Assembly of FAP93 at the proximal axoneme in Chlamydomonas cilia

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

Composition and function of ciliary inner-dynein-arm subunits studied in Chlamydomonas reinhardtii

Motile cilia (also interchangeably called "flagella") are conserved organelles extending from the surface of many animal cells and play essential functions in eukaryotes, including cell motility and environmental sensing. Large motor complexes, the ciliary dyneins, are present on ciliary outer-doublet microtubules and drive movement of cilia. Ciliary dyneins are classified into two general types: the outer dynein arms (ODAs) and the inner dynein arms (IDAs). While ODAs are important for generation of force and regulation of ciliary beat frequency, IDAs are essential for control of the size and shape of the bend, features collectively referred to as waveform. Also, recent studies have revealed unexpected links between IDA components and human diseases. In spite of their importance, studies on IDAs have been difficult since they are very complex and composed for several types of IDA motors, each unique in composition and location in the axoneme. Thanks in part to genetic, biochemical, and structural analysis of Chlamydomonas reinhardtii, we are beginning to understand the organization and function of the ciliary IDAs. In this review, we summarize the composition of Chlamydomonas IDAs particularly focusing on each subunit, and discuss the assembly, conservation, and functional role(s) of these IDA subunits. Furthermore, we raise several additional questions/challenges regarding IDAs, and discuss future perspectives of IDA studies.

Fifty years of microtubule sliding in cilia

Motility of cilia (also known as flagella in some eukaryotes) is based on axonemal doublet microtubule sliding that is driven by the dynein molecular motors. Dyneins are organized into intricately patterned inner and outer rows of arms, whose collective activity is to produce inter-microtubule movement. However, to generate a ciliary bend, not all dyneins can be active simultaneously. The switch point model accounts, in part, for how dynein motors are regulated during ciliary movement. On the basis of this model, supported by key direct experimental observations as well as more recent theoretical and structural studies, we are now poised to understand the mechanics of how ciliary dynein coordination controls axonemal bend formation and propagation.

The IDA3 adapter, required for intraflagellar transport of I1 dynein, is regulated by ciliary length

We determined how the ciliary motor I1 dynein is transported. A specialized adapter, IDA3, facilitates I1 dynein attachment to the ciliary transporter called intraflagellar transport (IFT). Loading of IDA3 and I1 dynein on IFT is regulated by ciliary length.

Chlamydomonas DYX1C1/PF23 is essential for axonemal assembly and proper morphology of inner dynein arms

Cytoplasmic assembly of ciliary dyneins, a process known as preassembly, requires numerous non-dynein proteins, but the identities and functions of these proteins are not fully elucidated. Here, we show that the classical Chlamydomonas motility mutant pf23 is defective in the Chlamydomonas homolog of DYX1C1. The pf23 mutant has a 494 bp deletion in the DYX1C1 gene and expresses a shorter DYX1C1 protein in the cytoplasm. Structural analyses, using cryo-ET, reveal that pf23 axonemes lack most of the inner dynein arms. Spectral counting confirms that DYX1C1 is essential for the assembly of the majority of ciliary inner dynein arms (IDA) as well as a fraction of the outer dynein arms (ODA). A C-terminal truncation of DYX1C1 shows a reduction in a subset of these ciliary IDAs. Sucrose gradients of cytoplasmic extracts show that preassembled ciliary dyneins are reduced compared to wild-type, which suggests an important role in dynein complex stability. The role of PF23/DYX1C1 remains unknown, but we suggest that DYX1C1 could provide a scaffold for macromolecular assembly.

Most animal cells have antenna-like organelles called “cilia”. These organelles have various important functions both in motility and sensing the environment. Motile cilia are essential for moving cells as well as moving fluids across a surface. The waveform of motile cilia requires large macromolecular motors; these are the ciliary dyneins. These dynein complexes are assembled in the cytoplasm in a pathway called preassembly, and then transported into cilia. Defects in this process cause a heterogeneous human disease called primary ciliary dyskinesia that results, for example, in the disruption of the motility of respiratory tract cilia, sperm and nodal cilia during development. The mechanisms of the preassembly pathway are not fully understood. In this study, we use a mutation in the well-conserved DYX1C1/PF23 gene of the green alga, Chlamydomonas reinhardtii. Loss of a conserved domain (DYX) reveals a failure to assemble most ciliary dyneins. Preassembly of inner arm dyneins is particularly affected. We find that if dynein arms are not assembled, dynein subunits in the cytoplasm are unstable. We suggest that DYX1C1 may play a role as a scaffold for other preassembly factors and the dynein subunits.

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.

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.

Analysis of Axonemal Assembly During Ciliary Regeneration in Chlamydomonas

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

Alcohol-induced ciliary dysfunction targets the outer dynein arm

Alcohol abuse results in an increased incidence of pulmonary infection, in part attributable to impaired mucociliary clearance. Analysis of motility in mammalian airway cilia has revealed that alcohol impacts the ciliary dynein motors by a mechanism involving altered axonemal protein phosphorylation. Given the highly conserved nature of cilia, it is likely that the mechanisms for alcohol-induced ciliary dysfunction (AICD) are conserved. Thus we utilized the experimental advantages offered by the model organism, Chlamydomonas, to determine the precise effects of alcohol on ciliary dynein activity and identify axonemal phosphoproteins that are altered by alcohol exposure. Analysis of live cells or reactivated cell models showed that alcohol significantly inhibits ciliary motility in Chlamydomonas via a mechanism that is part of the axonemal structure. Taking advantage of informative mutant cells, we found that alcohol impacts the activity of the outer dynein arm. Consistent with this finding, alcohol exposure results in a significant reduction in ciliary beat frequency, a parameter of ciliary movement that requires normal outer dynein arm function. Using mutants that lack specific heavy-chain motor domains, we have determined that alcohol impacts the β- and γ-heavy chains of the outer dynein arm. Furthermore, using a phospho-threonine-specific antibody, we determined that the phosphorylation state of DCC1 of the outer dynein arm-docking complex is altered in the presence of alcohol, and its phosphorylation correlates with AICD. These results demonstrate that alcohol targets specific outer dynein arm components and suggest that DCC1 is part of an alcohol-sensitive mechanism that controls outer dynein arm activity.

FAP206 is a microtubule-docking adapter for ciliary radial spoke 2 and dynein c

Radial spokes are conserved macromolecular complexes that are essential for ciliary motility. Little is known about the assembly and functions of the three individual radial spokes, RS1, RS2, and RS3. In Tetrahymena, a conserved ciliary protein, FAP206, docks RS2 and dynein c to the doublet microtubule.

Radial spokes are conserved macromolecular complexes that are essential for ciliary motility. A triplet of three radial spokes, RS1, RS2, and RS3, repeats every 96 nm along the doublet microtubules. Each spoke has a distinct base that docks to the doublet and is linked to different inner dynein arms. Little is known about the assembly and functions of individual radial spokes. A knockout of the conserved ciliary protein FAP206 in the ciliate Tetrahymena resulted in slow cell motility. Cryo–electron tomography showed that in the absence of FAP206, the 96-nm repeats lacked RS2 and dynein c. Occasionally, RS2 assembled but lacked both the front prong of its microtubule base and dynein c, whose tail is attached to the front prong. Overexpressed GFP-FAP206 decorated nonciliary microtubules in vivo. Thus FAP206 is likely part of the front prong and docks RS2 and dynein c to the microtubule.

Kinesin-13 regulates the quantity and quality of tubulin inside cilia

Kinesin-13, a microtubule-end depolymerase, has been shown to affect the length of cilia, but its ciliary function is unclear. In Tetrahymena thermophila, kinesin-13 positively regulates the axoneme length, influences the properties of ciliary tubulin, and affects the ciliary dynein-dependent motility.

Kinesin-13, an end depolymerizer of cytoplasmic and spindle microtubules, also affects the length of cilia. However, in different models, depletion of kinesin-13 either lengthens or shortens cilia, and therefore the exact function of kinesin-13 in cilia remains unclear. We generated null mutations of all kinesin-13 paralogues in the ciliate Tetrahymena. One of the paralogues, Kin13Ap, localizes to the nuclei and is essential for nuclear divisions. The remaining two paralogues, Kin13Bp and Kin13Cp, localize to the cell body and inside assembling cilia. Loss of both Kin13Bp and Kin13Cp resulted in slow cell multiplication and motility, overgrowth of cell body microtubules, shortening of cilia, and synthetic lethality with either paclitaxel or a deletion of MEC-17/ATAT1, the α-tubulin acetyltransferase. The mutant cilia assembled slowly and contained abnormal tubulin, characterized by altered posttranslational modifications and hypersensitivity to paclitaxel. The mutant cilia beat slowly and axonemes showed reduced velocity of microtubule sliding. Thus kinesin-13 positively regulates the axoneme length, influences the properties of ciliary tubulin, and likely indirectly, through its effects on the axonemal microtubules, affects the ciliary dynein-dependent motility.

A Structural Basis for How Motile Cilia Beat

The motile cilium is a mechanical wonder, a cellular nanomachine that produces a high-speed beat based on a cycle of bends that move along an axoneme made of 9+2 microtubules. The molecular motors, dyneins, power the ciliary beat. The dyneins are compacted into inner and outer dynein arms, whose activity is highly regulated to produce microtubule sliding and axonemal bending. The switch point hypothesis was developed long ago to account for how sliding in the presence of axonemal radial spoke-central pair interactions causes the ciliary beat. Since then, a new genetic, biochemical, and structural complexity has been discovered, in part, with Chlamydomonas mutants, with high-speed, high-resolution analysis of movement and with cryoelectron tomography. We stand poised on the brink of new discoveries relating to the molecular control of motility that extend and refine our understanding of the basic events underlying the switching of arm activity and of bend formation and propagation.

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.

The Chlamydomonas mutant pf27 reveals novel features of ciliary radial spoke assembly

To address the mechanisms of ciliary radial spoke assembly, we took advantage of the Chlamydomonas pf27 mutant. The radial spokes that assemble in pf27 are localized to the proximal quarter of the axoneme, but otherwise are fully assembled into 20S radial spoke complexes competent to bind spokeless axonemes in vitro. Thus, pf27 is not defective in radial spoke assembly or docking to the axoneme. Rather, our results suggest that pf27 is defective in the transport of spoke complexes. During ciliary regeneration in pf27, radial spoke assembly occurs asynchronously from other axonemal components. In contrast, during ciliary regeneration in wild-type Chlamydomonas, radial spokes and other axonemal components assemble concurrently as the axoneme grows. Complementation in temporary dikaryons between wild-type and pf27 reveals rescue of radial spoke assembly that begins at the distal tip, allowing further assembly to proceed from tip to base of the axoneme. Notably, rescued assembly of radial spokes occurred independently of the established proximal radial spokes in pf27 axonemes in dikaryons. These results reveal that 20S radial spokes can assemble proximally in the pf27 cilium but as the cilium lengthens, spoke assembly requires transport. We postulate that PF27 encodes an adaptor or modifier protein required for radial spoke–IFT interaction.

Actin is required for IFT regulation in Chlamydomonas reinhardtii

Assembly of cilia and flagella requires intraflagellar transport (IFT), a highly regulated kinesin-based transport system that moves cargo from the basal body to the tip of flagella [1]. The recruitment of IFT components to basal bodies is a function of flagellar length, with increased recruitment in rapidly growing short flagella [2]. The molecular pathways regulating IFT are largely a mystery. Because actin network disruption leads to changes in ciliary length and number, actin has been proposed to have a role in ciliary assembly. However, the mechanisms involved are unknown. In Chlamydomonas reinhardtii, conventional actin is found in both the cell body and the inner dynein arm complexes within flagella [3, 4]. Previous work showed that treating Chlamydomonas cells with the actin-depolymerizing compound cytochalasin D resulted in reversible flagellar shortening [5], but how actin is related to flagellar length or assembly remains unknown. Here we utilize small-molecule inhibitors and genetic mutants to analyze the role of actin dynamics in flagellar assembly in Chlamydomonas reinhardtii. We demonstrate that actin plays a role in IFT recruitment to basal bodies during flagellar elongation and that when actin is perturbed, the normal dependence of IFT recruitment on flagellar length is lost. We also find that actin is required for sufficient entry of IFT material into flagella during assembly. These same effects are recapitulated with a myosin inhibitor, suggesting that actin may act via myosin in a pathway by which flagellar assembly is regulated by flagellar length.

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.

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.

Sequential assembly of flagellar radial spokes

The unicellular alga Chlamydomonas can assemble two 10 μm flagella in 1 h from proteins synthesized in the cell body. Targeting and transporting these proteins to the flagella are simplified by preassembly of macromolecular complexes in the cell body. Radial spokes are flagellar complexes that are partially assembled in the cell body before entering the flagella. On the axoneme, radial spokes are "T" shaped structures with a head of five proteins and a stalk of 18 proteins that sediment together at 20S. In the cell body, radial spokes are partially assembled; about half of the radial spoke proteins (RSPs) form a 12S complex. In mutants lacking a single RSP, smaller spoke subassemblies were identified. When extracts from two such mutants were mixed in vitro the 12S complex was assembled from several smaller complexes demonstrating that portions of the stepwise assembly of radial spoke assembly can be carried out in vitro to elucidate the order of spoke assembly in the cell body.

A unified taxonomy for ciliary dyneins

The formation and function of eukaryotic cilia/flagella require the action of a large array of dynein microtubule motor complexes. Due to genetic, biochemical, and microscopic tractability, Chlamydomonas reinhardtii has become the premier model system in which to dissect the role of dyneins in flagellar assembly, motility, and signaling. Currently, 54 proteins have been described as components of various Chlamydomonas flagellar dyneins or as factors required for their assembly in the cytoplasm and/or transport into the flagellum; orthologs of nearly all these components are present in other ciliated organisms including humans. For historical reasons, the nomenclature of these diverse dynein components and their corresponding genes, mutant alleles, and orthologs has become extraordinarily confusing. Here, we unify Chlamydomonas dynein gene nomenclature and establish a systematic classification scheme based on structural properties of the encoded proteins. Furthermore, we provide detailed tabulations of the various mutant alleles and protein aliases that have been used and explicitly define the correspondence with orthologous components in other model organisms and humans.

An axonemal PP2A B-subunit is required for PP2A localization and flagellar motility

Analysis of Chlamydomonas axonemes revealed that the protein phosphatase, PP2A, is localized to the outer doublet microtubules and is implicated in regulation of dynein-driven motility. We tested the hypothesis that PP2A is localized to the axoneme by a specialized, highly conserved 55-kDa B-type subunit identified in the Chlamydomonas flagellar proteome. The B-subunit gene is defective in the motility mutant pf4. Consistent with our hypothesis, both the B- and C- subunits of PP2A fail to assemble in pf4 axonemes, while the dyneins and other axonemal structures are fully assembled in pf4 axonemes. Two pf4 intragenic revertants were recovered that restore PP2A to the axonemes and re-establish nearly wild-type motility. The revertants confirmed that the slow-swimming Pf4 phenotype is a result of the defective PP2A B-subunit. These results demonstrate that the axonemal B-subunit is, in part, an anchor protein required for PP2A localization and that PP2A is required for normal ciliary motility.

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.

Regulation of ciliary motility: conserved protein kinases and phosphatases are targeted and anchored in the ciliary axoneme

Recent evidence has revealed that the dynein motors and highly conserved signaling proteins are localized within the ciliary 9+2 axoneme. One key mechanism for regulation of motility is phosphorylation. Here, we review diverse evidence, from multiple experimental organisms, that ciliary motility is regulated by phosphorylation/dephosphorylation of the dynein arms through kinases and phosphatases that are anchored immediately adjacent to their axonemal substrates.

Distinct roles of 1alpha and 1beta heavy chains of the inner arm dynein I1 of Chlamydomonas flagella

We took advantage of Chlmaydomonas flagellar mutant strains lacking either the 1α or 1β motor domain in I1 dynein to distinguish the functional role of each. The 1β motor domain is an effective motor required for control of microtubule sliding, whereas the 1α motor domain may restrain microtubule sliding driven by other dyneins.

The Chlamydomonas I1 dynein is a two-headed inner dynein arm important for the regulation of flagellar bending. Here we took advantage of mutant strains lacking either the 1α or 1β motor domain to distinguish the functional role of each motor domain. Single- particle electronic microscopic analysis confirmed that both the I1α and I1β complexes are single headed with similar ringlike, motor domain structures. Despite similarity in structure, however, the I1β complex has severalfold higher ATPase activity and microtubule gliding motility compared to the I1α complex. Moreover, in vivo measurement of microtubule sliding in axonemes revealed that the loss of the 1β motor results in a more severe impairment in motility and failure in regulation of microtubule sliding by the I1 dynein phosphoregulatory mechanism. The data indicate that each I1 motor domain is distinct in function: The I1β motor domain is an effective motor required for wild-type microtubule sliding, whereas the I1α motor domain may be responsible for local restraint of microtubule sliding.

Tubulin glutamylation regulates ciliary motility by altering inner dynein arm activity

How microtubule-associated motor proteins are regulated is not well understood. A potential mechanism for spatial regulation of motor proteins is provided by posttranslational modifications of tubulin subunits that form patterns on microtubules. Glutamylation is a conserved tubulin modification [1] that is enriched in axonemes. The enzymes responsible for this posttranslational modification, glutamic acid ligases (E-ligases), belong to a family of proteins with a tubulin tyrosine ligase (TTL) homology domain (TTL-like or TTLL proteins) [2]. We show that in cilia of Tetrahymena, TTLL6 E-ligases generate glutamylation mainly on the B-tubule of outer doublet microtubules, the site of force production by ciliary dynein. Deletion of two TTLL6 paralogs caused severe deficiency in ciliary motility associated with abnormal waveform and reduced beat frequency. In isolated axonemes with a normal dynein arm composition, TTLL6 deficiency did not affect the rate of ATP-induced doublet microtubule sliding. Unexpectedly, the same TTLL6 deficiency increased the velocity of microtubule sliding in axonemes that also lack outer dynein arms, in which forces are generated by inner dynein arms. We conclude that tubulin glutamylation on the B-tubule inhibits the net force imposed on sliding doublet microtubules by inner dynein arms.

The regulation of dynein-driven microtubule sliding in Chlamydomonas flagella by axonemal kinases and phosphatases

The purpose of this chapter is to review the methodology and advances that have revealed conserved signaling proteins that are localized in the 9+2 ciliary axoneme for regulating motility. Diverse experimental systems have revealed that ciliary and eukaryotic flagellar motility is regulated by second messengers including calcium, pH, and cyclic nucleotides. In addition, recent advances in in vitro functional studies, taking advantage of isolated axonemes, pharmacological approaches, and biochemical analysis of axonemes have demonstrated that otherwise ubiquitous, conserved protein kinases and phosphatases are transported to and anchored in the axoneme. Here, we focus on the functional/pharmacological, genetic, and biochemical approaches in the model genetic system Chlamydomonas that have revealed highly conserved kinases, anchoring proteins (e.g., A-kinase anchoring proteins), and phosphatases that are physically located in the axoneme where they play a direct role in control of motility.

A novel ankyrin-repeat protein interacts with the regulatory proteins of inner arm dynein f (I1) of Chlamydomonas reinhardtii

How ciliary and flagellar motility is regulated is a challenging problem. The flagellar movement in Chlamydomonas reinhardtii is in part regulated by phosphorylation of a 138 kD intermediate chain (IC138) of inner arm dynein f (also called I1). In the present study, we found that the axoneme of mutants lacking dynein f lacks a novel protein having ankyrin repeat motifs, registered as FAP120 in the flagellar proteome database. FAP120 is also missing or decreased in the axonemes of bop5, a mutant that has a mutation in the structural gene of IC138 but assembles the dynein f complex. Intriguingly, the amounts of FAP120 in the axonemes of different alleles of bop5 and several dynein f-lacking mutants roughly parallel their contents of IC138. These results suggest a weak but stoichiometric interaction between FAP120 and IC138. We propose that FAP120 functions in the regulatoryprocess as part of a protein complex involving IC138. Cell Motil. Cytoskeleton 2008. (c) 2008 Wiley-Liss, Inc.

Regulation of dynein-driven microtubule sliding by the axonemal protein kinase CK1 in Chlamydomonas flagella

CK1 puts the brakes on dynein activity when added to purified axonemes in vitro, presumably to regulate how flagella bend.

Experimental analysis of isolated ciliary/flagellar axonemes has implicated the protein kinase casein kinase I (CK1) in regulation of dynein. To test this hypothesis, we developed a novel in vitro reconstitution approach using purified recombinant Chlamydomonas reinhardtii CK1, together with CK1-depleted axonemes from the paralyzed flagellar mutant pf17, which is defective in radial spokes and impaired in dynein-driven microtubule sliding. The CK1 inhibitors (DRB and CK1-7) and solubilization of CK1 restored microtubule sliding in pf17 axonemes, which is consistent with an inhibitory role for CK1. The phosphatase inhibitor microcystin-LR blocked rescue of microtubule sliding, indicating that the axonemal phosphatases, required for rescue, were retained in the CK1-depleted axonemes. Reconstitution of depleted axonemes with purified, recombinant CK1 restored inhibition of microtubule sliding in a DRB– and CK1-7–sensitive manner. In contrast, a purified “kinase-dead” CK1 failed to restore inhibition. These results firmly establish that an axonemal CK1 regulates dynein activity and flagellar motility.

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.

IC97 is a novel intermediate chain of I1 dynein that interacts with tubulin and regulates interdoublet sliding

Our goal is to understand the assembly and regulation of flagellar dyneins, particularly the Chlamydomonas inner arm dynein called I1 dynein. Here, we focus on the uncharacterized I1-dynein IC IC97. The IC97 gene encodes a novel IC without notable structural domains. IC97 shares homology with the murine lung adenoma susceptibility 1 (Las1) protein--a candidate tumor suppressor gene implicated in lung tumorigenesis. Multiple, independent biochemical assays determined that IC97 interacts with both alpha- and beta-tubulin subunits within the axoneme. I1-dynein assembly mutants suggest that IC97 interacts with both the IC138 and IC140 subunits within the I1-dynein motor complex and that IC97 is part of a regulatory complex that contains IC138. Microtubule sliding assays, using axonemes containing I1 dynein but devoid of IC97, show reduced microtubule sliding velocities that are not rescued by kinase inhibitors, revealing a critical role for IC97 in I1-dynein function and control of dynein-driven motility.

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.

Disruption of the A-kinase anchoring domain in flagellar radial spoke protein 3 results in unregulated axonemal cAMP-dependent protein kinase activity and abnormal flagellar motility

Biochemical studies of Chlamydomonas flagellar axonemes revealed that radial spoke protein (RSP) 3 is an A-kinase anchoring protein (AKAP). To determine the physiological role of PKA anchoring in the axoneme, an RSP3 mutant, pf14, was transformed with an RSP3 gene containing a mutation in the PKA-binding domain. Analysis of several independent transformants revealed that the transformed cells exhibit an unusual phenotype: a fraction of the cells swim normally; the remainder of the cells twitch feebly or are paralyzed. The abnormal/paralyzed motility is not due to an obvious deficiency of radial spoke assembly, and the phenotype cosegregates with the mutant RSP3. We postulated that paralysis was due to failure in targeting and regulation of axonemal cAMP-dependent protein kinase (PKA). To test this, reactivation experiments of demembranated cells were performed in the absence or presence of PKA inhibitors. Importantly, motility in reactivated cell models mimicked the live cell phenotype with nearly equal fractions of motile and paralyzed cells. PKA inhibitors resulted in a twofold increase in the number of motile cells, rescuing paralysis. These results confirm that flagellar RSP3 is an AKAP and reveal that a mutation in the PKA binding domain results in unregulated axonemal PKA activity and inhibition of normal motility.

Radial spoke proteins of Chlamydomonas flagella

The radial spoke is a ubiquitous component of '9+2' cilia and flagella, and plays an essential role in the control of dynein arm activity by relaying signals from the central pair of microtubules to the arms. The Chlamydomonas reinhardtii radial spoke contains at least 23 proteins, only 8 of which have been characterized at the molecular level. Here, we use mass spectrometry to identify 10 additional radial spoke proteins. Many of the newly identified proteins in the spoke stalk are predicted to contain domains associated with signal transduction, including Ca2+-, AKAP- and nucleotide-binding domains. This suggests that the spoke stalk is both a scaffold for signaling molecules and itself a transducer of signals. Moreover, in addition to the recently described HSP40 family member, a second spoke stalk protein is predicted to be a molecular chaperone, implying that there is a sophisticated mechanism for the assembly of this large complex. Among the 18 spoke proteins identified to date, at least 12 have apparent homologs in humans, indicating that the radial spoke has been conserved throughout evolution. The human genes encoding these proteins are candidates for causing primary ciliary dyskinesia, a severe inherited disease involving missing or defective axonemal structures, including the radial spokes.

Flagellar radial spoke protein 2 is a calmodulin binding protein required for motility in Chlamydomonas reinhardtii

Genetic and morphological studies have revealed that the radial spokes regulate ciliary and flagellar bending. Functional and biochemical analysis and the discovery of calmodulin in the radial spokes suggest that the regulatory mechanism involves control of axonemal protein phosphorylation and calcium binding to spoke proteins. To identify potential regulatory proteins in the radial spoke, in-gel kinase assays were performed on isolated axonemes and radial spoke fractions. The results indicated that radial spoke protein 2 (RSP2) can bind ATP and transfer phosphate in vitro. RSP2 was cloned and mapped to the PF24 locus, a gene required for motility. Sequencing revealed that pf24 contains a point mutation converting the first ATG to ATA, resulting in only trace amounts of RSP2 and confirming the RSP2 mapping. Surprisingly, the sequence does not include signature domains for conventional kinases, indicating that RSP2 may not perform as a protein kinase in vivo. However, the predicted RSP2 protein sequence contains Ca2+-dependent calmodulin binding motifs and a GAF domain, a domain found in diverse signaling proteins for binding small ligands including cyclic nucleotides. As predicted from the sequence, recombinant RSP2 binds calmodulin in a calcium-dependent manner. We postulate that RSP2 is a regulatory subunit of the radial spoke involved in localization of calmodulin for control of motility.

IC138 is a WD-repeat dynein intermediate chain required for light chain assembly and regulation of flagellar bending

Increased phosphorylation of dynein IC IC138 correlates with decreases in flagellar microtubule sliding and phototaxis defects. To test the hypothesis that regulation of IC138 phosphorylation controls flagellar bending, we cloned the IC138 gene. IC138 encodes a novel protein with a calculated mass of 111 kDa and is predicted to form seven WD-repeats at the C terminus. IC138 maps near the BOP5 locus, and bop5-1 contains a point mutation resulting in a truncated IC138 lacking the C terminus, including the seventh WD-repeat. bop5-1 cells display wild-type flagellar beat frequency but swim slower than wild-type cells, suggesting that bop5-1 is altered in its ability to control flagellar waveform. Swimming speed is rescued in bop5-1 transformants containing the wild-type IC138, confirming that BOP5 encodes IC138. With the exception of the roadblock-related light chain, LC7b, all the other known components of the I1 complex, including the truncated IC138, are assembled in bop5-1 axonemes. Thus, the bop5-1 motility phenotype reveals a role for IC138 and LC7b in the control of flagellar bending. IC138 is hyperphosphorylated in paralyzed flagellar mutants lacking radial spoke and central pair components, further indicating a role for the radial spokes and central pair apparatus in control of IC138 phosphorylation and regulation of flagellar waveform.

Chlamydomonas reinhardtiiproduces a profilin with unusual biochemical properties

We report the characterization of a profilin orthologue from Chlamydomonas reinhardtii. CrPRF, probably the only profilin isoform, is present in both the cell body and flagella. Examination of vegetative and gametic cells by immunofluorescence microscopy using multiple fixation procedures also revealed enrichment of CrPRF at the anterior of the cell near the base of flagella and near the base of the fertilization tubule in mating type plus gametes. Purified, recombinant CrPRF binds to actin with a Kd value ∼10–7 and displaces nuclei in a live cell ‘nuclear displacement’ assay, consistent with profilin’s ability to bind G-actin in vivo. However, when compared with other profilin isoforms, CrPRF has a relatively low affinity for poly-L-proline and for phosphatidylinositol (4,5) bisphosphate micelles. Furthermore, and surprisingly, CrPRF inhibits exchange of adenine nucleotide on G-actin in a manner similar to human ADF or DNase I. Thus, we postulate that a primary role for CrPRF is to sequester actin in Chlamydomonas. The unusual biochemical properties of CrPRF offer a new opportunity to distinguish specific functions for profilin isoforms.

Localization of calmodulin and dynein light chain LC8 in flagellar radial spokes

Genetic and in vitro analyses have revealed that radial spokes play a crucial role in regulation of ciliary and flagellar motility, including control of waveform. However, the mechanisms of regulation are not understood. Here, we developed a novel procedure to isolate intact radial spokes as a step toward understanding the mechanism by which these complexes regulate dynein activity. The isolated radial spokes sediment as 20S complexes that are the size and shape of radial spokes. Extracted radial spokes rescue radial spoke structure when reconstituted with isolated axonemes derived from the radial spoke mutant pf14. Isolated radial spokes are composed of the 17 previously defined spoke proteins as well as at least five additional proteins including calmodulin and the ubiquitous dynein light chain LC8. Analyses of flagellar mutants and chemical cross-linking studies demonstrated calmodulin and LC8 form a complex located in the radial spoke stalk. We postulate that calmodulin, located in the radial spoke stalk, plays a role in calcium control of flagellar bending.

Flagellar radial spoke protein 3 is an A-kinase anchoring protein (AKAP)

Previous physiological and pharmacological experiments have demonstrated that the Chlamydomonas flagellar axoneme contains a cAMP-dependent protein kinase (PKA) that regulates axonemal motility and dynein activity. However, the mechanism for anchoring PKA in the axoneme is unknown. Here we test the hypothesis that the axoneme contains an A-kinase anchoring protein (AKAP). By performing RII blot overlays on motility mutants defective for specific axonemal structures, two axonemal AKAPs have been identified: a 240-kD AKAP associated with the central pair apparatus, and a 97-kD AKAP located in the radial spoke stalk. Based on a detailed analysis, we have shown that AKAP97 is radial spoke protein 3 (RSP3). By expressing truncated forms of RSP3, we have localized the RII-binding domain to a region between amino acids 144-180. Amino acids 161-180 are homologous with the RII-binding domains of other AKAPs and are predicted to form an amphipathic helix. Amino acid substitution of the central residues of this region (L to P or VL to AA) results in the complete loss of RII binding. RSP3 is located near the inner arm dyneins, where an anchored PKA would be in direct position to modify dynein activity and regulate flagellar motility.

Peter Satir -- investigating the structural basis for cell function

Peter Satir has devoted his research career to elucidating the structural basis for ciliary motility. His ingenious use of structural analysis, combined with identification of powerful model systems, provided a model for the sliding microtubule hypothesis of ciliary bending and led to the discovery that dynein is a 'minus-end'-directed motor whose regulated activity underpins the bending motion of cilia. Here, we focus on ciliary motility to illustrate Satir's pioneering contributions to cell biology.

Casein kinase I is anchored on axonemal doublet microtubules and regulates flagellar dynein phosphorylation and activity

Flagellar dynein activity is regulated by phosphorylation. One critical phosphoprotein substrate in Chlamydomonas is the 138-kDa intermediate chain (IC138) of the inner arm dyneins (Habermacher, G., and Sale, W. S. (1997) J. Cell Biol. 136, 167-176). In this study, several approaches were used to determine that casein kinase I (CKI) is physically anchored in the flagellar axoneme and regulates IC138 phosphorylation and dynein activity. First, using a videomicroscopic motility assay, selective CKI inhibitors rescued dynein-driven microtubule sliding in axonemes isolated from paralyzed flagellar mutants lacking radial spokes. Rescue of dynein activity failed in axonemes isolated from these mutant cells lacking IC138. Second, CKI was unequivocally identified in salt extracts from isolated axonemes, whereas casein kinase II was excluded from the flagellar compartment. Third, Western blots indicate that within flagella, CKI is anchored exclusively to the axoneme. Analysis of multiple Chlamydomonas motility mutants suggests that the axonemal CKI is located on the outer doublet microtubules. Finally, CKI inhibitors that rescued dynein activity blocked phosphorylation of IC138. We propose that CKI is anchored on the outer doublet microtubules in position to regulate flagellar dynein.

Protein phosphatases PP1 and PP2A are located in distinct positions in the Chlamydomonas flagellar axoneme

We postulated that microcystin-sensitive protein phosphatases are integral components of the Chlamydomonas flagellar axoneme, positioned to regulate inner arm dynein activity. To test this, we took a direct biochemical approach. Microcystin-Sepharose affinity purification revealed a prominent 35-kDa axonemal protein, predicted to be the catalytic subunit of type-1 protein phosphatase (PP1c). We cloned the Chlamydomonas PP1c and produced specific polyclonal peptide antibodies. Based on western blot analysis, the 35-kDa PP1c is anchored in the axoneme. Moreover, analysis of flagella and axonemes from mutant strains revealed that PP1c is primarily, but not exclusively, anchored in the central pair apparatus, associated with the C1 microtubule. Thus, PP1 is part of the central pair mechanism that controls flagellar motility. Two additional axonemal proteins of 62 and 37 kDa were also isolated using microcystin-Sepharose affinity. Based on direct peptide sequence and western blots, these proteins are the A- and C-subunits of type 2A protein phosphatase (PP2A). The axonemal PP2A is not one of the previously identified components of the central pair apparatus, outer arm dynein, inner arm dynein, dynein regulatory complex or the radial spokes. We postulate PP2A is anchored on the doublet microtubules, possibly in position to directly control inner arm dynein activity.

Characterization of a Chlamydomonas insertional mutant that disrupts flagellar central pair microtubule-associated structures

Two alleles at a new locus, central pair-associated complex 1 (CPC1), were selected in a screen for Chlamydomonas flagellar motility mutations. These mutations disrupt structures associated with central pair microtubules and reduce flagellar beat frequency, but do not prevent changes in flagellar activity associated with either photophobic responses or phototactic accumulation of live cells. Comparison of cpc1 and pf6 axonemes shows that cpc1 affects a row of projections along C1 microtubules distinct from those missing in pf6, and a row of thin fibers that form an arc between the two central pair microtubules. Electron microscopic images of the central pair in axonemes from radial spoke-defective strains reveal previously undescribed central pair structures, including projections extending laterally toward radial spoke heads, and a diagonal link between the C2 microtubule and the cpc1 projection. By SDS-PAGE, cpc1 axonemes show reductions of 350-, 265-, and 79-kD proteins. When extracted from wild-type axonemes, these three proteins cosediment on sucrose gradients with three other central pair proteins (135, 125, and 56 kD) in a 16S complex. Characterization of cpc1 provides new insights into the structure and biochemistry of the central pair apparatus, and into its function as a regulator of dynein-based motility.

The Mr 140,000 intermediate chain of Chlamydomonas flagellar inner arm dynein is a WD-repeat protein implicated in dynein arm anchoring

Previous structural and biochemical studies have revealed that the inner arm dynein I1 is targeted and anchored to a unique site located proximal to the first radial spoke in each 96-nm axoneme repeat on flagellar doublet microtubules. To determine whether intermediate chains mediate the positioning and docking of dynein complexes, we cloned and characterized the 140-kDa intermediate chain (IC140) of the I1 complex. Sequence and secondary structural analysis, with particular emphasis on beta-sheet organization, predicted that IC140 contains seven WD repeats. Reexamination of other members of the dynein intermediate chain family of WD proteins indicated that these polypeptides also bear seven WD/beta-sheet repeats arranged in the same pattern along each intermediate chain protein. A polyclonal antibody was raised against a 53-kDa fusion protein derived from the C-terminal third of IC140. The antibody is highly specific for IC140 and does not bind to other dynein intermediate chains or proteins in Chlamydomonas flagella. Immunofluorescent microscopy of Chlamydomonas cells confirmed that IC140 is distributed along the length of both flagellar axonemes. In vitro reconstitution experiments demonstrated that the 53-kDa C-terminal fusion protein binds specifically to axonemes lacking the I1 complex. Chemical cross-linking indicated that IC140 is closely associated with a second intermediate chain in the I1 complex. These data suggest that IC140 contains domains responsible for the assembly and docking of the I1 complex to the doublet microtubule cargo.

The Chlamydomonas IDA7 locus encodes a 140-kDa dynein intermediate chain required to assemble the I1 inner arm complex

To identify new loci that are involved in the assembly and targeting of dynein complexes, we have screened a collection of motility mutants that were generated by insertional mutagenesis. One such mutant, 5B10, lacks the inner arm isoform known as the I1 complex. This isoform is located proximal to the first radial spoke in each 96-nm axoneme repeat and is an important target for the regulation of flagellar motility. Complementation tests reveal that 5B10 represents a new I1 locus, IDA7. Biochemical analyses confirm that ida7 axonemes lack at least five I1 complex subunits. Southern blots probed with a clone containing the gene encoding the 140-kDa intermediate chain (IC) indicate that the ida7 mutation is the result of plasmid insertion into the IC140 gene. Transformation with a wild-type copy of the IC140 gene completely rescues the mutant defects. Surprisingly, transformation with a construct of the IC140 gene lacking the first four exons of the coding sequence also rescues the mutant phenotype. These studies indicate that IC140 is essential for assembly of the I1 complex, but unlike other dynein ICs, the N-terminal region is not critical for its activity.

Regulation of flagellar dynein by phosphorylation of a 138-kD inner arm dynein intermediate chain

One of the challenges in understanding ciliary and flagellar motility is determining the mechanisms that locally regulate dynein-driven microtubule sliding. Our recent studies demonstrated that microtubule sliding, in Chlamydomonas flagella, is regulated by phosphorylation. However, the regulatory proteins remain unknown. Here we identify the 138-kD intermediate chain of inner arm dynein I1 as the critical phosphoprotein required for regulation of motility. This conclusion is founded on the results of three different experimental approaches. First, genetic analysis and functional assays revealed that regulation of microtubule sliding, by phosphorylation, requires inner arm dynein I1. Second, in vitro phosphorylation indicated the 138-kD intermediate chain of I1 is the only phosphorylated subunit. Third, in vitro reconstitution demonstrated that phosphorylation and dephosphorylation of the 138-kD intermediate chain inhibits and restores wild-type microtubule sliding, respectively. We conclude that change in phosphorylation of the 138-kD intermediate chain of I1 regulates dynein-driven microtubule sliding. Moreover, based on these and other data, we predict that regulation of I1 activity is involved in modulation of flagellar waveform.

Regulation of flagellar dynein by an axonemal type-1 phosphatase in Chlamydomonas

Physiological studies have demonstrated that flagellar radial spokes regulate inner arm dynein activity in Chlamydomonas and that an axonemal cAMP-dependent kinase inhibits dynein activity in radial spoke defective axonemes. These studies also suggested that an axonemal protein phosphatase is required for activation of flagellar dynein. We tested whether inhibitors of protein phosphatases would prevent activation of dynein by the kinase inhibitor PKI in Chlamydomonas axonemes lacking radial spokes. As predicted, preincubation of spoke defective axonemes (pf14 and pf17) with ATP gamma S maintained the slow dynein-driven microtubule sliding characteristic of paralyzed axonemes lacking spokes, and blocked activation of dynein-driven microtubule sliding by subsequent addition of PKI. Preincubation of spoke defective axonemes with the phosphatase inhibitors okadaic acid, microcystin-LR or inhibitor-2 also potently blocked PKI-induced activation of microtubule sliding velocity: the non-inhibitory okadaic acid analog, 1-norokadaone, did not. ATP gamma S or the phosphatase inhibitors blocked activation of dynein in a double mutant lacking the radial spokes and the outer dynein arms (pf14pf28). We concluded that the axoneme contains a type-1 phosphatase required for activation of inner arm dynein. We postulated that the radial spokes regulate dynein through the activity of the type-1 protein phosphatase. To test this, we performed in vitro reconstitution experiments using inner arm dynein from the double mutant pf14pf28 and dynein-depleted axonemes containing wild-type radial spokes (pf28). As described previously, microtubule sliding velocity was increased from approximately 2 microns/second to approximately 7 microns/second when inner arm dynein from pf14pf28 axonemes ws reconstituted with axonemes containing wild-type spokes. In contrast, pretreatment of inner arm dynein from pf14pf28 axonemes with ATP gamma S, or reconstitution in the presence of microcystin-LR, blocked increased velocity following reconstitution, despite the presence of wild-type radial spokes. We conclude that the radial spokes, through the activity of an axonemal type-1 phosphatase, activate inner arm dynein by dephosphorylation of a critical dynein component. Wild-type radial spokes also operate to inhibit the axonemal cAMP-dependent kinase, which would otherwise inhibit axonemal dynein and motility.

Regulation of dynein-driven microtubule sliding by an axonemal kinase and phosphatase in Chlamydomonas flagella

The following is a summary of physiological and pharmacological studies of the regulation of dynein-driven microtubule sliding in Chlamydomonas flagella. The experimental basis for the study is described, and data indicating that an axonemal cAMP-dependent protein kinase can regulate inner arm dynein activity are reviewed. In addition, preliminary data are summarized indicating that an axonemal type 1 phosphatase can also regulate dynein-drive microtubule sliding velocity. It is predicted that the protein kinase, phosphatase, and an inner dynein arm component form a regulatory complex in the axoneme.