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

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.

Axonemal motility in Chlamydomonas

Motile cilia and flagella rapidly propagate bending waves and produce water flow over the cell surface. Their function is important for the physiology and development of various organisms including humans. The movement is based on the sliding between outer doublet microtubules driven by axonemal dyneins, and is regulated by various axonemal components and environmental factors. For studies aiming to elucidate the mechanism of cilia/flagella movement and regulation, Chlamydomonas is an invaluable model organism that offers a variety of mutants. This chapter introduces standard methods for studying Chlamydomonas flagellar motility including analysis of swimming paths, measurements of swimming speed and beat frequency, motility reactivation in demembranated cells (cell models), and observation of microtubule sliding in disintegrating axonemes. Most methods may be easily applied to other organisms with slight modifications of the medium conditions.

Space-dependent formation of central pair microtubules and their interactions with radial spokes

Cilia and flagella contain nine outer doublet microtubules and a pair of central microtubules. The central pair of microtubules (CP) is important for cilia/flagella beating, as clearly shown by primary ciliary dyskinesia resulting from the loss of the CP. The CP is thought to regulate axonemal dyneins through interaction with radial spokes (RSs). However, the nature of the CP-RS interaction is poorly understood. Here we examine the appearance of CPs in the axonemes of a Chlamydomonas mutant, bld12, which produces axonemes with 8 to 11 outer-doublets. Most of its 8-doublet axonemes lack CPs. However, in the double mutant of bld12 and pf14, a mutant lacking the RS, most 8-doublet axonemes contain the CP. Thus formation of the CP apparently depends on the internal space limited by the outer doublets and RSs. In 10- or 11-doublet axonemes, only 3–5 RSs are attached to the CP and the doublet arrangement is distorted most likely because the RSs attached to the CP pull the outer doublets toward the axonemal center. The CP orientation in the axonemes varies in double mutants formed between bld12 and mutants lacking particular CP projections. The mutant bld12 thus provides the first direct and visual information about the CP-RS interaction, as well as about the mechanism of CP formation.

The IC138 and IC140 intermediate chains of the I1 axonemal dynein complex bind directly to tubulin

Dyneins are minus end directed microtubule motors that play a critical role in ciliary and flagellar movement. Ciliary dyneins, also known as axonemal dyneins, are characterized based on their location on the axoneme, either as outer dynein arms or inner dynein arms. The I1 dynein is the best-characterized subspecies of the inner dynein arms; however the interactions between many of the components of the I1 complex and the axoneme are not well defined. In an effort to elucidate the interactions in which the I1 components are involved, we performed zero-length crosslinking on axonemes and studied the crosslinked products formed by the I1 intermediate chains, IC138 and IC140. Our data indicate that IC138 and IC140 bind directly to microtubules. Mass-spectrometry analysis of the crosslinked product identified both α- and β-tubulin as the IC138 and IC140 binding partners. This was further confirmed by crosslinking experiments carried out on purified I1 fractions bound to Taxol-stabilized microtubules. Furthermore, the interaction between IC140 and tubulin is lost when IC138 is absent. Our studies support previous findings that intermediate chains play critical roles in the assembly, axonemal targeting and regulation of the I1 dynein complex.

A solid-state control system for dynein-based ciliary/flagellar motility

Ciliary and flagellar beating requires the coordinated action of multiple dyneins with different enzymatic and motor properties. In this issue, Yamamoto et al. (2013. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201211048) identify the MIA (modifier of inner arms) complex within the Chlamydomonas reinhardtii axoneme that physically links to a known regulatory structure and provides a signaling conduit from the radial spokes to an inner arm dynein essential for waveform determination.

Cryoelectron tomography of radial spokes in cilia and flagella

Cryo-EM tomography of wild-type and mutant cilia and flagella from Tetrahymena and Chlamydomonas reveals new information on the substructure of radial spokes.

Radial spokes (RSs) are ubiquitous components in the 9 + 2 axoneme thought to be mechanochemical transducers involved in local control of dynein-driven microtubule sliding. They are composed of >23 polypeptides, whose interactions and placement must be deciphered to understand RS function. In this paper, we show the detailed three-dimensional (3D) structure of RS in situ in Chlamydomonas reinhardtii flagella and Tetrahymena thermophila cilia that we obtained using cryoelectron tomography (cryo-ET). We clarify similarities and differences between the three spoke species, RS1, RS2, and RS3, in T. thermophila and in C. reinhardtii and show that part of RS3 is conserved in C. reinhardtii, which only has two species of complete RSs. By analyzing C. reinhardtii mutants, we identified the specific location of subsets of RS proteins (RSPs). Our 3D reconstructions show a twofold symmetry, suggesting that fully assembled RSs are produced by dimerization. Based on our cryo-ET data, we propose models of subdomain organization within the RS as well as interactions between RSPs and with other axonemal components.

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.

Identification of the human testis protein phosphatase 1 interactome

Protein phosphorylation is a critical regulatory mechanism in cellular signalling. To this end, PP1 is a major eukaryotic serine/threonine-specific phosphatase whose cellular functions, in turn, depend on complexes it forms with PP1 interacting proteins-PIPs. The importance of the testis/sperm-enriched variant, PP1γ2, in sperm motility and spermatogenesis has previously been shown. Given the key role of PIPs, it is imperative to identify the physiologically relevant PIPs in testis and sperm. Hence, we performed Yeast Two-Hybrid screens of a human testis cDNA library using as baits the different PP1 isoforms and also a proteomic approach aimed at identifying PP1γ2 binding proteins. To the best of our knowledge this is the largest data set of the human testis PP1 interactome. We report the identification of 77 proteins in human testis and 7 proteins in human sperm that bind PP1. The data obtained increased the known PP1 interactome by reporting 72 novel interactions. Confirmation of the interaction of PP1 with 5 different proteins was also further validated by co-immunoprecipitation or protein overlays. The data here presented provides important insights towards the function of these proteins and opens new possibilities for future research. In fact, such diversity in PP1 regulators makes them excellent targets for pharmacological intervention.

Posttranslational protein modifications in cilia and flagella

Tubulin and other flagellar and ciliary proteins are the substrates for a host of posttranslational modifications (PTMs), many of which have been highly conserved over evolutionary time. In addition to the binding of MAPs (microtubule-associated proteins) that provide a specific functionality, or the use of different tubulin isotypes to convey a specific function, most cells rely on an array of PTMs. These include phosphorylation, acetylation, glycylation, glutamylation, and methylation. The first and the last of this list are not unique to the tubulin in cilia and flagella, while the others are. This chapter will review briefly these varying modifications and will conclude with detailed methods for their detection and localization at the limit of resolution provided by electron microscopy.

Molecular mechanisms determining sperm motility initiation in two sparids (Sparus aurata and Lithognathus mormyrus)

Molecular mechanisms involved in sperm motility initiation in two sparids (Sparus aurata and Lithognathus mormyrus) have been studied. Our comparative study demonstrates that osmolality is the key signal in sperm motility activation in both species, whereas K(+) and Ca(2+) do not have any role. The straight-line velocity that resulted, however, was significantly different when measured in sperm activated with non-ionic and/or calcium-free solutions with respect to that measured in seawater-activated sperm. In both species, motility initiation depends on cAMP-dependent protein phosphorylation. The phosphorylation/dephosphorylation patterns that resulted in gilthead and striped sea bream were quite different. In gilthead sea bream, the phosphorylated proteins have molecular weights of 174, 147, 138, 70, and 9-15 kDa, whereas the dephosphorylated proteins have molecular weights of 76, 57, and 33 kDa. In striped sea bream, phosphorylation after sperm motility activation occurred on proteins of 174, 147, 103, 96, 61, 57, and 28 kDa, whereas only one protein of 70 kDa resulted from dephosphorylation. Matrix-assisted laser desorption ionization-time of flight analyses allowed identification of the following proteins: In gilthead sea bream, the 9-15 kDa proteins that were phosphorylated after motility activation include an A-kinase anchor protein (AKAP), an acetyl-coenzyme A synthetase, and a protein phosphatase inhibitor, and in striped sea bream, 103- and 61-kDa proteins that were phosphorylated after motility activation were identified as a phosphatase (myotubularin-related protein 1) and a kinase (DYRK3), respectively.

A conserved CaM- and radial spoke associated complex mediates regulation of flagellar dynein activity

For virtually all cilia and eukaryotic flagella, the second messengers calcium and cyclic adenosine monophosphate are implicated in modulating dynein- driven microtubule sliding to regulate beating. Calmodulin (CaM) localizes to the axoneme and is a key calcium sensor involved in regulating motility. Using immunoprecipitation and mass spectrometry, we identify members of a CaM-containing complex that are involved in regulating dynein activity. This complex includes flagellar-associated protein 91 (FAP91), which shares considerable sequence similarity to AAT-1, a protein originally identified in testis as an A-kinase anchor protein (AKAP)- binding protein. FAP91 directly interacts with radial spoke protein 3 (an AKAP), which is located at the base of the spoke. In a microtubule sliding assay, the addition of antibodies generated against FAP91 to mutant axonemes with reduced dynein activity restores dynein activity to wild-type levels. These combined results indicate that the CaM- and spoke-associated complex mediates regulatory signals between the radial spokes and dynein arms.

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.

Phototactic activity inChlamydomonas'non-phototactic' mutants deficient in Ca2+-dependent control of flagellar dominance or in inner-arm dynein

In the mechanism underlying the phototactic behavior of Chlamydomonas, Ca2+ has been thought to control the dominance between the two flagella so as to steer the cell to correct directions. A newly isolated mutant, lsp1, that displays weak phototaxis was found to be defective in this Ca2+-dependent shift in flagellar dominance; in demembranated and reactivated cell models, the trans flagellum (the flagellum farthest from the eyespot) beat more strongly than the other (the cis flagellum) in about half of the cells regardless of the Ca2+ concentration between <10-9 M and 10-6 M, a range over which wild-type cell models display switching of flagellar dominance. This is unexpected because ptx1, another mutant that is also deficient in flagellar dominance control, has been reported to lack phototactic ability. We therefore re-examined ptx1 and another reportedly non-phototactic mutant, ida1, which lacks inner arm dynein subspecies f (also called I1). Both were found to retain reduced phototactic abilities. These results indicate that both Ca2+-dependent flagellar dominance control and inner-arm dynein subspecies f are important for phototaxis, but are not absolutely necessary. Analysis of the flagellar beat frequency in lsp1 cell models showed that both of the flagella beat at the frequency of the cis flagellum in wild type. In addition, lsp1 and ptx1 were found to be deficient in determining the sign of phototactic migration. Hence, the Ca2+-dependent flagellar dominance control detected in demembranated cells might be involved in the determination of the sign of phototaxis. The gene responsible for the lsp1 mutation was identified by phenotype rescue experiments and found to have sequences for phosphorylation.

A Novel Tctex2-related Light Chain Is Required for Stability of Inner Dynein Arm I1 and Motor Function in the Chlamydomonas Flagellum

Tctex1 and Tctex2 were originally described in mice as putative distorters/sterility factors involved in the non-Mendelian transmission of t haplotypes. Subsequently, these proteins were found to be light chains of both cytoplasmic and axonemal dyneins. We have now identified a novel Tctex2-related protein (Tctex2b) within the Chlamydomonas flagellum. Tctex2b copurifies with inner arm I1 after both sucrose gradient centrifugation and anion exchange chromatography. Unlike the Tctex2 homologue within the outer dynein arm, analysis of a Tctex2b-null strain indicates that this protein is not essential for assembly of inner arm I1. However, a lack of Tctex2b results in an unstable dynein particle that disassembles after high salt extraction from the axoneme. Cells lacking Tctex2b swim more slowly than wild type and exhibit a reduced flagellar beat frequency. Furthermore, using a microtubule sliding assay we observed that dynein motor function is reduced in vitro. These data indicate that Tctex2b is required for the stability of inner dynein arm I1 and wild-type axonemal dynein function.

Characterization of a cAMP-dependent protein kinase catalytic subunit from rainbow trout spermatozoa

The cyclic AMP-dependent phosphorylation of proteins is essential for the initiation of sperm motility in salmonid fishes. This study isolated cDNA for the catalytic subunit of a cAMP-dependent protein kinase (PKA-C) from rainbow trout testis. The deduced amino acid sequence shows 75-80% identity to sequences previously reported in other organisms. However, the N-terminal regions of PKA-C from the testis as well as ovary in the trout appear slightly shorter than those from other tissues, suggesting that small PKA-C might be specific to germ cells. An immunofluorescence study using polyclonal antibody against trout testis PKA-C shows that it localizes along sperm flagellum. Furthermore, immunoelectron microscopy revealed that PKA-C is anchored to the outer arm dynein of flagellar axonemes. These results suggest that PKA-C is involved in regulating the flagellar motility of sperm via phosphorylation of a subunit of the outer arm dynein.

Structural-functional relationships of the dynein, spokes, and central-pair projections predicted from an analysis of the forces acting within a flagellum

In the axoneme of eukaryotic flagella the dynein motor proteins form crossbridges between the outer doublet microtubules. These motor proteins generate force that accumulates as linear tension, or compression, on the doublets. When tension or compression is present on a curved microtubule, a force per unit length develops in the plane of bending and is transverse to the long axis of the microtubule. This transverse force (t-force) is evaluated here using available experimental evidence from sea urchin sperm and bull sperm. At or near the switch point for beat reversal, the t-force is in the range of 0.25-1.0 nN/ micro m, with 0.5 nN/ micro m the most likely value. This is the case in both beating and arrested bull sperm and in beating sea urchin sperm. The total force that can be generated (or resisted) by all the dyneins on one micron of outer doublet is also approximately 0.5 nN. The equivalence of the maximum dynein force/ micro m and t-force/ micro m at the switch point may have important consequences. Firstly, the t-force acting on the doublets near the switch point of the flagellar beat is sufficiently strong that it could terminate the action of the dyneins directly by strongly favoring the detached state and precipitating a cascade of detachment from the adjacent doublet. Secondly, after dynein release occurs, the radial spokes and central-pair apparatus are the structures that must carry the t-force. The spokes attached to the central-pair projections will bear most of the load. The central-pair projections are well-positioned for this role, and they are suitably configured to regulate the amount of axoneme distortion that occurs during switching. However, to fulfill this role without preventing flagellar bend formation, moveable attachments that behave like processive motor proteins must mediate the attachment between the spoke heads and the central-pair structure.

Sperm PP1gamma2 is regulated by a homologue of the yeast protein phosphatase binding protein sds22

Serine/threonine phosphatase PP1gamma2 is a testis-specific protein phosphatase isoform in spermatozoa. This enzyme appears to play a key role in motility initiation and stimulation. Catalytic activity of PP1gamma2 is higher in immotile compared with motile spermatozoa. Inhibition of PP1gamma2 activity causes both motility initiation and motility stimulation. Protein phosphatases, in general, are regulated by their binding proteins. The objective of this article is to understand the mechanisms by which PP1gamma2 is regulated, first by identifying its regulatory proteins. We had previously shown that a portion of bovine sperm PP1gamma2 is present in the cytosolic fraction of sperm sonicates. We purified PP1gamma2 from soluble bovine sperm extracts by immunoaffinity chromatography. Gel electrophoresis of the purified enzyme showed that it was complexed to a protein 43 M(r) x 10(-3) in size. Microsequencing revealed that this protein is a mammalian homologue of sds22, which is a yeast PP1 binding protein. Phosphatase activity measurements showed that PP1gamma2 complexed to sds22 is catalytically inactive. The complex cannot be activated by limited proteolysis. The complex is unable to bind to microcystin sepharose. This suggests that sds22 may block the microcystin binding site in PP1gamma2. A proportion of PP1gamma2 in sperm extracts, which is presumably not complexed to sds22, is catalytically active. Fluorescence immunocytochemistry was used to determine the intrasperm localization of PP1gamma2 and sds22. Both proteins are present in the tail. They are also present in distinct locations in the head. Our data suggest that PP1gamma2 binding to sds22 inhibits its catalytic activity. Mechanisms regulating sds22 binding to PP1gamma2 are likely to be important in understanding the biochemical basis underlying development and regulation of sperm function.

Molecular structure of dynein and motility of a giant sperm axoneme provided with only the outer dynein arm

The peculiar sperm axoneme of the dipteran Asphondylia ruebsaameni is characterized by an extraordinarily high number of microtubule doublets (up to 2,500) arranged in double parallel spirals. Doublets of the inner row of each spiral are tilted, so that their outer arms point towards the B-tubule of the next doublet in the outer row. Doublets are provided with only the outer arm, and no structure related to the central pair/radial spoke complex is present. When analyzed by quick-freeze, deep-etch electron microscopy, the structure of the dynein arms was shown to share the same organization described in other organisms; however, it appears to be somewhat more complex than that previously found in a related dipteran species, Monarthropalpus flavus, since the foot region of the arms displays a globular extra-domain that is intercalated between adjacent arms. Treatment of demembranated sperm with ATP and vanadate induced conformational changes in the dynein arms. SDS-page suggested the presence of a single dynein high molecular weight band or, in the gels with the best electrophoretic resolution, of two very closely spaced bands. This polypeptide positively reacted with a polyclonal antibody raised against a specific amino acid sequence located in the phosphate-binding loop of the dynein catalytic site. Dynein heavy chain-related DNA sequences corresponding to the catalytic phosphate-binding region were amplified by RT-PCR. Two distinct fragments (Asph-ax1 and Asph-ax2) encoding axonemal dynein sequences were identified. Southern blot analysis performed on genomic DNA using these sequences as a probe showed that they are part of different genes. An intron was identified in the Asph-ax1 fragment at a position corresponding to the site of a nucleotide deletion in the putative pseudogene of Monarthropalpus. Asphondylia spermatozoa exhibited in vivo a whirling movement both in the deferent duct and in the spermatheca, but they were unable to undergo processive movement in vitro. They propagated a three-dimensional wave only when constrained in a bent configuration by some mechanical means. The phylogenetic relationships between the two dipteran species, Monarthopalpus and Asphondylia, based on these biochemical and molecular data are also discussed.

Regulation of ciliary beat frequency by the nitric oxide-cyclic guanosine monophosphate signaling pathway in rat airway epithelial cells

Nitric oxide (NO) upregulates ciliary beat frequency (CBF). The present study evaluates mechanisms of the NO-cyclic guanosine monophosphate (cGMP) pathway regulation of CBF. Rat tracheal explants were loaded with 4,5-diaminofluorescein diacetate for the demonstration of NO production by ciliated epithelial cells after L-arginine (L-Arg) stimulation. CBF was measured using phase contrast microscopy and videotape analysis. The roles of NO, soluble guanylate cyclase (sGC), cGMP-dependent protein kinase (PK) G, and phosphodiesterase (PDE) V in regulation of CBF were evaluated. NO synthase (NOS) was activated with L-Arg or inhibited with N(G)-monomethyl-L-Arg. sGC was stimulated with NO donors 1-hydroxy-2-oxo-3- (N-ethyl-2-aminoethyl)-3-ethyl-1-triazene and S-nitroso-L-glutathione or mimicked by 8-bromo-guanosine 3', 5'-cyclic monophosphate (8-Br-cGMP) and inhibited with 1H-[1,2, 4]oxadiazole[4,3-a]quinoxalin-1-one. The effects of the PKG inhibition with KT5823 and PDE V inhibition with Zaprinast were also examined. The studies demonstrate that ciliated epithelial cells produce NO, which is correlated with CBF stimulation. L-Arg dose- and time-dependently increases CBF, and NO donors, 8-Br-cGMP, and Zaprinast also enhance CBF. Inhibitors of NOS, sGC, and PKG can block the stimulant effect of L-Arg on CBF. Thus, NO is a regulator of CBF acting via sGC and PKG. The NO-cGMP signaling pathway regulates CBF in an autocrine manner in cultured rat ciliated airway epithelium.

The use of phosphocreatine plus ADP as energy source for motility of membrane-deprived trout spermatozoa

Live trout spermatozoa initiate flagellar motility for a short period of time (30 s at 18 degrees C), during which their mean beat frequency (BF) decreases steadily from 60 to 20 Hz; motility then stops abruptly. When demembranated, the motility of axonemes lasts much longer, up to 20 min, with high beat frequency, provided that ATP (millimolar concentration) and cAMP (micromolar) are added. In the present paper, the motility of demembranated trout sperm was investigated in the absence of added ATP in various incubation conditions relative to other substrates. Without the addition of exogenous creatine kinase, the addition of phosphocreatine (PCr) and ADP shows the appearance of a progressive activation of all sperm models with BF increasing with time up to high values. Without the addition of cAMP, the BF increases to lower values but flagella propagated poorly coordinated waves for only a few min. Similar progressive activation is also observed when only ADP is added (without any previous in vivo activation) and BF increases up to moderate values. In this latter case, no activation occurs without addition of cAMP. The respective roles of creatine kinase and adenylate kinase in this process were investigated by addition of specific inhibitors such as fluorodinitrobenzene and P1,P5-di(adenosine-5')pentaphosphate in the above described conditions. We conclude from these observations that all the elements necessary for a coupling between ADP/PCr/creatine kinase on one hand and ATP/ADP/dynein on the other appear to be present in trout spermatozoa: thus the existence of a shuttle sustaining this coupling is strongly suggested.

The role of central apparatus components in flagellar motility and microtubule assembly

In order to generate the complex waveforms typical of beating cilia and flagella, the action of the dynein arms must be regulated. This regulation not only depends on the presence of multiple dynein isoforms, but also clearly involves other structures in the axoneme such as the radial spokes and central apparatus; mutants lacking these structures have paralyzed flagella. In this article, we review recent progress in identifying protein components of the central apparatus and discuss the role of these components in regulation of flagellar motility and central apparatus assembly. The central apparatus is composed of two single microtubules and their associated structures which include the central pair projections, the central pair bridges linking the two tubules, and the central pair caps which are attached to the distal or plus ends of the microtubules. To date, the genes encoding four components of the central apparatus have been cloned, PF15, PF16, PF20 and KLP1. PF16, PF20 and KLP1 have been sequenced and their gene products localized. Two additional components have been identified immunologically, a 110 kD polypeptide recognized by an antibody generated against highly conserved kinesin peptide sequence, and a 97 kD polypeptide recognized by CREST antisera. Based on a variety of data, one model that has emerged to explain the role of the central apparatus in flagellar motility is that the central apparatus ultimately regulates dynein through interactions with the radial spokes. The challenge now is to determine the precise mechanism by which the polypeptides comprising the central apparatus and the radial spokes interact to transduce a regulatory signal to the dynein arms. In terms of assembly, the central apparatus microtubules assemble with their plus ends distal to the cell body but, unlike the nine doublet microtubules, they are not nucleated from the basal bodies. Since some central apparatus defective mutants fail to assemble the entire central apparatus, their gene products may eventually prove to have microtubule nucleating or stabilizing properties. By continuing to identify the genes that encode central apparatus components, we will begin to understand the contribution of these microtubules to flagellar motility and gain insight into their nucleation, assembly, and stability.

Calcium regulation of microtubule sliding in reactivated sea urchin sperm flagella

The changes in the bending pattern of flagella induced by an increased intracellular Ca(2+) concentration are caused by changes in the pattern and velocity of microtubule sliding. However, the mechanism by which Ca(2+) regulates microtubule sliding in flagella has been unclear. To elucidate it, we studied the effects of Ca(2+) on microtubule sliding in reactivated sea urchin sperm flagella that were beating under imposed head vibration. We found that the maximum microtubule sliding velocity obtainable by imposed vibration, which was about 170–180 rad/second in the presence of 250 microM MgATP and <10(−9) M Ca(2+), was decreased by 10(−6)-10(−5) M Ca(2+) by about 15–20%. Similar decrease of the sliding velocity was observed at 54 and 27 microM MgATP. The Ca(2+)-induced decrease of the sliding velocity was due mainly to a decrease in the reverse bend angle. When the plane of beat was artificially rotated by rotating the plane of vibration of the pipette that held the sperm head, the asymmetric bending pattern also rotated at 10(−5) M Ca(2+) as well as at <10(−9) M Ca(2+). The rotation of the bending pattern was observed at MgATP higher than 54 microM (approximately 100 microM ATP). These results indicate that the Ca(2+)-induced decrease of the sliding velocity is mediated by a rotatable component or components (probably the central pair) at high MgATP, but is not due to specific dynein arms on particular doublets. We further investigated the effects of a mild trypsin treatment and of trifluoperazine on the Ca(2+)-induced decrease in sliding velocity. Axonemes treated for 3 minutes with a low concentration (0.1 microgram/ml) of trypsin beat with a more symmetrical waveform than before the treatment. Also, their microtubule sliding velocity and reverse bend angle were not affected by high Ca(2+) concentrations. Trifluoperazine (25-50 microM) had no effect on the decrease of the sliding velocity in beating flagella at 10(−5) M Ca(2+). However, the flagella that had been ‘quiescent’ at 10(−4) M Ca(2+) resumed asymmetrical beating following an application of 10–50 microM trifluoperazine. In such beating flagella, both the sliding velocity and the reverse bend angle were close to their respective values at 10(−5) M Ca(2+). Trypsin treatment induced a similar recovery of beating in quiescent flagella at 10(-)(4) M Ca(2+), albeit with a more symmetrical waveform. These results provide first evidence that, at least at ATP concentrations higher than approximately 100 microM, 10(−6)-10(−5) M Ca(2+) decreases the maximum sliding velocity of microtubules in beating flagella through a trypsin-sensitive regulatory mechanism which possibly involves the central pair apparatus. They also suggest that calmodulin may be associated with the mechanism underlying flagellar quiescence induced by 10(−4) M Ca(2+).

Effects of the central pair apparatus on microtubule sliding velocity in sea urchin sperm flagella

To produce oscillatory bending movement in cilia and flagella, the activity of dynein arms must be regulated. The central-pair microtubules, located at the centre of the axoneme, are often thought to be involved in the regulation, but this has not been demonstrated definitively. In order to determine whether the central-pair apparatus are directly involved in the regulation of the dynein arm activity, we analyzed the movement of singlet microtubules that were brought into contact with dynein arms on bundles of doublets obtained by sliding disintegration of elastase-treated flagellar axonemes. An advantage of this new assay system was that we could distinguish the bundles that contained the central pair apparatus from those that did not, the former being clearly thicker than the latter. We found that microtubule sliding occurred along both the thinner and the thicker bundles, but its velocity differed between the two kinds of bundles in an ATP concentration dependent manner. At high ATP concentrations, such as 0.1 and 1 mM, the sliding velocity on the thinner bundles was significantly higher than that on the thicker bundles, while at lower ATP concentrations the sliding velocity did not change between the thinner and the thicker bundles. We observed similar bundle width-related differences in sliding velocity after removal of the outer arms. These results provide first evidence suggesting that the central pair and its associated structures may directly regulate the activity of the inner (and probably also the outer) arm dynein.

The Chlamydomonas Dhc1 gene encodes a dynein heavy chain subunit required for assembly of the I1 inner arm complex

Multiple members of the dynein heavy chain (Dhc) gene family have been recovered in several organisms, but the relationships between these sequences and the Dhc isoforms that they encode are largely unknown. To identify Dhc loci and determine the specific functions of the individual Dhc isoforms, we have screened a collection of motility mutants generated by insertional mutagenesis in Chlamydomonas. In this report, we characterize one strain, pf9-3, in which the insertion event was accompanied by a deletion of approximately 13 kb of genomic DNA within the transcription unit of the Dhc1 gene. Northern blot analysis confirms that pf9-3 is a null mutation. Biochemical and structural studies of isolated axonemes demonstrate that the pf9-3 mutant fails to assemble the I1 inner arm complex, a two-headed dynein isoform composed of two Dhcs (1 alpha and 1 beta) and three intermediate chains. To determine if the Dhc1 gene product corresponds to one of the Dhcs of the I1 complex, antibodies were generated against a Dhc1-specific peptide sequence. Immunoblot analysis reveals that the Dhc1 gene encodes the 1 alpha Dhc subunit. These studies thus, identify the first inner arm Dhc locus to be described in any organism and further demonstrate that the 1 alpha Dhc subunit plays an essential role in the assembly of the I1 inner arm complex.

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.

The sup-pf-2 mutations of Chlamydomonas alter the activity of the outer dynein arms by modification of the gamma-dynein heavy chain

The sup-pf-2 mutation is a member of a group of dynein regulatory mutations that are capable of restoring motility to paralyzed central pair or radial spoke defective strains. Previous work has shown that the flagellar beat frequency is reduced in sup-pf-2, but little else was known about the sup-pf-2 phenotype (Huang, B., Z. Ramanis, and D.J.L. Luck. 1982. Cell. 28:115-125; Brokaw, C.J., and D.J.L. Luck. 1985. Cell Motil. 5:195-208). We have reexamined sup-pf-2 using improved biochemical and structural techniques and by the analysis of additional sup-pf-2 alleles. We have found that the sup-pf-2 mutations are associated with defects in the outer dynein arms. Biochemical analysis of sup-pf-2-1 axonemes indicates that both axonemal ATPase activity and outer arm polypeptides are reduced by 40-50% when compared with wild type. By thin-section EM, these defects correlate with an approximately 45% loss of outer dynein arm structures. Interestingly, this loss is biased toward a subset of outer doublets, resulting in a radial asymmetry that may reflect some aspect of outer arm assembly. The defects in outer arm assembly do not appear to result from defects in either the outer doublet microtubules or the outer arm docking structures, but rather appear to result from defects in outer dynein arm components. Analysis of new sup-pf-2 mutations indicates that the severity of the outer arm assembly defects varies with different alleles. Complementation tests and linkage analysis reveal that the sup-pf-2 mutations are alleles of the PF28/ODA2 locus, which is thought to encode the gamma-dynein heavy chain subunit of the outer arm. The sup-pf-2 mutations therefore appear to alter the activity of the outer dynein arms by modification of the gamma-dynein heavy chain.

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.