoa were constrained in a bent configuration by some mechanical impediment. We propose that, in the absence of both the central pair/radial spoke complex and the inner arms, a curvature-dependent activation acts to trigger motility in these spermatozoa.

The Rib43a protein is associated with forming the specialized protofilament ribbons of flagellar microtubules in Chlamydomonas

Ciliary and flagellar microtubules contain a specialized set of three protofilaments, termed ribbons, that are composed of tubulin and several associated proteins. Previous studies of sea urchin sperm flagella identified three of the ribbon proteins as tektins, which form coiled-coil filaments in doublet microtubules and which are associated with basal bodies and centrioles. To study the function of tektins and other ribbon proteins in the assembly of flagella and basal bodies, we have begun an analysis of ribbons from the unicellular biflagellate, Chlamydomonas reinhardtii, and report here the molecular characterization of the ribbon protein rib43a. Using antibodies against rib43a to screen an expression library, we recovered a full-length cDNA clone that encodes a 42,657-Da polypeptide. On Northern blots, the rib43a cDNA hybridized to a 1. 7-kb transcript, which was up-regulated upon deflagellation, consistent with a role for rib43a in flagellar assembly. The cDNA was used to isolate RIB43a, an approximately 4.6-kb genomic clone containing the complete rib43a coding region, and restriction fragment length polymorphism analysis placed the RIB43a gene on linkage group III. Sequence analysis of the RIB43a gene indicates that the substantially coiled-coil rib43a protein shares a high degree of sequence identity with clones from Trypanosoma cruzi and Homo sapiens (genomic, normal fetal kidney, and endometrial and germ cell tumors) but little sequence similarity to other proteins including tektins. Affinity-purified antibodies against native and bacterially expressed rib43a stained both flagella and basal bodies by immunofluorescence microscopy and stained isolated flagellar ribbons by immuno-electron microscopy. The structure of rib43a and its association with the specialized protofilament ribbons and with basal bodies is relevant to the proposed role of ribbons in forming and stabilizing doublet and triplet microtubules and in organizing their three-dimensional structure.

The reactivation of demembranated human spermatozoa lacking outer dynein arms is independent of pH

The effect of pH, Mg-ATP, and free calcium on activity of the inner dynein arm was investigated using demembranated human spermatozoa lacking the outer dynein arms (LODA). The results were compared with those obtained for demembranated-reactivated normal spermatozoa to evaluate the functional properties of the inner and outer dynein arms in axonemal motility. The reactivation of Triton X-100-demembranated LODA spermatozoa was analysed at various pHs and concentrations of Mg-ATP and calcium using video recordings. The percentage of reactivated LODA spermatozoa as a function of Mg-ATP concentration was not dependent on pH, whereas reactivation of normal human spermatozoa is pH dependent. This suggests that there may be a pH-dependent regulatory mechanism associated with the outer dynein arms. A delay in the principal bend propagation of normal and LODA reactivated cells was found at pH 7.1. This disappeared at pH 7.8 in normal but not in LODA populations. This suggests a role for outer dynein arms in the initiation of the propagation of flagellar bends at alkaline pH. The level of LODA and normal sperm reactivation both depended on the calcium concentration in the medium. At lower free calcium concentrations, the reactivation level and beat frequency of reactivated cells were higher. Our results suggest a functional difference between outer and inner dynein arms of human spermatozoa based on a differential pH sensitivity. Moreover, calcium seems to exert its regulatory action elsewhere than on the outer dynein arms.

Functional interactions among cytoskeleton, membranes, and cell wall in the pollen tube of flowering plants

The pollen tube is a cellular system that plays a fundamental role during the process of fertilization in higher plants. Because it is so important, the pollen tube has been subjected to intensive studies with the aim of understanding its biology. The pollen tube represents a fascinating model for studying interactions between the internal cytoskeletal machinery, the membrane system, and the cell wall. These compartments, often studied as independent units, show several molecular interactions and can influence the structure and organization of each other. The way the cell wall is constructed, the dynamics of the endomembrane system, and functions of the cytoskeleton suggest that these compartments are a molecular "continuum," which represents a link between the extracellular environment and the pollen tube cytoplasm. Several experimental approaches have been used to understand how these interactions may translate the pollen-pistil interactions into differential processes of pollen tube growth.

Antibodies that can discriminate between dynein heavy chains and their HUV1 photoproducts

Dyneins are multi-subunit enzymes that transduce chemical energy into the mechanical energy that makes cilia and flagella beat and moves organelles towards the minus end of microtubules. The ATPase activity is borne by heavy chains, and recent molecular analysis indicates that dynein heavy chain genes form an ancient multigene family: the similarity between the same isoform of two distantly related species is greater than that between different isoforms of the same species. We have exploited sequence identities between a Paramecium axonemal dynein heavy chain gene cloned in our laboratory and sequences of dynein heavy chains from other species to prepare antibodies against active-site peptides capable of recognizing dynein heavy chains regardless of species or isoform. One of the antibodies is perfectly specific for the larger product of V1 photolysis (HUV1) and thus incorporates a unique property of the hydrolytic ATP binding site of all known dynein heavy chains, the capacity for photocleavage in the presence of micromolar vanadate. Our characterization of these reagents suggests that they will be useful for biochemical and in situ studies of known dyneins as well as identification of potential new members of the family.

Role of disturbance of ependymal ciliary movement in development of hydrocephalus in rats

We have developed a new in vitro method of quantitatively analyzing ciliary movement in the ependymal wall of the aqueduct in rats. An axial slice of the midbrain containing ependymal wall was placed in a culture dish filled with a culture medium containing latex beads 1 micron in diameter at a concentration of 10(7) beads/ml. The movement of the beads caused by flow of culture medium generated by the to-and-fro ciliary movement was recorded by a high speed video system attached to an inverted phase-contrast microscope. Ciliary movement was expressed by the speed of the latex beads (micron/s). Aqueductal ciliary movement in congenitally hydrocephalic HTX rats, congenitally hydrocephalic WIC-Hyd rats, and other normal rats was evaluated. The results suggest that in congenitally hydrocephalic WIC-Hyd rats the degree of hydrocephalus related strongly to the degree of ciliary dyskinesia, but in congenitally hydrocephalic HTX rats it did not. Considering this discrepancy, we attempted to see whether or not hydrocephalus was caused by artificial disturbance of ependymal ciliary movement in vivo. We found that continuous infusion of metavanadate, an inhibitor of ciliary movement, into the III ventricle of normal Sprague-Dawley rats for 7 days induced dilatation of the ventricular system. Although the question whether or not disturbance of aqueductal ependymal ciliary movement is related to the development of human congenital hydrocephalus is debatable, the results of the present in vitro and in vivo experimental investigations appear to suggest that the disturbance of ciliary movement in the aqueduct could at least be one of the factors contributing to the inducement of hydrocephalus in experimental conditions.

A strategy for the reconstruction of structures possessing axial symmetry: sectioned axonemes in sperm flagella

The images of complex biological structures seen in the electron microscope, possessing an n-fold rotational symmetry, can be enhanced by averaging the axially repeating motif in order to improve their signal-to-noise ratio; this requires that the slices with n-fold symmetry do not exhibit distortions relative to one another. A strategy is proposed to detect the relative distortions and to remove them in order to obtain reliable results from the averaging process. Extensive use is made of cross-correlation analysis and of interpolation; because the procedure involves iterated resampling of the image, it is essential to adopt interpolation algorithms which preserve the spectral power. The procedure is illustrated by the analysis of transverse sections of the sperm flagellum axoneme of a stick insect; it might be used for the reconstruction of microtubules, nuclear pore complexes, virus capsids, and other supramolecular aggregates. The reconstructed images of axoneme sections reveal new information about the interactions between adjacent doublets; in particular, a double conjunction connecting the inner dynein arm with the nearest B-tubule has been consistently observed.

Excitable dynein model with multiple active sites for large-amplitude oscillations and bend propagation in flagella

The formal excitable dynein model proposed by Murase et al. (1989, J. theor. Biol. 139, 413-430) is modified to produce large-amplitude oscillations and excitability. The present model assumes that (i) each dynein arm has multiple active sites, which are distributed along most of the 24-nm distance between adjacent B-subtubule attachment sites; and (ii) any given dynein molecule tends to produce force continuously during interdoublet sliding in one direction and to produce little force during sliding in the opposite direction. Since no sliding motion occurs without superthreshold perturbations in the form of the sliding displacement, this new model also possesses an excitable nature. Once passive elastic components (e.g. nexin links and radial spokes) are incorporated into this model, oscillations with large amplitudes result. To test the ability of the model for bend propagation without a curvature-control mechanism, forced oscillations are applied to the basal end of the flagellum by the sliding displacement. It is found that bend propagation can occur even in the absence of a curvature-control mechanism.

Microtubule binding and translocation by inner dynein arm subtype I1

Structural, biochemical, and genetic evidence has demonstrated there are three inner dynein arm subforms, I1, I2, and I3, which differ in organization and composition (see Piperno et al.: J. Cell Biol. 110:379-389, 1990). Using dynein extracted from Chlamydomonas outer dynein armless mutant pf28, we have begun to define the structural and functional properties of isolated inner arm subforms. Inner dynein arm I1 was purified either by sucrose density gradient centrifugation or microtubule binding affinity. I1, composed of heavy chains 1 alpha and 1 beta, sedimented at 21S and selectively bound to and cross-linked purified microtubules in an ATP-sensitive manner. Deep etch electron microscopy revealed that the 21S sedimenting fraction contained two-headed structures in which large globular heads are connected by long, flexible-stem domains. In contrast, components derived from I2 and I3 sedimented as a mixture of 11S particles with single globular heads which did not bind to purified microtubules. Both the 21S and 11S sedimenting fractions supported microtubule translocation in in vitro motility assays. In 1 mM MgATP the I1-containing fraction produced very slow microtubule-gliding velocities (0.76 microns/sec) compared to the I2,I3-containing fraction (4.1 microns/sec).

Fine structure of guinea pig vestibular kinocilium

The fine structure of the utricular kinocilium of the guinea pig was examined with transmission electron microscopy after treatment with tannic acid to enhance resolution of internal morphology. The utricular kinocilium was devoid of inner dynein arms and a central pair of microtubules, while a set of outer dynein arms and radial spokes was found. This supports the hypothesis that the vestibular kinocilium is non-motile. Internal electron-dense particles at the attachment sites of the stereo-kinociliar bonds were situated in the immediate periphery of the outer dynein arms, although no visible connection existed between these structures. Findings obtained in the present study seem to give insight on the mechanism of mechanosensory transduction in the vestibular sensory cells.

The role of axonemal components in ciliary motility

1. The axoneme is the detergent-insoluble cytoskeleton of the cilium. 2. All axonemes generate movement by the same fundamental mechanism: microtubule sliding utilizing ATP hydrolysis during a mechanochemical cycling of dynein arms on the axonemal doublets. 3. Structure, fundamental biochemistry and physiology of the axoneme are conserved evolutionarily, but the phenotypes of beating movements and the responses to specific cytoplasmic signals differ greatly from organism to organism. 4. A model of asynchronous dynein arm activity--the switch point hypothesis--has been proposed to account for cyclic beating in the face of unidirectional sliding. The model suggests that the diversity of beat phenotype may be explicable by changes in the timing of switching between active and inactive states of doublet arm activity. Evidence of axonemal splitting in arrested axonemes provides new support for the hypothesis.

Isolated beta-heavy chain subunit of dynein translocates microtubules in vitro

Our goal was to assess the microtubule translocating ability of individual ATPase subunits of outer arm dynein. Solubilized outer arm dynein from sea urchin sperm (Stronglocentrotus purpuratus) was dissociated into subunits by low ionic strength buffer and fractionated by zonal centrifugation. Fractions were assessed by an in vitro functional assay wherein microtubules move across a glass surface to which isolated dynein fractions had been absorbed. Microtubule gliding activity was coincident with the 12-S beta-heavy chain-intermediate chain 1 ATPase fractions (beta/IC1). Neither the alpha-heavy chain nor the intermediate chains 2 and 3 fractions coincided with microtubule gliding activity. The beta/IC1 ATPase induced very rapid gliding velocities (9.7 +/- 0.88 micron/s, range 7-11.5 micron/s) in 1 mM ATP-containing motility buffers. In direct comparison, isolated intact 21-S outer arm dynein, from which the beta/IC1 fraction was derived, induced slower microtubule gliding rates (21-S dynein, 5.6 +/- 0.7 micron/s; beta/IC1, 8.7 +/- 1.2 micron/s). These results demonstrate that a single subdomain in dynein, the beta/IC1 ATPase, is sufficient for microtubule sliding activity.

Dynein as a microtubule translocator in ciliary motility: current studies of arm structure and activity pattern

The dynein arms of ciliary doublet microtubules cause adjacent axonemal doublets to slide apart with fixed polarity. This suggests that there is a unique mechanochemistry to the dynein arm with unidirectional force generation in all active arms and also that not all arms are active at once during a ciliary beat. Negative stain and thin-section images of arms in axonemes treated with beta, gamma methylene adenosine triphosphate (AMP-PCP) show a consistent subunit construction where the globular head of the arm interacts with subfiber B of doublet N+1. This interpretation differs from that provided by freeze etch and STEM interpretations of in situ arm construction and has implications for the mechanochemical cycle of the arm. A computer model of the arms in relation to other axonemal structures has been constructed to test these interpretations. Attachment of the head of the arm subfiber B is directly demonstrable in splayed axonemes in AMP-PCP. About half of the doublets in an axoneme show such attachments, while half do not. This might imply that about half the doublets in an axoneme are active at any given instant and can be identified as such. This information may be useful in probing questions of how active arms differ biochemically from inactive arms and of how microtubule translocators in general become active.

Isolation of a sixth dynein subunit adenosine triphosphatase of Chlamydomonas axonemes

This study of the axoneme led to the identification of a previously unknown adenosine triphosphatase (ATPase), which is likely a major component of inner dynein arms. The ATPase was isolated from a soluble fraction of axonemes obtained from pf 28, a Chlamydomonas mutant lacking the outer dynein arms. The activity hydrolyzed up to 2.3 mumol of ATP.min-1.mg-1 of protein (at pH 7.2, in the presence of both Ca++ and Mg++), had a sedimentation coefficient of 11S in sucrose gradient, and cosedimented with four polypeptides of apparent molecular weight 325,000, 315,000 140,000, and 42,000. Several arguments indicate that the new ATPase is a component of the inner dynein arms. Three or four polypeptides cosedimenting with the activity belong to a group of axonemal components that are deficient in the axonemes of pf 23 and pf 30, two mutants that display different levels of inner dynein arm deficiency. The 42,000 component is axonemal actin, a subunit of two other inner dynein ATPases. The two polypeptides of molecular weight greater than 300,000 have electrophoretic mobility similar to that of high molecular weight components of outer and inner dynein arms. In spite of some similarities each ATPase isolated from inner or outer arms is composed of a different set of polypeptides. Different ATPases may be required for the modulation of localized sliding of adjacent outer double microtubules in the axoneme.

Direction of force generated by the inner row of dynein arms on flagellar microtubules

Our goal was to determine the direction of force generation of the inner dynein arms in flagellar axonemes. We developed an efficient means of extracting the outer row of dynein arms in demembranated sperm tail axonemes, leaving the inner row of dynein arms structurally and functionally intact. Sperm tail axonemes depleted of outer arms beat at half the beat frequency of sperm tails with intact arms over a wide range of ATP concentrations. The isolated, outer arm-depleted axonemes were induced to undergo microtubule sliding in the presence of ATP and trypsin. Electron microscopic analysis of the relative direction of microtubule sliding (see Sale, W. S. and P. Satir, 1977, Proc. Natl. Acad. Sci. USA, 74:2045-2049) revealed that the doublet microtubule with the row of inner dynein arms, doublet N, always moved by sliding toward the proximal end of the axoneme relative to doublet N + 1. Therefore, the inner arms generate force such that doublet N pushes doublet N + 1 tipward. This is the same direction of microtubule sliding induced by ATP and trypsin in axonemes having both inner and outer dynein arms. The implications of this result for the mechanism of ciliary bending and utility in functional definition of cytoplasmic dyneins are discussed.

High-pressure liquid chromatography fractionation of Chlamydomonas dynein extracts and characterization of inner-arm dynein subunits

A rapid procedure for fractionating salt-stable dynein subunits from high-salt extracts of Chlamydomonas axonemes has been developed using a high-pressure liquid chromatography system with an anion exchange column and gradient salt elution. Five distinct fractions are shown to be highly enriched for five distinct subunits or subunit complexes by SDS/polyacrylamide gel electrophoresis. ATPase activity and electron microscopy. Peaks 1 and 4 contain, respectively, the single-headed gamma-subunit and the two-headed alpha/beta-heteropolymer that form the outer arm in situ and are dissociated by salt exposure; both peaks are absent from the outer arm-less mutant pf-28. Peaks 2, 3 and 5 contain, respectively, two distinct single-headed species and a double-headed species that derive from inner arms; all three peaks are missing from the inner arm-less mutant pf-23. Sucrose-gradient sedimentation analysis confirms these assignments and provides additional information on the intermediate-chain and light-chain composition of the inner-arm species. Electron microscopy of the purified inner-arm species visualized by the quick-freeze deep-etch technique complements a previous analysis of outer-arm species. Each protein is shown to have a unique morphology, and both the inner- and outer-arm proteins clearly belong to a common family whose structural divergence presumably reflects functional specialization.

Correlation of absent inner dynein arms and mucociliary clearance in a patient with Kartagener's syndrome

Recent experimental studies have demonstrated extensive biochemical differences between the outer and inner dynein arms of cilia. The inner dynein arms are now emerging as the "prime movers" for ciliary clearance. We studied ciliary motility as evidenced by radioisotope mucociliary clearance, and the ultrastructure of respiratory cilia in a patient with Kartagener's syndrome. Cilia exhibited complete absence of only the inner dynein arms, while retaining outer arms, and mucociliary clearance was totally absent. We also studied neutrophil chemotaxis and other immunologic functions in our patient and found them to be normal. Our findings demonstrate an association between the structural abnormality of absent inner dynein arms alone and ciliary immotility in Kartagener's syndrome. The neutrophil migration abnormalities may have a different mechanism which needs further study.

Bending patterns of Chlamydomonas flagella: IV. Mutants with defects in inner and outer dynein arms indicate differences in dynein arm function

Mutants with outer dynein arm defects or deficiencies all show a major reduction in beat frequency to about half the normal value; some of these mutants show an additional decrease in sliding velocity associated with reduced shear amplitude and an additional reduction in beat frequency, as well as other more minor modifications of the normal forward mode bending pattern. New mutants (ida98, pf30), which appear to be deficient in a subset of inner dynein arms show a reduction in sliding velocity that is primarily associated with a reduction in shear amplitude, with only a small reduction in beat frequency. These differences in motility phenotype between inner and outer dynein arm mutants suggest that inner and outer dynein arms may have distinct functions. The relatively large decrease in sliding velocity associated with partial loss of inner arms is consistent with earlier observations on pf23, a nonmotile mutant lacking inner arms, suggesting that inner arms may have an essential function in motility. The ability to generate reverse mode bending patterns is retained in some inner or outer dynein arm mutants, but appears to be decreased in those mutants which show reduced shear amplitude for the forward mode bending pattern.

Different structural states of a microtubule cross-linking molecule, captured by quick-freezing motile axostyles in protozoa

Freeze-etch preparation of the laminated bundles of microtubules in motile axostyles demonstrates that the cross-bridges populating individual layers or laminae are structurally similar to the dynein arms of cilia and flagellae. Also, like dynein, they are extracted by high salt and undergo a change in tilt upon removal of endogenous ATP (while the axostyle as a whole straightens and becomes stiff). On the other hand, the bridges running between adjacent microtubule laminae in the axostyle turn out to be much more delicate and wispy in appearance, and display no similarity to dynein arms. Thus we propose that the internal or "intra-laminar" cross-bridges are the active force-generating ATPases in this system, and that they generate overall bends or changes in the helical pitch of the axostyle by altering the longitudinal and lateral register of microtubules in each lamina individually; e.g., by "warping" each lamina and creating longitudinal shear forces within it. The cross-links between adjacent laminae, on the other hand, would then simply be force-transmitting elements that serve to translate the shearing forces generated within individual laminae into overall helical shape changes. (This hypothesis differs from the views of earlier workers who considered a more active role for the later cross-links, postulating that they cause an active sliding between adjacent layers that somehow leads to axostyle movement.) Also described here are physical connections between adjacent intra-laminar cross-bridges, structurally analogous to the overlapping components of the outer dynein arms of cilia and flagella. As with dynein, these may represent a mechanism for propagating local changes from cross-bridge to cross-bridge down the axostyle, as occurs during the passage of bends down the length of the organelle.