Association between actin and light chains in Chlamydomonas flagellar inner-arm dyneins

Inner dynein arms in cilia and flagella contain actin as a subunit; however, the function of this actin is totally unknown. Here we performed chemical crosslinking experiments to examine the interaction of actin with other subunits. Six of the seven Chlamydomonas inner-arm dynein species separated by anion-exchange chromatography contain actin and either one of the two previously identified light chains, p28 and centrin, in a mutually exclusive manner. Western blotting of chemically crosslinked dyneins indicated that actin is directly associated with p28 and centrin but not with the dynein heavy chains (HCs). In contrast, p28 and centrin both appeared to interact directly with the N-terminal half of the HCs. Thus it is likely that actin is associated with the heavy chains through p28/centrin. These light chains may well function in the assembly or targeting of the inner arm to the correct axonemal location.     

Identification of dynein heavy chain genes expressed in human and mouse testis: chromosomal localization of an axonemal dynein gene

Dynein heavy chains are involved in microtubule-dependent transport processes. While cytoplasmic dyneins are involved in chromosome or vesicle movement, axonemal dyneins are essential for motility of cilia and flagella. Here we report the isolation of dynein heavy chain (DHC)-like sequences in man and mouse. Using polymerase chain reaction and reverse-transcribed human and mouse testis RNA cDNA fragments encoding the conserved ATP binding region of dynein heavy chains were amplified. We identified 11 different mouse and eight human dynein-like sequences in testis which show high similarity to known dyneins of different species such as rat, sea urchin or green algae. Sequence similarities suggest that two of the mouse clones and one human clone encode putative cytoplasmic dynein heavy chains, whereas the other sequences show higher similarity to axonemal dyneins. Two of nine axonemal dynein isoforms identified in the mouse testis are more closely related to known outer arm dyneins, while seven clones seem to belong to the inner arm dynein group. Of the isolated human isoforms three clones were classified as outer arm and four clones as inner arm dynein heavy chains. Each of the DHC cDNAs corresponds to an individual gene as determined by Southern blot experiments. The alignment of the deduced protein sequences between human (HDHC) and mouse (MDHC) dynein fragments reveals higher similarity between single human and mouse sequences than between two sequences of the same species. Human and mouse cDNA fragments were used to isolate genomic clones. Two of these clones, gHDHC7 and gMDHC7, are homologous genes encoding axonemal inner arm dyneins. While the human clone is assigned to 3p21, the mouse gene maps to chromosome 14.     

Characterization of outer arm dynein in sea anemone, Anthopleura midori.

Outer arm dynein was purified from sperm flagella of a sea anemone, Anthopleura midori, and its biochemical and biophysical properties were characterized. The dynein, obtained at a 20S ATPase peak by sucrose density gradient centrifugation, consisted of two heavy chains, three intermediate chains, and seven light chains. The specific ATPase activity of dynein was 1.3 micromol Pi/mg/min. Four polypeptides (296, 296, 225, and 206 kDa) were formed by UV cleavage at 365 nm of dynein in the presence of vanadate and ATP. In addition, negatively stained images of dynein molecules and the hook-shaped image of the outer arm of the flagella indicated that sea anemone outer arm dynein is two-headed. In contrast to protist dyneins, which are three-headed, outer arm dyneins of flagella and cilia in multicellular animals are two-headed molecules corresponding to the two heavy chains. Phylogenetic considerations were made concerning the diversity of outer arm dyneins.     

Vanadate-sensitized cleavage of dynein heavy chains by 365-nm irradiation of demembranated sperm flagella and its effect on the flagellar motility

Irradiation of demembranated flagella of sea urchin sperm at 365 nm in the presence of 0.05-1 mM MgATP and 5-10 microM vanadate (Vi) cleaves the alpha and beta heavy chains of the outer arm dynein at the same site and at about the same rate as reported previously for the solubilized dynein (Gibbons, I. R., Lee-Eiford, A., Mocz, G., Phillipson, C. A., Tang, W.-J. Y., and Gibbons, B. H. (1987) J. Biol. Chem. 262, 2780-2786). The decrease in intact alpha and beta heavy chain material is biphasic, with about 80% being lost with a half-time of 8-10 min, and the remainder more slowly. Five other axonemal polypeptides of Mr greater than 350,000 are lost similarly, concomitant with the appearance of at least 9 new peptides of Mr 150,000-250,000. The motility of irradiated sperm flagella upon subsequent dilution into reactivation medium containing 1 mM ATP and 2.5 mM catechol shows a progressive decrease in flagellar beat frequency for irradiation times that produce up to about 50% cleavage of the dynein heavy chains; more prolonged irradiation causes irreversible loss of motility. Competition between photocleaved and intact outer arm dynein for rebinding to dynein-depleted sperm flagella shows that cleavage has little effect upon the ability for rebinding, although the cleaved dynein partially inhibits subsequent motility. Substitution of MnATP for the MgATP in the irradiation medium prevents the loss of all of the axonemal polypeptides during irradiation for up to 60 min and also protects the potential for subsequent flagellar motility. It is concluded that loss of the five axonemal polypeptides upon irradiation results from a Vi-sensitized photocleavage similar to that which occurs in the alpha and beta heavy chains of outer arm dynein and that these polypeptides represent Vi-inhibitable ATPase subunits of dyneins located in the inner arms and possibly elsewhere in the flagellar axoneme.     

The architecture of outer dynein arms in situ

Outer dynein arms, the force generators for axonemal motion, form arrays on microtubule doublets in situ, although they are bouquet-like complexes with separated heads of multiple heavy chains when isolated in vitro. To understand how the three heavy chains are folded in the array, we reconstructed the detailed 3D structure of outer dynein arms of Chlamydomonas flagella in situ by electron cryo-tomography and single-particle averaging. The outer dynein arm binds to the A-microtubule through three interfaces on two adjacent protofilaments, two of which probably represent the docking complex. The three AAA rings of heavy chains, seen as stacked plates, are connected in a striking manner on microtubule doublets. The tail of the alpha-heavy chain, identified by analyzing the oda11 mutant, which lacks alpha-heavy chain, extends from the AAA ring tilted toward the tip of the axoneme and towards the inside of the axoneme at 50 degrees , suggesting a three-dimensional power stroke. The neighboring outer dynein arms are connected through two filamentous structures: one at the exterior of the axoneme and the other through the alpha-tail. Although the beta-tail seems to merge with the alpha-tail at the internal side of the axoneme, the gamma-tail is likely to extend at the exterior of the axoneme and join the AAA ring. This suggests that the fold and function of gamma-heavy chain are different from those of alpha and beta-chains.     

The dynein heavy chain family

Dynein is the large molecular motor that translocates to the (-) ends of microtubules. Dynein was first isolated from Tetrahymena cilia four decades ago. The analysis of the primary structure of the dynein heavy chain and the discovery that many organisms express multiple dynein heavy chains have led to two insights. One, dynein, whose motor domain comprises six AAA modules and two potential mechanical levers, generates movement by a mechanism that is fundamentally different than that which underlies the motion of myosin and kinesin. And two, organisms with cilia or flagella express approximately 14 different dynein heavy chain genes, each gene encodes a distinct dynein protein isoform, and each isoform appears to be functionally specialized. Sequence comparisons demonstrate that functionally equivalent isoforms of dynein heavy chains are well conserved across species. Alignments of portions of the motor domain result in seven clusters: (i) cytoplasmic dynein Dyhl; (ii) cytoplasmic dynein Dyh2; (iii) axonemal outer arm dynein alpha; (iv) outer arm dyneins beta and gamma; (v) inner arm dynein 1alpha; (vi) inner arm dynein 1beta; and (vii) a group of apparently single-headed inner arm dyneins. Some of the dynein groups contained more than one representative from a single organism, suggesting that these may be tissue-specific variants.     

Redox-based control of the gamma heavy chain ATPase from Chlamydomonas outer arm dynein

The outer dynein arm from Chlamydomonas flagella contains two redox-active thioredoxin-related light chains associated with the alpha and beta heavy chains; these proteins belong to a distinct subgroup within the thioredoxin family. This observation suggested that some aspect of dynein activity might be modulated through redox poise. To test this, we have examined the effect of sulfhydryl oxidation on the ATPase activity of isolated dynein and axonemes from wildtype and mutant strains lacking various heavy chain combinations. The outer, but not inner, dynein arm ATPase was stimulated significantly following treatment with low concentrations of dithionitrobenzoic acid; this effect was readily reversible by dithiol, and to a lesser extent, monothiol reductants. Mutational and biochemical dissection of the outer arm revealed that ATPase activation in response to DTNB was an exclusive property of the gamma heavy chain, and that enzymatic enhancement was modulated by the presence of other dynein components. Furthermore, we demonstrate that the LC5 thioredoxin-like light chain binds to the N-terminal stem domain of the alpha heavy chain and that the beta heavy chain-associated LC3 protein also interacts with the gamma heavy chain. These data suggest the possibility of a dynein-associated redox cascade and further support the idea that the gamma heavy chain plays a key regulatory role within the outer arm.     

The inner dynein arms I2 interact with a "dynein regulatory complex" in Chlamydomonas flagella.

We provide indirect evidence that six axonemal proteins here referred to as "dynein regulatory complex" (drc) are located in close proximity with the inner dynein arms I2 and I3. Subsets of drc subunits are missing from five second-site suppressors, pf2, pf3, suppf3, suppf4, and suppf5, that restore flagellar motility but not radial spoke structure of radial spoke mutants. The absence of drc components is correlated with a deficiency of all four heavy chains of inner arms I2 and I3 from axonemes of suppressors pf2, pf3, suppf3, and suppf5. Similarly, inner arm subunits actin, p28, and caltractin/centrin, or subsets of them, are deficient in pf2, pf3, and suppf5. Recombinant strains carrying one of the mutations pf2, pf3, or suppf5 and the inner arm mutation ida4 are more defective for I2 inner arm heavy chains than the parent strains. This evidence indicates that at least one subunit of the drc affects the assembly of and interacts with the inner arms I2.     

Partially functional outer-arm dynein in a novel Chlamydomonas mutant expressing a truncated gamma heavy chain

The outer dynein arm of Chlamydomonas flagella contains three heavy chains (alpha, beta, and gamma), each of which exhibits motor activity. How they assemble and cooperate is of considerable interest. Here we report the isolation of a novel mutant, oda2-t, whose gamma heavy chain is truncated at about 30% of the sequence. While the previously isolated gamma chain mutant oda2 lacks the entire outer arm, oda2-t retains outer arms that contain alpha and beta heavy chains, suggesting that the N-terminal sequence (corresponding to the tail region) is necessary and sufficient for stable outer-arm assembly. Thin-section electron microscopy and image analysis localize the gamma heavy chain to a basal region of the outer-arm image in the axonemal cross section. The motility of oda2-t is lower than that of the wild type and oda11 (lacking the alpha heavy chain) but higher than that of oda2 and oda4-s7 (lacking the motor domain of the beta heavy chain). Thus, the outer-arm dynein lacking the gamma heavy-chain motor domain is partially functional. The availability of mutants lacking individual heavy chains should greatly facilitate studies on the structure and function of the outer-arm dynein.     

Molecular architecture of inner dynein arms in situ in Chlamydomonas reinhardtii flagella

The inner dynein arm regulates axonemal bending motion in eukaryotes. We used cryo-electron tomography to reconstruct the three-dimensional structure of inner dynein arms from Chlamydomonas reinhardtii. All the eight different heavy chains were identified in one 96-nm periodic repeat, as expected from previous biochemical studies. Based on mutants, we identified the positions of the AAA rings and the N-terminal tails of all the eight heavy chains. The dynein f dimer is located close to the surface of the A-microtubule, whereas the other six heavy chain rings are roughly colinear at a larger distance to form three dyads. Each dyad consists of two heavy chains and has a corresponding radial spoke or a similar feature. In each of the six heavy chains (dynein a, b, c, d, e, and g), the N-terminal tail extends from the distal side of the ring. To interact with the B-microtubule through stalks, the inner-arm dyneins must have either different handedness or, more probably, the opposite orientation of the AAA rings compared with the outer-arm dyneins.     

Chlamydomonas flagellar outer row dynein assembly protein ODA7 interacts with both outer row and I1 inner row dyneins

We previously found that a mutation at the ODA7 locus in Chlamydomonas prevents axonemal outer row dynein assembly by blocking association of heavy chains and intermediate chains in the cytoplasm. We have now cloned the ODA7 locus by walking in the Chlamydomonas genome from nearby molecular markers, confirmed the identity of the gene by rescuing the mutant phenotype with genomic clones, and identified the ODA7 gene product as a 58-kDa leucine-rich repeat protein unrelated to outer row dynein LC1. Oda7p is missing from oda7 mutant flagella but is present in flagella of other outer row or inner row dynein assembly mutants. However, Oda7 levels are greatly reduced in flagella that lack both outer row dynein and inner row I1 dynein. Biochemical fractionation and rebinding studies support a model in which Oda7 participates in a previously uncharacterized structural link between inner and outer row dyneins.     

Twenty-five dyneins in Tetrahymena: A re-examination of the multidynein hypothesis

Dyneins are responsible for essential movements in eukaryotic cells. The motor activity of each dynein complex resides in its complement of heavy chains. In the present study, we examined 136 heavy chain sequences from the completed genomes of 11 diverse model organisms, including examples from Viridiplantae, Excavata, Chromalveolata, and Metazoa. In many cases, we discovered dynein heavy chains previously not identified. For example, Tetrahymena expresses a total of 25 DYH genes rather than the previously identified 14. The Tetrahymena DYH genes are nonaxonemal DYH1 and DYH2; axonemal outer arm alpha, beta, and gamma; axonemal two-headed inner arm 1alpha and 1beta; and 18 single-headed inner arm heavy chains. The heavy chains divide into nine classes; six of these are highly conserved in sequence and number of isoforms in a given organism. The other three are single-headed inner arm dyneins, whose numbers vary significantly in different organisms. These findings lead to two conclusions. One, the last common ancestor of all eukaryotes expressed nine different dynein heavy chains. Two, subsequent to the divergences leading to different organisms, additional dynein heavy chains emerged. These newer dyneins are not well conserved across species and the variation may reflect different motility requirements in different organisms. Together, these results suggest that each of the nine classes of dyneins is functionally distinct, but members within some of the classes are not specialized. An understanding of the relationships among the various dynein heavy chains is important when deducing functions across species.     

The LC7 light chains of Chlamydomonas flagellar dyneins interact with components required for both motor assembly and regulation

Members of the LC7/Roadblock family of light chains (LCs) have been found in both cytoplasmic and axonemal dyneins. LC7a was originally identified within Chlamydomonas outer arm dynein and associates with this motor's cargo-binding region. We describe here a novel member of this protein family, termed LC7b that is also present in the Chlamydomonas flagellum. Levels of LC7b are reduced approximately 20% in axonemes isolated from strains lacking inner arm I1 and are approximately 80% lower in the absence of the outer arms. When both dyneins are missing, LC7b levels are diminished to <10%. In oda9 axonemal extracts that completely lack outer arms, LC7b copurifies with inner arm I1, whereas in ida1 extracts that are devoid of I1 inner arms it associates with outer arm dynein. We also have observed that some LC7a is present in both isolated axonemes and purified 18S dynein from oda1, suggesting that it is also a component of both the outer arm and inner arm I1. Intriguingly, in axonemal extracts from the LC7a null mutant, oda15, which assembles approximately 30% of its outer arms, LC7b fails to copurify with either dynein, suggesting that it interacts with LC7a. Furthermore, both the outer arm gamma heavy chain and DC2 from the outer arm docking complex completely dissociate after salt extraction from oda15 axonemes. EDC cross-linking of purified dynein revealed that LC7b interacts with LC3, an outer dynein arm thioredoxin; DC2, an outer arm docking complex component; and also with the phosphoprotein IC138 from inner arm I1. These data suggest that LC7a stabilizes both the outer arms and inner arm I1 and that both LC7a and LC7b are involved in multiple intradynein interactions within both dyneins.     

Functional reconstitution of Chlamydomonas outer dynein arms from alpha- beta and gamma subunits: requirement of a third factor

The outer dynein arm of Chlamydomonas flagella, when isolated under Mg(2+)-free conditions, tends to dissociate into an 11 to 12S particle (12S dynein) containing the gamma heavy chain and a 21S particle (called 18S dynein) containing the alpha and beta heavy chains. We show here that functional outer arms can be reconstituted by the addition of 12S and 18S dyneins to the axonemes of the outer armless mutants oda1- oda6. A third factor that sediments at integral 7S is required for efficient reconstitution of the outer arms on the axonemes of oda1 and oda3. However, this factor is not necessary for reconstitution on the axonemes of oda2, oda4, oda5, and oda6. SDS-PAGE analysis indicates that the axonemes of the former two mutants lack a integral of 70-kD polypeptide that is present in those of the other mutants as well as in the 7S fraction from the wild-type extract. Furthermore, electron micrographs of axonemal cross sections revealed that the latter four mutants, but not oda1 or oda3, have small pointed structures on the outer doublets, at a position in cross section where outer arms normally occur. We suggest that the 7S factor constitutes the pointed structure on the outer doublets and facilitates attachment of the outer arm. The discovery of this structure raises a new question as to how the attachment site for the outer arm dynein is determined within the axoneme.     

Phosphoregulation of an Inner Dynein Arm Complex in Chlamydomonas reinhardtii Is Altered in Phototactic Mutant Strains

To gain a further understanding of axonemal dynein regulation, mutant strains of Chlamydomonas reinhardtii that had defects in both phototactic behavior and flagellar motility were identified and characterized. ptm1, ptm2, and ptm3 mutant strains exhibited motility phenotypes that resembled those of known inner dynein arm region mutant strains, but did not have biochemical or genetic phenotypes characteristic of other inner dynein arm mutations. Three other mutant strains had defects in the f class of inner dynein arms. Dynein extracts from the pf9-4 strain were missing the entire f complex. Strains with mutations in pf9/ida1, ida2, or ida3 failed to assemble the f dynein complex and did not exhibit phototactic behavior. Fractionated dynein from mia1-1 and mia2-1 axonemes exhibited a novel f class inner dynein arm biochemical phenotype; the 138-kD f intermediate chain was present in altered phosphorylation forms. In vitro axonemal dynein activity was reduced by the mia1-1 and mia2-1 mutations. The addition of kinase inhibitor restored axonemal dynein activity concomitant with the dephosphorylation of the 138-kD f intermediate chain. Dynein extracts from uni1-1 axonemes, which specifically assemble only one of the two flagella, contained relatively high levels of the altered phosphorylation forms of the 138-kD intermediate chain. We suggest that the f dynein complex may be phosphoregulated asymmetrically between the two flagella to achieve phototactic turning. C hlamydomonas reinhardtii flagella use an asymmetric beat stroke, similar to a breast stroke, to propel cells forward. To generate the asymmetric beat stroke, dynein activity must be regulated both along the length and around the circumference of the flagella. If all dyneins were active at the same time, the flagella would exist in a state of rigor. The dyneins are located in two rows along the length of the doublet microtubules. The inner dynein arms are heterogeneous in composition with at least eight heavy chains and various intermediate and light chains arranged in an elaborate morphology that repeats every 96 nm (Kagami and Kamiya, 1992; Mastronarde et al., 1992). In contrast, the outer dynein arms are biochemically and morphologically homogeneous (Huang et al., 1979; Mitchell and Rosenbaum, 1985; Kamiya, 1988); each outer dynein arm contains three dynein heavy chains and 10 intermediate and light chains. The inner and outer arms appear to have different functions in the formation of the beat stroke; the inner arms generate the waveform of the beat stroke, whereas the outer arms provide additional force to the waveform (Brokaw and Kamiya, 1987).Previous workers had shown that dynein regulation is imposed, in part, by activities of the radial spokes and the central pair complex. Mutant strains that are missing or have altered radial spokes or central pair complexes are paralyzed even if they have a full complement of dyneins (Adams et al., 1981; Piperno et al., 1981). Many extragenic suppressors of this paralysis phenotype do not restore the missing structures, but rather suppress by altering either inner arm or outer arm region structures (Huang et al., 1982a ; Piperno et al., 1992; Porter et al., 1992, 1994). These data suggest that direct or indirect interactions exist between the dynein arms and the radial spokes or central pair complexes.Over 80 proteins in Chlamydomonas flagella are phosphorylated (Piperno et al., 1981), which makes dynein regulation by phosphorylation an attractive model. Hasegawa et al. (1987) showed that a higher percentage of demembranated axonemes reactivate with ATP after treatments that lower cAMP levels or inhibit cAMP-dependent protein kinase (cAPK)¹. In flagella from other organisms, cAMP has an opposite role (for reviews see Tash and Means, 1983; Tash, 1989). An increased frequency of reactivation also occurs after the NP-40–soluble components are extracted from the axonemes, which suggests that the cAPK, target phosphoproteins, and endogenous phosphatases are all integral axonemal components (Hasegawa et al., 1987). In quantitative sliding disintegration assays, the inner dynein arm activity of axonemes that are missing the radial spokes is increased in the presence of pharmacological or specific peptide inhibitors of cAPK (Smith and Sale, 1992; Howard et al., 1994). Reconstitution experiments with axonemes that are missing the radial spokes suggest that radial spokes normally function to activate the inner dynein arms by inhibiting a cAPK (Smith and Sale, 1992; Howard et al., 1994). It is not known if the cAPK directly phosphorylates inner dynein arm components or phosphorylates another axonemal component that then acts on the inner dynein arms (Howard et al., 1994).The f (originally called I1) inner arms are biochemically the best studied inner dynein arm complex. This complex is comprised of two dynein heavy chains and three intermediate chains of 140, 138, and 110 kD; it can be purified by sucrose density centrifugation (Piperno and Luck, 1981; Smith and Sale, 1991; Porter et al., 1992) or ion-exchange chromatography (Kagami and Kamiya, 1992). The purified complex has low ATPase activity and only rarely translocates microtubules in vitro (Smith and Sale, 1991; Kagami and Kamiya, 1992). Deep-etch EM of the purified f inner arm shows a two-headed complex that is connected to a common base by thin stalks (Smith and Sale, 1991). Longitudinal EM image analyses have shown that this complex is located just proximally of the first radial spoke in each 96-nm repeating unit (Piperno et al., 1990; Mastronarde et al., 1992). Mutations at three different loci (PF9/ IDA1, IDA2, and IDA3) result in the complete loss of the f complex (Kamiya et al., 1991; Kagami and Kamiya, 1992; Porter et al., 1992). The PF9/IDA1 locus encodes a dynein heavy chain that is believed to be one of the two heavy chains that are components of the f complex (Porter, 1996).We undertook a new approach to identify axonemal components involved in dynein regulation; we isolated and characterized mutant strains that were unable to perform phototaxis. In Chlamydomonas, phototaxis is a behavior by which cells orient to the direction of incident light. Light direction is detected by the eyespot, an asymmetrically located organelle, and a signal is transmitted to the flagella using voltage-gated ion channels (Harz and Hegemann, 1991). For cells to perform phototaxis, the waveforms of the two flagella are altered coordinately. The trans flagellum, which is located farther from the eyespot, beats with a larger front amplitude than the cis flagellum to turn the cell toward the light (Rüffer and Nultsch, 1991). It seemed likely that the alterations in the beat amplitudes needed for correct phototactic behavior could be caused by differential dynein regulation in the cis and trans flagella. Therefore, we hypothesized that there should be a class of phototactic mutant strains that is not able to perform phototaxis because of defects in the regulation of dyneins. Three of the eight phototactic mutant strains that we characterized had biochemical defects in the f class of inner dynein arms. One of these strains, pf9-4, was missing the entire f complex, and the other two strains, mia1-1 and mia2-1, exhibited a novel f class inner dynein arm biochemical phenotype. These observations suggest that the f inner dynein arm is a target for regulation during phototaxis.     

Flagellar quiescence in Chlamydomonas: Characterization and defective quiescence in cells carrying sup-pf-1 and sup-pf-2 outer dynein arm mutations

Chlamydomonas reinhardtii can use their flagella for two distinct types of movement: swimming through liquid or gliding on a solid substrate. Cells switching from swimming to gliding motility undergo a reversible flagellar quiescence. This phenomenon appears to involve the outer dynein arms, since mutants having altered outer arm beta and gamma dyneins (sup-pf-1 and sup-pf-2) show a diminished ability to quiesce. Sup-pf-1 and sup-pf-2 were originally isolated as gain-of-function mutations that suppress the flagellar paralysis resulting from radial spoke or central pair defects. Defective quiescence is also a gain-of-function phenomenon, as cells completely lacking outer arm heavy chains show a normal quiescence phenotype. These data suggest that regulation of outer arm dynein activity is essential for flagellar quiescence and furthermore that regulation of quiescence involves a signal transduction pathway that shares elements with the radial spoke/central pair system.     

Further studies on knockout mice lacking a functional dynein heavy chain (MDHC7). 1. Evidence for a structural deficit in the axoneme

Male mice had previously been generated in which the inner dynein arm heavy chain 7 gene (MDHC7) was inactivated by the substitution of four exons encoding the ATP-binding site (P1-loop) with the neomycin resistance gene, giving a putative non-functional gene product. We have used additional techniques of electron microscopy to determine what effect the truncated, non-functional heavy chain has on the assembly of the inner dynein arm complex. From a comparison of MDHC7-/- with the wild-type morphology, we have found that the expected loss of a C-terminal (globular) domain is associated with inner dynein arm 3, a change from two visible "heads" to one. This deficit was seen in replicas of rapidly-frozen, deeply-etched spermatozoa, and was confirmed in filtered images of 20-nm-thin sections, cut in longitudinal planes. Assembly of the other IDAs appeared unaffected. This study is the first to reveal the location of a specific dynein heavy chain within the 96-nm repeat pattern of the inner dynein arms of the mammalian axoneme.     

Biochemical Analysis of a Mutant Tetrahymena Lacking Outer Dynein Arms

ABSTRACT. Tetrahymena thermophila mutants homozygous for the oad mutation become nonmotile when grown at the restrictive temperature, and axonemes isolated from nonmotile mutants lack approximately 90% of their outer dynein arms. Electrophoretic analyses of axonemes isolated from nonmotile mutants ( oad axonemes) indicate they contain significantly fewer of the 22 S dynein heavy chains that axonemes isolated from wild-type cells (wild-type axonemes) contain. The 22 S dynein heavy chains that remain in axonemes isolated from nonmotile, oad mutants are assembled into 22 S dynein particles that exhibit wild-type levels of ATPase activity. Two-dimensional gel electrophoresis of oad axonemes show that they are deficient in no proteins other than those proteins thought to be components of 22 S dynein. This report is the first formal proof that outer dynein arms in Tetrahymena cilia are composed of 22 S dynein.     

Inner arm dynein ATPase fraction of sea urchin sperm flagella causes active sliding of axonemal outer doublet microtubule.

In order to clarify the role of the inner arms of the axoneme in sperm flagellar movement, we prepared an ATPase fraction (12S) from the outer arm-depleted axonemes of sea urchin sperm flagella. When both arm-depleted axonemes were incubated with the 12S ATPase, they exhibited the sliding disintegration of outer doublet microtubules. Electron microscopy revealed that the ATPase rebound to the original inner arm sites of the axoneme. Therefore, it is quite likely that the 12S ATPase is one of the components of the inner arms. We referred to it as "inner arm dynein".     

Arrangement of inner dynein arms in wild-type and mutant flagella of Chlamydomonas

We have used computer averaging of electron micrographs from longitudinal and cross-sections of wild-type and mutant axonemes to determine the arrangement of the inner dynein arms in Chlamydomonas reinhardtii. Based on biochemical and morphological data, the inner arms have previously been described as consisting of three distinct subspecies, I1, I2, and I3. Our longitudinal averages revealed 10 distinguishable lobes of density per 96-nm repeating unit in the inner row of dynein arms. These lobes occurred predominantly but not exclusively in two parallel rows. We have analyzed mutant strains that are missing I1 and I2 subspecies. Cross-sectional averages of pf9 axonemes, which are missing the I1 subspecies, showed a loss of density in both the inner and outer portions of the inner arm. Averages from longitudinal images showed that three distinct lobes were missing from a single region; two of the lobes were near the outer arms but one was more inward. Serial 24-nm cross-sections of pf9 axonemes showed a complete gap at the proximal end of the repeating unit, confirming that the I1 subunit spans both inner and outer portions of the inner arm region. Examination of pf23 axonemes, which are missing both I1 and I2 subspecies, showed an additional loss almost exclusively in the inner portion of the inner arm. In longitudinal view, this additional loss occurred in three separate locations and consisted of three inwardly placed lobes, one adjacent to each of the two radial spokes and the third at the distal end of the repeating unit. These same lobes were absent ida4 axonemes, which lack only the I2 subspecies. The I2 subspecies thus does not consist of a single dynein arm subunit in the middle of the repeating unit. The radial spoke suppressor mutation, pf2, is missing four polypeptides of previously unknown location. Averages of these axonemes were missing a portion of the structures remaining in pf23 axonemes. This result suggests that polypeptides of the radial spoke control system are close to the inner dynein arms.