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

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.

STYXL1 regulates CCT complex assembly and flagellar tubulin folding in sperm formation

Tubulin-based microtubule is a core component of flagella axoneme and essential for sperm motility and male fertility. Structural components of the axoneme have been well explored. However, how tubulin folding is regulated in sperm flagella formation is still largely unknown. Here, we report a germ cell-specific co-factor of CCT complex, STYXL1. Deletion of Styxl1 results in male infertility and microtubule defects of sperm flagella. Proteomic analysis of Styxl1-/- sperm reveals abnormal downregulation of flagella-related proteins including tubulins. The N-terminal rhodanese-like domain of STYXL1 is important for its interactions with CCT complex subunits, CCT1, CCT6 and CCT7. Styxl1 deletion leads to defects in CCT complex assembly and tubulin polymerization. Collectively, our findings reveal the vital roles of germ cell-specific STYXL1 in CCT-facilitated tubulin folding and sperm flagella development.

Mucociliary Wnt signaling promotes cilia biogenesis and beating

It is widely thought that Wnt/Lrp6 signaling proceeds through the cytoplasm and that motile cilia are signaling-inert nanomotors. Contrasting both views, we here show in the mucociliary epidermis of X. tropicalis embryos that motile cilia transduce a ciliary Wnt signal that is distinct from canonical β-catenin signaling. Instead, it engages a Wnt-Gsk3-Ppp1r11-Pp1 signaling axis. Mucociliary Wnt signaling is essential for ciliogenesis and it engages Lrp6 co-receptors that localize to cilia via a VxP ciliary targeting sequence. Live-cell imaging using a ciliary Gsk3 biosensor reveals an immediate response of motile cilia to Wnt ligand. Wnt treatment stimulates ciliary beating in X. tropicalis embryos and primary human airway mucociliary epithelia. Moreover, Wnt treatment improves ciliary function in X. tropicalis ciliopathy models of male infertility and primary ciliary dyskinesia (ccdc108, gas2l2). We conclude that X. tropicalis motile cilia are Wnt signaling organelles that transduce a distinct Wnt-Pp1 response.

Primary cilia are WNT-transducing organelles whose biogenesis is controlled by a WNT-PP1 axis

WNT signaling is important in development, stem cell maintenance, and disease. WNT ligands typically signal via receptor activation across the plasma membrane to induce β-catenin-dependent gene activation. Here, we show that in mammalian primary cilia, WNT receptors relay a WNT/GSK3 signal that β-catenin-independently promotes ciliogenesis. Characterization of a LRP6 ciliary targeting sequence and monitoring of acute WNT co-receptor activation (phospho-LRP6) support this conclusion. Ciliary WNT signaling inhibits protein phosphatase 1 (PP1) activity, a negative regulator of ciliogenesis, by preventing GSK3-mediated phosphorylation of the PP1 regulatory inhibitor subunit PPP1R2. Concordantly, deficiency of WNT/GSK3 signaling by depletion of cyclin Y and cyclin-Y-like protein 1 induces primary cilia defects in mouse embryonic neuronal precursors, kidney proximal tubules, and adult mice preadipocytes.

IIIG9 inhibition in adult ependymal cells changes adherens junctions structure and induces cellular detachment

Ependymal cells have multiple apical cilia that line the ventricular surfaces and the central canal of spinal cord. In cancer, the loss of ependymal cell polarity promotes the formation of different types of tumors, such as supratentorial anaplastic ependymomas, which are highly aggressive in children. IIIG9 (PPP1R32) is a protein restricted to adult ependymal cells located in cilia and in the apical cytoplasm and has unknown function. In this work, we studied the expression and localization of IIIG9 in the adherens junctions (cadherin/β-catenin-positive junctions) of adult brain ependymal cells using confocal and transmission electron microscopy. Through in vivo loss-of-function studies, ependymal denudation (single-dose injection experiments of inhibitory adenovirus) was observed, inducing the formation of ependymal cells with a "balloon-like" morphology. These cells had reduced cadherin expression (and/or delocalization) and cleavage of the cell death marker caspase-3, with "cilia rigidity" morphology (probably vibrational beating activity) and ventriculomegaly occurring prior to these events. Finally, after performing continuous infusions of adenovirus for 14 days, we observed total cell denudation and reactive parenchymal astrogliosis. Our data confirmed that IIIG9 is essential for the maintenance of adherens junctions of polarized ependymal cells. Eventually, altered levels of this protein in ependymal cell differentiation may increase ventricular pathologies, such as hydrocephalus or neoplastic transformation.

Heme-binding protein CYB5D1 is a radial spoke component required for coordinated ciliary beating

Coordinated beating is crucial for the function of multiple cilia. However, the molecular mechanism is poorly understood. Here, we characterize a conserved ciliary protein CYB5D1 with a heme-binding domain and a cordon-bleu ubiquitin-like domain. Mutation or knockdown of Cyb5d1 in zebrafish impaired coordinated ciliary beating in the otic vesicle and olfactory epithelium. Similarly, the two flagella of an insertional mutant of the CYB5D1 ortholog in Chlamydomonas (Crcyb5d1) showed an uncoordinated pattern due to a defect in the cis-flagellum. Biochemical analyses revealed that CrCYB5D1 is a radial spoke stalk protein that binds heme only under oxidizing conditions. Lack of CrCYB5D1 resulted in a reductive shift in flagellar redox state and slowing down of the phototactic response. Treatment of Crcyb5d1 with oxidants restored coordinated flagellar beating. Taken together, these data suggest that CrCYB5D1 may integrate environmental and intraciliary signals and regulate the redox state of cilia, which is crucial for the coordinated beating of multiple cilia.

Central Apparatus, the Molecular Kickstarter of Ciliary and Flagellar Nanomachines

Motile cilia and homologous organelles, the flagella, are an early evolutionarily invention, enabling primitive eukaryotic cells to survive and reproduce. In animals, cilia have undergone functional and structural speciation giving raise to typical motile cilia, motile nodal cilia, and sensory immotile cilia. In contrast to other cilia types, typical motile cilia are able to beat in complex, two-phase movements. Moreover, they contain many additional structures, including central apparatus, composed of two single microtubules connected by a bridge-like structure and assembling numerous complexes called projections. A growing body of evidence supports the important role of the central apparatus in the generation and regulation of the motile cilia movement. Here we review data concerning the central apparatus structure, protein composition, and the significance of its components in ciliary beating regulation.

Proteome of the central apparatus of a ciliary axoneme

The central apparatus is an essential component of “9+2” cilia. Zhao et al. identify more than 40 new potential components of the central apparatus of Chlamydomonas. Many are conserved and will facilitate genetic screening of patients with a form of primary ciliary dyskinesia that is difficult to diagnose.

Nearly all motile cilia have a “9+2” axoneme containing a central apparatus (CA), consisting of two central microtubules with projections, that is essential for motility. To date, only 22 proteins are known to be CA components. To identify new candidate CA proteins, we used mass spectrometry to compare axonemes of wild-type Chlamydomonas and a CA-less mutant. We identified 44 novel candidate CA proteins, of which 13 are conserved in humans. Five of the latter were studied more closely, and all five localized to the CA; therefore, most of the other candidates are likely to also be CA components. Our results reveal that the CA is far more compositionally complex than previously recognized and provide a greatly expanded knowledge base for studies to understand the architecture of the CA and how it functions. The discovery of the new conserved CA proteins will facilitate genetic screening to identify patients with a form of primary ciliary dyskinesia that has been difficult to diagnose.

PKA, PP1, and DC1 phosphorylation mediate alcohol-induced ciliary dysfunction in Chlamydomonas reinhardtii

Excessive alcohol consumption impairs mucociliary clearance, in part, by compromising ciliary movement. Our previous study found alcohol reduces ciliary beat frequency in Chlamydomonas through a mechanism that involves the β and γ heavy chains of the outer dynein arm (ODA). Moreover, we identified DC1, a subunit of the ODA-docking complex (ODA-DC), as the first ciliary target for alcohol. DC1 phosphorylation is alcohol sensitive and correlates with alcohol-induced ciliary dysfunction (AICD). Furthermore, DC1 phosphorylation is disrupted in the absence of the central pair and ODA. These results implicate a role for DC1 phosphorylation in regulating the ODA activity and mediating AICD. In our current study, we identified four alcohol-sensitive phosphosites in DC1: S33, T73, T351, and S628. Mutations of these sites rescue the assembly of the ODA-DC and ODA, resulting in wild-type swimming velocities. When cells were challenged with alcohol, we determined that three sites, S33, T351, and S628, are critical for mediating the ciliary slowing effects of alcohol. This result is consistent with our pharmacological studies, which reveal that both PP1 and PKA activities are required for AICD.

Redox regulation of motile cilia in airway disease

Motile cilia on airway cells are necessary for clearance of mucus-trapped particles out of the lung. Ciliated airway epithelial cells are uniquely exposed to oxidants through trapping of particles, debris and pathogens in mucus and the direct exposure to inhaled oxidant gases. Dynein ATPases, the motors driving ciliary motility, are sensitive to the local redox environment within each cilium. Several redox-sensitive cilia-localized proteins modulate dynein activity and include Protein Kinase A, Protein Kinase C, and Protein Phosphatase 1. Moreover, cilia are rich in known redox regulatory proteins and thioredoxin domain-containing proteins that are critical in maintaining a balanced redox environment. Importantly, a nonsense mutation in TXNDC3, which contains a thioredoxin motif, has recently been identified as disease-causing in Primary Ciliary Dyskinesia, a hereditary motile cilia disease resulting in impaired mucociliary clearance. Here we review current understanding of the role(s) oxidant species play in modifying airway ciliary function. We focus on oxidants generated in the airways, cilia redox targets that modulate ciliary beating and imbalances in redox state that impact health and disease. Finally, we review disease models such as smoking, asthma, alcohol drinking, and infections as well as the direct application of oxidants that implicate redox balance as a modulator of cilia motility.

Ciliary Length Sensing Regulates IFT Entry via Changes in FLA8/KIF3B Phosphorylation to Control Ciliary Assembly

The length of cilia is robustly regulated [1]. Previous data suggest that cells possess a sensing system to control ciliary length [2-5]. However, the details of the mechanism are currently not known [6, 7]. Such a system requires a mechanism that responds to ciliary length, and consequently, disruption of that response system should alter ciliary length [1]. The assembly rate of cilium mediated by intraflagellar transport (IFT) gradually decreases as the cilium elongates and eventually is balanced by the constant rate of disassembly, at which point cilium elongation stops [8, 9]. Because the rate of IFT entry into the cilium also decreases as the cilium elongates [10], regulation of IFT entry could provide the mechanism for length control. Previously, we showed that phosphorylation of the FLA8/KIF3B subunit of the anterograde kinesin-II IFT motor blocks IFT entry and flagellar assembly in Chlamydomonas [11]. Here, we show in Chlamydomonas that cellular signaling in response to alteration of flagellar length regulates phosphorylation of FLA8/KIF3B, which restricts IFT entry and, thus, flagellar assembly to control flagellar length. Cellular levels of phosphorylated FLA8 (pFLA8) are tightly linked to flagellar length: FLA8 phosphorylation is reduced in cells with short flagella and elevated in cells with long flagella. Depletion of the phosphatases CrPP1 and CrPP6 increases the level of cellular pFLA8, leading to short flagella due to decreased IFT entry. The results demonstrate that ciliary length control is achieved by a cellular sensing system that controls IFT entry through phosphorylation of the anterograde IFT motor.

A microtubule-dynein tethering complex regulates the axonemal inner dynein f (I1)

FAP44 and FAP43/FAP244 form a complex that tethers the Inner dynein subspecies f to the microtubule in Chlamydomonas flagella. The tether complex regulates flagellar motility by restraining conformational change in the dynein motor.

Motility of cilia/flagella is generated by a coordinated activity of thousands of dyneins. Inner dynein arms (IDAs) are particularly important for the formation of ciliary/flagellar waveforms, but the molecular mechanism of IDA regulation is poorly understood. Here we show using cryoelectron tomography and biochemical analyses of Chlamydomonas flagella that a conserved protein FAP44 forms a complex that tethers IDA f (I1 dynein) head domains to the A-tubule of the axonemal outer doublet microtubule. In wild-type flagella, IDA f showed little nucleotide-dependent movement except for a tilt in the f β head perpendicular to the microtubule-sliding direction. In the absence of the tether complex, however, addition of ATP and vanadate caused a large conformational change in the IDA f head domains, suggesting that the movement of IDA f is mechanically restricted by the tether complex. Motility defects in flagella missing the tether demonstrates the importance of the IDA f-tether interaction in the regulation of ciliary/flagellar beating.

Expression of a Novel Ciliary Protein, IIIG9, During the Differentiation and Maturation of Ependymal Cells

IIIG9 is the regulatory subunit 32 of protein phosphatase 1 (PPP1R32), a key phosphatase in the regulation of ciliary movement. IIIG9 localization is restricted to cilia in the trachea, fallopian tube, and testicle, suggesting its involvement in the polarization of ciliary epithelium. In the adult brain, IIIG9 mRNA has only been detected in ciliated ependymal cells that cover the ventricular walls. In this work, we prepared a polyclonal antibody against rat IIIG9 and used this antibody to show for the first time the ciliary localization of this protein in adult ependymal cells. We demonstrated IIIG9 localization at the apical border of the ventricular wall of 17-day-old embryonic (E17) and 1-day-old postnatal (PN1) brains and at the level of ependymal cilia at 10- and 20-day-old postnatal (PN10-20) using temporospatial distribution analysis and comparing the localization with a ciliary marker. Spectral confocal and super-resolution Structured Illumination Microscopy (SIM) analysis allowed us to demonstrate that IIIG9 shows a punctate pattern that is preferentially located at the borders of ependymal cilia in situ and in cultures of ependymocytes obtained from adult rat brains. Finally, by immunogold ultrastructural analysis, we showed that IIIG9 is preferentially located between the axoneme and the ciliary membrane. Taken together, our data allow us to conclude that IIIG9 is localized in the cilia of adult ependymal cells and that its expression is correlated with the process of ependymal differentiation and with the maturation of radial glia. Similarly, its particular localization within ependymal cilia suggests a role of this protein in the regulation of ciliary movement.

The Central Apparatus of Cilia and Eukaryotic Flagella

The motile cilium is a complex organelle that is typically comprised of a 9+2 microtubule skeleton; nine doublet microtubules surrounding a pair of central singlet microtubules. Like the doublet microtubules, the central microtubules form a scaffold for the assembly of protein complexes forming an intricate network of interconnected projections. The central microtubules and associated structures are collectively referred to as the central apparatus (CA). Studies using a variety of experimental approaches and model organisms have led to the discovery of a number of highly conserved protein complexes, unprecedented high-resolution views of projection structure, and new insights into regulation of dynein-driven microtubule sliding. Here, we review recent progress in defining mechanisms for the assembly and function of the CA and include possible implications for the importance of the CA in human health.

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

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

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.

Protein phosphatases PP2A, PP4 and PP6: mediators and regulators in development and responses to environmental cues

The three closely related groups of serine/threonine protein phosphatases PP2A, PP4 and PP6 are conserved throughout eukaryotes. The catalytic subunits are present in trimeric and dimeric complexes with scaffolding and regulatory subunits that control activity and confer substrate specificity to the protein phosphatases. In Arabidopsis, three scaffolding (A subunits) and 17 regulatory (B subunits) proteins form complexes with five PP2A catalytic subunits giving up to 255 possible combinations. Three SAP-domain proteins act as regulatory subunits of PP6. Based on sequence similarities with proteins in yeast and mammals, two putative PP4 regulatory subunits are recognized in Arabidopsis. Recent breakthroughs have been made concerning the functions of some of the PP2A and PP6 regulatory subunits, for example the FASS/TON2 in regulation of the cellular skeleton, B' subunits in brassinosteroid signalling and SAL proteins in regulation of auxin transport. Reverse genetics is starting to reveal also many more physiological functions of other subunits. A system with key regulatory proteins (TAP46, TIP41, PTPA, LCMT1, PME-1) is present in all eukaryotes to stabilize, activate and inactivate the catalytic subunits. In this review, we present the status of knowledge concerning physiological functions of PP2A, PP4 and PP6 in Arabidopsis, and relate these to yeast and mammals.

Ciliary transport regulates PDGF-AA/αα signaling via elevated mammalian target of rapamycin signaling and diminished PP2A activity

Primary cilia are built and maintained by intraflagellar transport (IFT), whereby the two IFT complexes, IFTA and IFTB, carry cargo via kinesin and dynein motors for anterograde and retrograde transport, respectively. Many signaling pathways, including platelet- derived growth factor (PDGF)-AA/αα, are linked to primary cilia. Active PDGF-AA/αα signaling results in phosphorylation of Akt at two residues: P-Akt(T308) and P-Akt(S473), and previous work showed decreased P-Akt(S473) in response to PDGF-AA upon anterograde transport disruption. In this study, we investigated PDGF-AA/αα signaling via P-Akt(T308) and P-Akt(S473) in distinct ciliary transport mutants. We found increased Akt phosphorylation in the absence of PDGF-AA stimulation, which we show is due to impaired dephosphorylation resulting from diminished PP2A activity toward P-Akt(T308). Anterograde transport mutants display low platelet-derived growth factor receptor (PDGFR)α levels, whereas retrograde mutants exhibit normal PDGFRα levels. Despite this, neither shows an increase in P-Akt(S473) or P-Akt(T308) upon PDGF-AA stimulation. Because mammalian target of rapamycin complex 1 (mTORC1) signaling is increased in ciliary transport mutant cells and mTOR signaling inhibits PDGFRα levels, we demonstrate that inhibition of mTORC1 rescues PDGFRα levels as well as PDGF-AA-dependent phosphorylation of Akt(S473) and Akt(T308) in ciliary transport mutant MEFs. Taken together, our data indicate that the regulation of mTORC1 signaling and PP2A activity by ciliary transport plays key roles in PDGF-AA/αα signaling.

Subcellular localization and function of mouse radial spoke protein 3 in mammalian cells and central nervous system

Radial spoke protein 3 (RSP3) was first identified in Chlamydomonas as a component of the radial spoke. The mammalian homologue of the Chlamydomonas RSP3 gene is mainly expressed in testis and developing central nervous system (CNS). However, the subcellular localization and function of mammalian RSP3 in the developing brain and mammalian cells remain poorly understood. Here we show that the mouse RSP3 accumulates at the perinuclear region of Chinese hamster ovary (CHO) and 293T cells. Detailed analysis shows that the mouse RSP3 is not co-localized with the endoplasmic reticulum or Golgi apparatus markers in CHO cells. Using in utero electroporation, we found that over-expression of mammalian RSP3 increases the percentage of neurons reaching the upper cortical plate. In vivo analysis shows that the mouse RSP3 mainly accumulates in the proximal cytoplasmic dilation of the leading process of the migrating cortical neurons. Furthermore, we find that the mammalian RSP3 concentrates in the ependymal cilia as a component of the cilia. Thus, our data provide the first evidence for the subcellular localization and function of mammalian RSP3 in mammalian cells and developing CNS.

The global phosphoproteome of Chlamydomonas reinhardtii reveals complex organellar phosphorylation in the flagella and thylakoid membrane

Chlamydomonas reinhardtii is the most intensively-studied and well-developed model for investigation of a wide-range of microalgal processes ranging from basic development through understanding triacylglycerol production. Although proteomic technologies permit interrogation of these processes at the protein level and efforts to date indicate phosphorylation-based regulation of proteins in C. reinhardtii is essential for its underlying biology, characterization of the C. reinhardtii phosphoproteome has been limited. Herein, we report the richest exploration of the C. reinhardtii proteome to date. Complementary enrichment strategies were used to detect 4588 phosphoproteins distributed among every cellular component in C. reinhardtii. Additionally, we report 18,160 unique phosphopeptides at <1% false discovery rate, which comprise 15,862 unique phosphosites - 98% of which are novel. Given that an estimated 30% of proteins in a eukaryotic cell are subject to phosphorylation, we report the majority of the phosphoproteome (23%) of C. reinhardtii. Proteins in key biological pathways were phosphorylated, including photosynthesis, pigment production, carbon assimilation, glycolysis, and protein and carbohydrate metabolism, and it is noteworthy that hyperphosphorylation was observed in flagellar proteins. This rich data set is available via ProteomeXchange (ID: PXD000783) and will significantly enhance understanding of a range of regulatory mechanisms controlling a variety of cellular process and will serve as a critical resource for the microalgal community.

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.

Identification and characterization of two distinct PPP1R2 isoforms in human spermatozoa

Background

Protein Ser/Thr Phosphatase PPP1CC2 is an alternatively spliced isoform of PPP1C that is highly enriched in testis and selectively expressed in sperm. Addition of the phosphatase inhibitor toxins okadaic acid or calyculin A to caput and caudal sperm triggers and stimulates motility, respectively. Thus, the endogenous mechanisms of phosphatase inhibition are fundamental for controlling sperm function and should be characterized. Preliminary results have shown a protein phosphatase inhibitor activity resembling PPP1R2 in bovine and primate spermatozoa.

Results

Here we show conclusively, for the first time, that PPP1R2 is present in sperm. In addition, we have also identified a novel protein, PPP1R2P3. The latter was previously thought to be an intron-less pseudogene. We show that the protein corresponding to the pseudogene is expressed. It has PPP1 inhibitory potency similar to PPP1R2. The potential phosphosites in PPP1R2 are substituted by non-phosphorylable residues, T73P and S87R, in PPP1R2P3. We also confirm that PPP1R2/PPP1R2P3 are phosphorylated at Ser121 and Ser122, and report a novel phosphorylation site, Ser127. Subfractionation of sperm structures show that PPP1CC2, PPP1R2/PPP1R2P3 are located in the head and tail structures.

Conclusions

The conclusive identification and localization of sperm PPP1R2 and PPP1R2P3 lays the basis for future studies on their roles in acrosome reaction, sperm motility and hyperactivation. An intriguing possibility is that a switch in PPP1CC2 inhibitory subunits could be the trigger for sperm motility in the epididymis and/or sperm hyperactivation in the female reproductive tract.

TCTEX1D4, a novel protein phosphatase 1 interactor: connecting the phosphatase to the microtubule network

Reversible phosphorylation plays an important role as a mechanism of intracellular control in eukaryotes. PPP1, a major eukaryotic Ser/Thr-protein phosphatase, acquires its specificity by interacting with different protein regulators, also known as PPP1 interacting proteins (PIPs). In the present work we characterized a physiologically relevant PIP in testis. Using a yeast two-hybrid screen with a human testis cDNA library, we identified a novel PIP of PPP1CC2 isoform, the T-complex testis expressed protein 1 domain containing 4 (TCTEX1D4) that has recently been described as a Tctex1 dynein light chain family member. The overlay assays confirm that TCTEX1D4 interacts with the different spliced isoforms of PPP1CC. Also, the binding domain occurs in the N-terminus, where a consensus PPP1 binding motif (PPP1BM) RVSF is present. The distribution of TCTEX1D4 in testis suggests its involvement in distinct functions, such as TGFβ signaling at the blood–testis barrier and acrosome cap formation. Immunofluorescence in human ejaculated sperm shows that TCTEX1D4 is present in the flagellum and in the acrosome region of the head. Moreover, TCTEX1D4 and PPP1 co-localize in the microtubule organizing center (MTOC) and microtubules in cell cultures. Importantly, the TCTEX1D4 PPP1BM seems to be relevant for complex formation, for PPP1 retention in the MTOC and movement along microtubules.

These novel results open new avenues to possible roles of this dynein, together with PPP1. In essence TCTEX1D4/PPP1C complex appears to be involved in microtubule dynamics, sperm motility, acrosome reaction and in the regulation of the blood–testis barrier.

Conserved structural motifs in the central pair complex of eukaryotic flagella

Cilia and flagella are conserved hair-like appendages of eukaryotic cells that function as sensing and motility generating organelles. Motility is driven by thousands of axonemal dyneins that require precise regulation. One essential motility regulator is the central pair complex (CPC) and many CPC defects cause paralysis of cilia/flagella. Several human diseases, such as immotile cilia syndrome, show CPC abnormalities, but little is known about the detailed three-dimensional (3D) structure and function of the CPC. The CPC is located in the center of typical [9+2] cilia/flagella and is composed of two singlet microtubules (MTs), each with a set of associated projections that extend toward the surrounding nine doublet MTs. Using cryo-electron tomography coupled with subtomogram averaging, we visualized and compared the 3D structures of the CPC in both the green alga Chlamydomonas and the sea urchin Strongylocentrotus at the highest resolution published to date. Despite the evolutionary distance between these species, their CPCs exhibit remarkable structural conservation. We identified several new projections, including those that form the elusive sheath, and show that the bridge has a more complex architecture than previously thought. Organism-specific differences include the presence of MT inner proteins in Chlamydomonas, but not Strongylocentrotus, and different overall outlines of the highly connected projection network, which forms a round-shaped cylinder in algae, but is more oval in sea urchin. These differences could be adaptations to the mechanical requirements of the rotating CPC in Chlamydomonas, compared to the Strongylocentrotus CPC which has a fixed orientation.

Arabidopsis thaliana histone deacetylase 14 (HDA14) is an α-tubulin deacetylase that associates with PP2A and enriches in the microtubule fraction with the putative histone acetyltransferase ELP3

It is now emerging that many proteins are regulated by a variety of covalent modifications. Using microcystin-affinity chromatography we have purified multiple protein phosphatases and their associated proteins from Arabidopsis thaliana. One major protein purified was the histone deacetylase HDA14. We demonstrate that HDA14 can deacetylate α-tubulin, associates with α/β-tubulin and is retained on GTP/taxol-stabilized microtubules, at least in part, by direct association with the PP2A-A2 subunit. Like HDA14, the putative histone acetyltransferase ELP3 was purified on microcystin-Sepharose and is also enriched at microtubules, potentially functioning in opposition to HDA14 as the α-tubulin acetylating enzyme. Consistent with the likelihood of it having many substrates throughout the cell, we demonstrate that HDA14, ELP3 and the PP2A A-subunits A1, A2 and A3 all reside in both the nucleus and cytosol of the cell. The association of a histone deacetylase with PP2A suggests a direct link between protein phosphorylation and acetylation.

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

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

Integrated control of axonemal dynein AAA(+) motors

Axonemal dyneins are AAA(+) enzymes that convert ATP hydrolysis to mechanical work. This leads to the sliding of doublet microtubules with respect to each other and ultimately the generation of ciliary/flagellar beating. However, in order for useful work to be generated, the action of individual dynein motors must be precisely controlled. In addition, cells modulate the motility of these organelles through a variety of second messenger systems and these signals too must be integrated by the dynein motors to yield an appropriate output. This review describes the current status of efforts to understand dynein control mechanisms and their connectivity focusing mainly on studies of the outer dynein arm from axonemes of the unicellular biflagellate green alga Chlamydomonas.

Three-dimensional structure of the radial spokes reveals heterogeneity and interactions with dyneins in Chlamydomonas flagella

Cryo–electron tomography of Chlamydomonas flagella reveals previously uncharacterized features of the radial spokes, including structural heterogeneity and direct interactions with dyneins and between the spoke heads. A “radial spoke 3 stand-in” occupies what would be the site of a third spoke in organisms with spoke triplets.

Radial spokes (RSs) play an essential role in the regulation of axonemal dynein activity and thus of ciliary and flagellar motility. However, few details are known about the complexes involved. Using cryo–electron tomography and subtomogram averaging, we visualized the three-dimensional structure of the radial spokes in Chlamydomonas flagella in unprecedented detail. Unlike many other species, Chlamydomonas has only two spokes per axonemal repeat, RS1 and RS2. Our data revealed previously uncharacterized features, including two-pronged spoke bases that facilitate docking to the doublet microtubules, and that inner dyneins connect directly to the spokes. Structures of wild type and the headless spoke mutant pf17 were compared to define the morphology and boundaries of the head, including a direct RS1-to-RS2 interaction. Although the overall structures of the spokes are very similar, we also observed some differences, corroborating recent findings about heterogeneity in the docking of RS1 and RS2. In place of a third radial spoke we found an uncharacterized, shorter electron density named “radial spoke 3 stand-in,” which structurally bears no resemblance to RS1 and RS2 and is unaltered in the pf17 mutant. These findings demonstrate that radial spokes are heterogeneous in structure and may play functionally distinct roles in axoneme regulation.

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.

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.

Mechanisms of mammalian ciliary motility: Insights from primary ciliary dyskinesia genetics

Motile cilia and flagella are organelles that, historically, have been poorly understood and inadequately investigated. However, cilia play critical roles in fluid clearance in the respiratory system and the brain, and flagella are required for sperm motility. Genetic studies involving human patients and mouse models of primary ciliary dyskinesia over the last decade have uncovered a number of important ciliary proteins and have begun to elucidate the mechanisms underlying ciliary motility. When combined with genetic, biochemical, and cell biological studies in Chlamydomonas reinhardtii, these mammalian genetic analyses begin to reveal the mechanisms by which ciliary motility is regulated.

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.

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.

Analysis of flagellar phosphoproteins from Chlamydomonas reinhardtii

Cilia and flagella are cell organelles that are highly conserved throughout evolution. For many years, the green biflagellate alga Chlamydomonas reinhardtii has served as a model for examination of the structure and function of its flagella, which are similar to certain mammalian cilia. Proteome analysis revealed the presence of several kinases and protein phosphatases in these organelles. Reversible protein phosphorylation can control ciliary beating, motility, signaling, length, and assembly. Despite the importance of this posttranslational modification, the identities of many ciliary phosphoproteins and knowledge about their in vivo phosphorylation sites are still missing. Here we used immobilized metal affinity chromatography to enrich phosphopeptides from purified flagella and analyzed them by mass spectrometry. One hundred forty-one phosphorylated peptides were identified, belonging to 32 flagellar proteins. Thereby, 126 in vivo phosphorylation sites were determined. The flagellar phosphoproteome includes different structural and motor proteins, kinases, proteins with protein interaction domains, and many proteins whose functions are still unknown. In several cases, a dynamic phosphorylation pattern and clustering of phosphorylation sites were found, indicating a complex physiological status and specific control by reversible protein phosphorylation in the flagellum.

Sperm structure of Limoniidae and their phylogenetic relationship with Tipulidae (Diptera, Nematocera)

The sperm ultrastructure of a few species of Limoniidae (Limonia nigropunctata; L. nubeculosa; Chionea n. sp.; C. alpina; C. lutescens) was studied. The two species of Limonia have a monolayered acrosome with crystallized material, a three-lobed nucleus in cross section, a ring of centriole adjunct material and a flagellum which consists of a 9+9+1 axoneme and a single mitochondrial derivative. The central axonemal tubule is provided with 15 protofilaments in its tubular wall, while the accessory tubules have 13 protofilaments and are flanked by the electron-dense intertubular material. The three species of Chionea share a monolayered acrosome, a nucleus with two longitudinal grooves, a centriole adjunct material which surrounds the centriole and the initial part of the axoneme. The axoneme is of conventional type, with 9+9+2 microtubular pattern, with accessory tubules provided with 13 protofilaments and intertubular material. However, in C. lutescens the accessory tubules start with 15 protofilaments and transform into a tubule with 13 protofilaments. These data are discussed in the light of the phylogenetic relationship between Limoniidae and Tipulidae. For this purpose, the sperm ultrastructure of Nephrotoma appendiculata was also considered comparatively.

Ultrastructural and biochemical analysis of a new mutation in Chlamydomonas reinhardtii affecting the central pair apparatus

We present a new Chlamydomonas reinhardtii flagellar mutant in which central pair projections are missing and the central pair microtubules are twisted along the length of the flagellum. We have named this mutant tcp1 for twisted central pair. Immunoblots using an antibody that recognizes the heavy chain of sea urchin kinesin reveal that a 70 kDa protein present in wild-type and pf18 (central pairless) axonemes is absent in tcp1, suggesting the presence of an uncharacterized kinesin associated with the central pair apparatus. We demonstrate that the kinesin-like protein Klp1 is not attached to central pair microtubules in tcp1, but rather is located in, or is part of, a region we have termed the internal axonemal matrix. It is proposed that this matrix acts as a scaffold for axonemal proteins that may also be associated with the central pair apparatus.

Inhibition by ATP and activation by ADP in the regulation of flagellar movement in sea urchin sperm

ATP and ADP are known to play inhibitory and activating roles, respectively, in the regulation of dynein motile activity of flagella. To elucidate how these nucleotide functions are related to the regulation of normal flagellar beating, we examined their effects on the motility of reactivated sea urchin sperm flagella at low pH. At pH 7.0-7.2 which is lower than the physiological pH of 8, about 90% of reactivated flagella were motionless at 1 mM ATP, while about 60% were motile at 0.02 mM ATP. The motionless flagella at 1 mM ATP maintained a single large bend or an S-shaped bend, indicating formation of dynein crossbridges in the axoneme. The ATP-dependent inhibition of flagellar movement was released by ADP, and was absent in outer arm-depleted flagella. Similar inhibition was also observed at 0.02 mM ATP when demembranated flagella were reactivated in the presence of Li+ or pretreated with protein phosphatase 1 (PP1). ADP also released this type of ATP-inhibition. In PP1-pretreated axonemes the binding of a fluorescent analogue of ADP to dynein decreased. Under elastase-treatment at pH 8.0, the beating of demembranated flagella at 1 mM ATP and 0.02 mM ATP lasted for approximately 100 and 45 s, respectively. The duration of beating at 0.02 mM ATP was prolonged by Li+, and that at 1 mM ATP was shortened by removal of outer arms. These results indicate that the regulation of on/off switching of dynein motile activity of flagella involves ATP-induced inhibition and ADP-induced activation, probably through phosphorylation/dephosphorylation of outer arm-linked protein(s).

Role(s) of the serine/threonine protein phosphatase 1 on mammalian sperm motility

Mammalian spermatozoa acquire the capacity for motility and fertilization during the transit through the epididymis under the control of different factors, such as cAMP, intracellular pH, intracellular calcium and phosphorylation of sperm proteins. As the acquisition of functional competence including gaining motility during epididymal transit occurs in the complete absence of contemporaneous gene transcription and translation on the part of the spermatozoa, it is widely accepted that post-translational modifications are the only means by which spermatozoa can acquire functionality. Serine-threonine protein phosphatase 1 (PP1) together with their testis/sperm-specific interacting proteins might be involved in this regulatory mechanism. PP1alpha, PP1beta/delta, PP1gamma1 and PP1gamma2 are all expressed in the testis whereas PP1gamma2 is the only isoform expressed on spermatozoa. I2, I3, sds22, 14-3-3 and hsp90 are associated with PP1gamma2 in spermatozoa located on the sperm head and tail. Activity of PP1gamma2 and the binding pattern to these regulatory proteins changes in spermatozoa recruited from the caput and those from the cauda part of the epididymis. In this review, we summarize the possible roles of PP1 on spermatozoa during spermatogenesis and flagellar motility control. We suggest that PP1 might take part in the inhibition of the sperm motility activation by interacting with AKAPs and CAMKII. A hypothesized signaling pathway of mammalian sperm motility activation and PP1's function has been proposed.

Protein phosphatase 2A plays a crucial role in Giardia lamblia differentiation

The ability of Giardia lamblia to undergo two distinct differentiations in response to physiologic stimuli is central to its pathogenesis. The giardial cytoskeleton changes drastically during encystation and excystation. However, the signal transduction pathways mediating these transformations are poorly understood. We tested the hypothesis that PP2A, a highly conserved serine/threonine protein phosphatase, might be important in giardial differentiation. We found that in vegetatively growing trophozoites, gPP2A-C protein localizes to basal bodies/centrosomes, and to cytoskeletal structures unique to Giardia: the ventral disk, and the dense rods of the anterior, posterior-lateral, and caudal flagella. During encystation, gPP2A-C protein disappears from only the anterior flagellar dense rods. During excystation, gPP2A-C localizes to the cyst wall in excysting cysts but is not found in the wall of cysts with emerging excyzoites. Transcriptome and immunoblot analyses indicated that gPP2A-C mRNA and protein are upregulated in mature cysts and during the early stage of excystation that models passage through the host stomach. Stable expression of gPP2A-C antisense RNA did not affect vegetative growth, but strongly inhibited the formation of encystation secretory vesicles (ESV) and water-resistant cysts. Moreover, the few cysts that formed were highly defective in excystation. Thus, gPP2A-C localizes to universal cytoskeletal structures and to structures unique to Giardia. It is also important for encystation and excystation, crucial giardial transformations that entail entry into and exit from dormancy.

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.

Analysis of the phosphoproteome of Chlamydomonas reinhardtii provides new insights into various cellular pathways

The unicellular flagellated green alga Chlamydomonas reinhardtii has emerged as a model organism for the study of a variety of cellular processes. Posttranslational control via protein phosphorylation plays a key role in signal transduction, regulation of gene expression, and control of metabolism. Thus, analysis of the phosphoproteome of C. reinhardtii can significantly enhance our understanding of various regulatory pathways. In this study, we have grown C. reinhardtii cultures in the presence of an inhibitor of Ser/Thr phosphatases to increase the phosphoprotein pool. Phosphopeptides from these cells were enriched by immobilized metal-ion affinity chromatography and analyzed by nano-liquid chromatography-electrospray ionization-mass spectrometry (MS) with MS-MS as well as neutral-loss-triggered MS-MS-MS spectra. In this way, we were able to identify 360 phosphopeptides from 328 different phosphoproteins of C. reinhardtii, thus providing new insights into a variety of cellular processes, including metabolic and signaling pathways. Comparative analysis of the phosphoproteome also yielded new functional information on proteins controlled by redox regulation (thioredoxin target proteins) and proteins of the chloroplast 70S ribosome, the centriole, and especially the flagella, for which 32 phosphoproteins were identified. The high yield of phosphoproteins of the latter correlates well with the presence of several flagellar kinases and indicates that phosphorylation/dephosphorylation represents one of the key regulatory mechanisms of eukaryotic cilia. Our data also provide new insights into certain cilium-related mammalian diseases.

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.

Microtubule sliding movement in tilapia sperm flagella axoneme is regulated by Ca2+/calmodulin-dependent protein phosphorylation

Demembranated euryhaline tilapia Oreochromis mossambicus sperm were reactivated in the presence of concentrations in excess of 10(-6) M Ca(2+). Motility features changed when Ca(2+) concentrations were increased from 10(-6) to 10(-5) M. Although the beat frequency did not increase, the shear angle and wave amplitude of flagellar beating increased, suggesting that the sliding velocity of microtubules in the axoneme, which represents dynein activity, rises with an increase in Ca(2+). Thus, it is possible that Ca(2+) binds to flagellar proteins to activate flagellar motility as a result of the enhanced dynein activity. One Ca(2+)-binding protein (18 kDa, pI 4.0), calmodulin (CaM), was detected by (45)Ca overlay assay and immunologically. A CaM antagonist, W-7, suppressed the reactivation ratio and swimming speed, suggesting that the 18 kDa Ca(2+)-binding protein is CaM and that CaM regulates flagellar motility. CaMKIV was detected immunologically as a single 48 kDa band in both the fraction of low ion extract of the axoneme and the remnant of the axoneme, suggesting that CaMKIV binds to distinct positions in the axoneme. It is possible that CaMKIV phosphorylates the axonemal proteins in a Ca(2+)/CaM-dependent manner for regulating the dynein activity. A (32)P-uptake in the axoneme showed that 48, 75, 120, 200, 250, 380, and 400 kDa proteins were phosphorylated in a Ca(2+)/CaM kinase-dependent manner. Proteins (380 kDa) were phosphorylated in the presence of 10(-5) M Ca(2+). It is possible that an increase in Ca(2+) induces Ca(2+)/CaM kinase-dependent regulation, including protein phosphorylation for activation/regulation of dynein activity in flagellar axoneme.

Two Anti-radial Spoke Monoclonal Antibodies Inhibit Chlamydomonas Axonemal Motility by Different Mechanisms

In the 9 + 2 axoneme, radial spokes are structural components attached to the A-tubules of the nine outer doublet microtubules. They protrude toward the central pair microtubule complex with which they have transient but regular interactions for the normal flagellar motility to occur. Flagella of Chlamydomonas mutants deficient in entire radial spokes or spoke heads are paralyzed. In this study the importance of two radial spoke proteins in the flagellar movement is exemplified by the potent inhibitory action of two monoclonal antibodies on the axonemal motility of demembranated-reactivated Chlamydomonas models. We show that one of these proteins is localized on the stalk of the radial spokes, whereas the other is a component of the head of the same structure and most likely correspond to radial spoke protein 2 and 1, respectively. Fine motility analysis by videomicrography further indicates that these two anti-radial spoke protein antibodies at low concentration affect motility of demembranated-reactivated Chlamydomonas by changing the flagellar waveform without modifying axonemal beat frequency. They also modify wave amplitude differently during motility inhibition. This brings more direct evidence for the involvement of both radial spoke stalk and head in the fine tuning of the waveform during flagellar motility.

Regulation of flagellar dynein activity by a central pair kinesin

The motility of cilia and flagella is powered by dynein ATPases associated with outer doublet microtubules. However, a flagellar kinesin-like protein that may function as a motor associates with the central pair complex. We determined that Chlamydomonas reinhardtii central pair kinesin Klp1 is a phosphoprotein and, like conventional kinesins, binds to microtubules in vitro in the presence of adenosine 5'-[beta,gamma-imido]triphosphate, but not ATP. To characterize the function of Klp1, we generated RNA interference expression constructs that reduce in vivo flagellar Klp1 levels. Klp1 knockdown cells have flagella that either beat very slowly or are paralyzed. EM image averages show disruption of two structures associated with the C2 central pair microtubule, C2b and C2c. Greatest density is lost from part of projection C2c, which is in a position to interact with doublet-associated radial spokes. Klp1 therefore retains properties of a motor protein and is essential for normal flagellar motility. We hypothesize that Klp1 acts as a conformational switch to signal spoke-dependent control of dynein activity.