The 78,000-M(r) intermediate chain of Chlamydomonas outer arm dynein is a microtubule-binding protein.

A previous study (King et al., 1991. J. Biol. Chem. 266:8401-8407) showed that the 78,000-M(r) intermediate chain (IC78) from the Chlamydomonas outer arm dynein is in direct contact with alpha-tubulin in situ, suggesting that this protein may be involved in binding the dynein to the doublet microtubules. Molecular genetic analysis of this chain recently demonstrated that it is a WD repeat protein essential for outer arm assembly (Wilkerson et al., 1995.J. Cell Biol. 129:169-178). We have now transcribed and translated IC78 in vitro, and demonstrate that this molecule binds axonemes and microtubules, whereas a homologous protein (the 69,000-M(r) intermediate chain [IC69] of Chlamydomonas outer arm dynein) does not. Thus, IC78 is a bona fide microtubule-binding protein. Taken together with the previous results, these findings indicate that IC78 is likely to provide at least some of the adhesive force that holds the dynein to the doublet microtubule, and support the general hypothesis that the dynein intermediate chains are involved in targeting different dyneins to the specific cell organelles with which they associate. Analysis of the binding activities of various IC78 deletion constructs translated in vitro identified discrete regions of IC78 that affected the binding to microtubules; two of these regions are specifically missing in IC69. Previous studies also showed that IC78 is in direct contact with IC69; the current work indicates that the region of IC78 that mediates this interaction is coincident with two of IC78's WD repeats. This supports the hypothesis that these repeats are involved in protein-protein interactions within the dynein complex.

A Chlamydomonas homologue of the putative murine t complex distorter Tctex-2 is an outer arm dynein light chain.

Molecular analysis of a 19,000-Mr protein from the Chlamydomonas flagellum reveals that it is homologous to the t complex-encoded protein Tctex-2, which is a candidate for one of the distorter products that cause the extreme transmission ratio distortion (meiotic drive) of the murine t complex. The 19,000-Mr protein is extracted from the axoneme with 0.6 M NaCl and comigrates with the outer dynein arm in sucrose density gradients. This protein also is specifically missing in axonemes prepared from a mutant that does not assemble the outer arm. These data raise the possibility that Tctex-2 is a sperm flagellar dynein component. Combined with the recent identification of Tctex-1 (another distorter candidate) as a light chain of cytoplasmic dynein, these results lead to a biochemical model for how differential defects in spermiogenesis that result in the phenomenon of meiotic drive might be generated in wild-type vs t-bearing sperm.

Drosophila roadblock and Chlamydomonas LC7: a conserved family of dynein-associated proteins involved in axonal transport, flagellar motility, and mitosis.

Eukaryotic organisms utilize microtubule-dependent motors of the kinesin and dynein superfamilies to generate intracellular movement. To identify new genes involved in the regulation of axonal transport in Drosophila melanogaster, we undertook a screen based upon the sluggish larval phenotype of known motor mutants. One of the mutants identified in this screen, roadblock (robl), exhibits diverse defects in intracellular transport including axonal transport and mitosis. These defects include intra-axonal accumulations of cargoes, severe axonal degeneration, and aberrant chromosome segregation. The gene identified by robl encodes a 97-amino acid polypeptide that is 57% identical (70% similar) to the 105-amino acid Chlamydomonas outer arm dynein-associated protein LC7, also reported here. Both robl and LC7 have homology to several other genes from fruit fly, nematode, and mammals, but not Saccharomyces cerevisiae. Furthermore, we demonstrate that members of this family of proteins are associated with both flagellar outer arm dynein and Drosophila and rat brain cytoplasmic dynein. We propose that roadblock/LC7 family members may modulate specific dynein functions.

LC2, the chlamydomonas homologue of the t complex-encoded protein Tctex2, is essential for outer dynein arm assembly.

Tctex2 is thought to be one of the distorter genes of the mouse t haplotype. This complex greatly biases the segregation of the chromosome that carries it such that in heterozygous +/t males, the t haplotype is transmitted to >95% of the offspring, a phenomenon known as transmission ratio distortion. The LC2 outer dynein arm light chain of Chlamydomonas reinhardtii is a homologue of the mouse protein Tctex2. We have identified Chlamydomonas insertional mutants with deletions in the gene encoding LC2 and demonstrate that the LC2 gene is the same as the ODA12 gene, the product of which had not been identified previously. Complete deletion of the LC2/ODA12 gene causes loss of all outer arms and a slow jerky swimming phenotype. Transformation of the deletion mutant with the cloned LC2/ODA12 gene restores the outer arms and rescues the motility phenotype. Therefore, LC2 is required for outer arm assembly. The fact that LC2 is an essential subunit of flagellar outer dynein arms allows us to propose a detailed mechanism whereby transmission ratio distortion is explained by the differential binding of mutant (t haplotype encoded) and wild-type dyneins to the axonemal microtubules of t-bearing or wild-type sperm, with resulting differences in their motility.

The M(r) = 8,000 and 11,000 outer arm dynein light chains from Chlamydomonas flagella have cytoplasmic homologues.

We report here the molecular cloning of the M(r) = 8,000 and 11,000 dynein light chains (DLCs) from the outer arm of Chlamydomonas flagella. These two molecules, which are associated with the intermediate chains at the base of the soluble dynein particle, have predicted masses of 10.3 and 13.8 kDa, respectively, and are 40% identical. Southern blot analysis indicates that one gene exists for each DLC in the Chlamydomonas genome and only a single message was observed for each on Northern blots. Secondary structure predictions suggest that both molecules contain a highly amphiphilic alpha helix that is presumably involved in protein-protein interactions. Several DLC homologues were identified in the GenBank databases. One, predicted from the genomic sequence of Caenorhabditis elegans, is 88.8% identical with the M(r) = 8,000 Chlamydomonas DLC. A second, from rice callus cDNA, is 47% identical with the same DLC. As neither nematodes nor higher plants have motile cilia or flagella at any stage of their life cycles, these DLC homologues presumably must function within the cytoplasm where they may represent previously unrecognized components of cytoplasmic dynein.

Identification of a Ca(2+)-binding light chain within Chlamydomonas outer arm dynein.

We describe here the molecular cloning of the M(r) 18,000 dynein light chain from the outer arm of Chlamydomonas flagella. In vivo, this molecule is directly associated with the gamma dynein heavy chain. Sequence analysis indicates that this light chain is a novel member of the calmodulin superfamily of Ca2+ binding regulatory proteins; this molecule is 42, 37 and 36% identical to calmodulin, centrin/caltractin and troponin C, respectively, and also shows significant similarity to myosin light chains. Although four helix-loop-helix elements are evident, only two conform precisely to the EF hand consensus and are therefore predicted to bind Ca2+ in vivo. In vitro Ca2+ binding studies indicate that this dynein light chain (expressed as a C-terminal fusion with maltose binding protein) has at least one functional Ca2+ binding site with an apparent affinity for Ca2+ of approximately 3 x 10(-5) M. Within the Chlamydomonas flagellum, the transition from an assymmetric to a symmetric waveform (which implies an alteration in dynein activity) is mediated by an increase in intraflagellar Ca2+ from 10(-6) to 10(-1) M; this transition is altered in mutants that lack the outer arm. The data presented here suggest that a Ca(2+)-dependent alteration in the interaction of this dynein light chain with the motor containing heavy chain may affect outer arm function during flagellar reversal.

Dimerization of the highly conserved light chain shared by dynein and myosin V.

The Mr 8,000 light chain originally identified in Chlamydomonas flagellar dynein is also a component of both cytoplasmic dynein and myosin V. Furthermore, this small protein has been implicated as an inhibitor of neuronal nitric oxide synthase, suggesting that it may play multiple regulatory roles within the cell. Covalent cross-linking of both dynein and myosin V using 1,5-difluoro-2, 4-dinitrobenzene revealed that this light chain exists as a dimer in situ. This observation was confirmed using two additional amine-selective cross-linking reagents (dimethyl pimelimidate and disuccinimidyl suberate). When expressed as a C-terminal fusion with maltose-binding protein, the presence of the light chain caused the recombinant molecule to dimerize. Analysis of fusions containing truncated light chains identified the predicted amphiphilic helix (residues 14-32) as sufficient to cause dimerization; cross-linking required a second helical segment (residues 33-46). Together the data presented suggest that two light chains interact to form a parallel dimeric structure. This arrangement has significant implications for the potential functions of this highly conserved molecule and suggests a mechanism by which it might dissociate nitric oxide synthase.

Two functional thioredoxins containg redox-senesitive vicinal dithiols from the Chlamydomonas outer dynein arm.

We describe here the molecular cloning and analysis of the Mr 14,000 and 16,000 outer arm dynein light chains (DLCs) from Chlamydomonas flagella. Within the outer arm, the Mr 14,000 DLC apparently is associated with the intermediate chains at the base of the soluble dynein particle; the Mr 16,000 DLC interacts directly with the a dynein heavy chain. Sequence analysis indicates that both molecules are novel members of the thioredoxin superfamily and share approximately 30% sequence identity with thioredoxin from Penicillium. Both DLCs have a perfect copy of the thioredoxin active site (WCGPCK); the Mr 16,000 DLC also contains the canonical P-loop motif (AX4GKS). There is a single gene for both DLCs within Chlamydomonas and only single messages that were upregulated more than 10-fold upon deflagellation were observed on Northern blots. Both recombinant DLCs were specifically eluted from a phenylarsine oxide matrix with beta-mercaptoethanol indicating that they contain vicinal dithiols competent to undergo reversible oxidation/reduction. Furthermore, we demonstrate that outer (but not inner) arm dynein may he purified on the basis of its affinity for phenylarsine oxide suggesting that the predicted redox-sensitive vicinal dithiols exist within the native complex.

Light chain 1 from the Chlamydomonas outer dynein arm is a leucine-rich repeat protein associated with the motor domain of the gamma heavy chain.

The LC1 light chain from Chlamydomonas outer arm dynein is tightly bound to the gamma heavy chain. Molecular cloning revealed that LC1 is a member of the SDS22+ subclass of the leucine-rich repeat protein family and as such is likely involved in mediating interactions between dynein and the components of a signal transduction pathway. Through the combination of covalent cross-linking and vanadate-mediated photolysis, LC1 was found to associate with that portion of the gamma HC that is C-terminal to the P1 loop. This region comprises most of the globular head domain of the heavy chain and includes the stalk-like structure that is involved in microtubule binding. Attachment of LC1 to this region represents the only known example of an accessory polypeptide directly associated with a dynein motor domain. Additional cross-linking experiments revealed that LC1 also interacts directly in situ with an approximately 45 kDa axonemal component; this interaction is disrupted by the standard high salt treatment used to remove the outer arm from the axoneme. These data suggest that LC1 acts to mediate the association between this 45 kDa axonemal polypeptide and the motor unit of the gamma HC.

Requirements for integrins during Drosophila development.

The common beta subunit of the PS antigens of Drosophila is homologous with vertebrate integrins and is encoded by the lethal(1)myospheroid gene. We have generated flies mosaic for wild-type and mutant alleles of lethal(1)myospheroid using adult gynandromorphs and radiation-induced somatic crossing over. The defects observed in the gynandromorphs demonstrate widespread requirements for PS integrins during development especially in ventrally derived structures, which also show strong expression of PS beta integrin. Smaller lethal(1)myospheroid clones induced during larval development result in blister and vein defects in the wings and aberrant development of photoreceptor cells, demonstrating roles for PS integrins during development of both wings and eyes. PS integrins are required for the close apposition of the dorsal and ventral wing epithelia and for the proper arrangement of photoreceptor cells. However, many other adhesive and morphogenetic processes proceed normally in the absence of integrins containing the beta subunit encoded by lethal(1)myospheroid.

Defects in mesoderm, neural tube and vascular development in mouse embryos lacking fibronectin.

To examine the role of fibronectin in vivo, we have generated mice in which the fibronectin gene is inactivated. Heterozygotes have one half normal levels of plasma fibronectin, yet appear normal. When homozygous, the mutant allele causes early embryonic lethality, proving that fibronectin is required for embryogenesis. However, homozygous mutant embryos implant and initiate gastrulation normally including extensive mesodermal movement. Neural folds also form but the mutant embryos subsequently display shortened anterior-posterior axes, deformed neural tubes and severe defects in mesodermally derived tissues. Notochord and somites are absent; the heart and embryonic vessels are variable and deformed, and the yolk sac, extraembryonic vasculature and amnion are also defective. These abnormalities can be interpreted as arising from fundamental deficits in mesodermal migration, adhesion, proliferation or differentiation as a result of the absence of fibronectin. The nature of these embryonic defects leads to reevaluation of suggested roles for fibronectin during early development based on results obtained in vitro and in embryos of other species.

The Tctex1/Tctex2 class of dynein light chains. Dimerization, differential expression, and interaction with the LC8 protein family.

The Tctex1/Tctex2 family of dynein light chains associates with the intermediate chains at the base of the soluble dynein particle. These components are essential for dynein assembly and participate in specific motor-cargo interactions. To further address the role of these light chains in dynein activity, the structural and biochemical properties of several members of this polypeptide class were examined. Gel filtration chromatography and native gel electrophoresis indicate that recombinant Chlamydomonas flagellar Tctex1 exists as a dimer in solution. Furthermore, yeast two-hybrid analysis suggests that this association also occurs in vivo. In contrast, both murine and Chlamydomonas Tctex2 are monomeric. To investigate protein-protein interactions involving these light chains, outer arm dynein from Chlamydomonas flagella was cross-linked using dimethylpimelimidate. Immunoblot analysis of the resulting products revealed the interaction of LC2 (Tctex2) with LC6, which is closely related to the highly conserved LC8 protein found in many enzyme systems, including dynein. Northern dot blot analysis demonstrated that Tctex1/Tctex2 family light chains are differentially expressed both in a tissue-specific and developmentally regulated manner in humans. These data provide further support for the existence of functionally distinct populations of cytoplasmic dynein with differing light chain content.

Brain cytoplasmic and flagellar outer arm dyneins share a highly conserved Mr 8,000 light chain.

Sequence comparisons with the Mr 8,000 light chain from Chlamydomonas outer arm dynein revealed the presence of highly conserved homologues (up to 90% identity) in the expressed sequence tag data base (King, S. M. & Patel-King, R. S. (1995a) J. Biol. Chem. 270, 11445-11452). Several of these homologous sequences were derived from organisms and/or tissues that lack motile cilia/flagella, suggesting that these proteins may function in the cytoplasm. In Drosophila, lack of the homologous protein results in embryonic lethality (Dick, T., Ray, K., Salz, H. K. & Chia, W.(1996) Mol. Cell. Biol., 16, 1966-1977). Fractionation of mammalian brain homogenates reveals three distinct cytosolic pools of the homologous protein, one of which specifically copurifies with cytoplasmic dynein following both ATP-sensitive microtubule affinity/sucrose density gradient centrifugation and immunoprecipitation with a monoclonal antibody specific for the 74-kDa intermediate chain (IC74). Quantitative densitometry indicates that there is one copy of the Mr 8,000 polypeptide per IC74. Dual channel confocal immunofluorescent microscopy revealed that the Mr 8,000 protein is significantly colocalized with cytoplasmic dynein but not with kinesin in punctate structures (many of which are associated with microtubules) within mammalian oligodendrocytes. Thus, it appears that flagellar outer arm and brain cytoplasmic dyneins share a highly conserved light chain polypeptide that, at least in Drosophila, is essential for viability.

Identification and molecular characterization of the p24 dynactin light chain.

Intracellular transport along microtubules uses the motor proteins cytoplasmic dynein and kinesin. Cytoplasmic dynein is responsible for movement to the minus ends of microtubules and the evidence indicates that dynein interacts with another protein complex, dynactin. In order to better understand how these proteins function, we have sought to identify and clone the subunit polypeptides of these two complexes, in particular their light chains. Dynactin is made up of eight subunits of approximately 24,000 to 160,000 Da. In order to clone the p24 subunit, the components of purified dynactin were resolved by SDS polyacrylamide gel electrophoresis. The amino acid sequence of a tryptic peptide from the 24,000-Mr region of the gel was obtained and a candidate polypeptide identified by a screen of the databases. This polypeptide has a predicted molecular weight of 20,822 Da. Using an antibody to a different region of this protein, we demonstrate that it copurifies with microtubules and elutes from the microtubule pellet with characteristics similar to those of the dynactin complex and distinct from those of cytoplasmic dynein. This polypeptide co-sediments with dynactin on sucrose density gradients and it also co-immunoprecipitates with dynactin, but not with kinesin or cytoplasmic dynein. Together these results demonstrate that this polypeptide is the p24 subunit of dynactin. Analysis of the predicted amino acid sequence of p24 shows that it is a unique protein that has no significant similarity to known enzymes or other proteins. Structural analysis indicates that most of this protein will form an alpha-helix and that portions of the molecule may participate in the formation of coiled-coils. Since stoichiometric analysis of dynactin indicates that there is one molecule of p24 per dynactin complex, these characteristics suggest that this polypeptide may be involved in protein-protein interactions, perhaps in the assembly of the dynactin complex.

The mouse t-complex-encoded protein Tctex-1 is a light chain of brain cytoplasmic dynein.

Mammalian brain cytoplasmic dynein contains three light chains of Mr = 8,000, 14,000, and 22,000 (King, S. M., Barbarese, E., Dillman, J. F., III, Patel-King, R. S., Carson, J. H., and Pfister, K. K. (1996) J. Biol. Chem. 271, 19358-19366). Peptide sequence data (16/16 residues correct) implicate the Mr = 14,000 polypeptide as Tctex-1, a protein encoded within the mouse t-complex. Tctex-1 cosediments with microtubules and is eluted with ATP or salt but not with GTP as expected for a dynein subunit. The ATP-eluted protein precisely cosediments with known cytoplasmic dynein proteins in sucrose density gradients. Tctex-1 also is immunoprecipitated from brain and other tissue homogenates by a monoclonal antibody raised against the 74-kDa cytoplasmic dynein intermediate chain. Quantitative densitometry indicates that Tctex-1 is a stoichiometric component of the dynein complex. As Tctex-1 is a candidate for involvement in the transmission ratio distortion (meiotic drive) of mouse t-haplotypes, these results suggest that cytoplasmic dynein dysfunction may play an important role in non-mendelian chromosome segregation.

Cytoplasmic dynein contains a family of differentially expressed light chains.

Cytoplasmic dynein contains a series of accessory proteins associated with the motor containing heavy chains.1 These include three distinct classes of light chains (Mr < approximately 22 000). Here we demonstrate that a previously cloned protein termed rp3 is a bona fide Mr 14 000 light chain component of this microtubule motor complex. The rp3 polypeptide is approximately 55% identical to the Tctex1 dynein light chain, and together, these two proteins define one branch of a diverse family of Mr 14 000 light chains associated with both cytoplasmic and flagellar dyneins. The Tctex1 and rp3 light chains are differentially expressed in various tissues: rp3 is most prevalent in liver and brain cytoplasmic dynein, whereas those tissues contain the least amounts of Tctex1. Immunofluorescence analysis was consistent with the tissue-specific distribution of these proteins and revealed that both rp3 and Tctex1 are present in multiple perinuclear punctate particles. Furthermore, in two cell lines, rp3 was found associated with an elongated structure located in the layer of cytoplasm above the nucleus. Electrophoretic/immunological analysis indicates that there are only single isoforms for these proteins in brain and PC-12 cells, suggesting that alterations in the Mr 14 000 light chains of dynein are achieved at the level of the individual proteins and not by posttranslational modification. Dissection of the cytoplasmic dynein complex revealed that Tctex1, an Mr 8000 LC dimer, and IC74 associate to define a basal-located intermediate chain/light chain complex analogous to that found in flagellar outer arm dynein.