Further evidence of the similarity of microtubule protein from mitotic apparatus and sperm tail of the sea urchin, as a substrate in thiol-disulfide exchange reaction

It has been demonstrated that thiol-disulfide exchange reaction occurs between outer fiberprotein from sperm tail and cortical protein from egg cortex of the sea urchin. This fact indicates homology between microtubule proteins of the outer fiber and mitotic apparatus.     

IMMUNOCHEMICAL STUDIES OF 22S PROTEIN FROM ISOLATED MITOTIC APPARATUS

Evidence is presented that the "22S protein" of mitotic apparatus isolated from sea urchin eggs is not microtubule protein. An antibody preparation active against 22S protein is described, and immunochemical studies of the distribution of 22S protein in various cellular fractions and among morphological features of mitotic apparatus are reported. The protein is ubiquitous in the metaphase egg fractions that were tested but is not found in sperm flagella. It is immunologically distinct from proposed microtubule protein isolated from mitotic apparatus by the method of Sakai, and from proposed microtubule protein obtained after extraction with mild acid. It exists in nontubule material of isolated mitotic apparatus but is not detectable in microtubules.     

Cytoskeletal architecture of isolated mitotic spindle with special reference to microtubule-associated proteins and cytoplasmic dynein

We have studied cytoskeletal architectures of isolated mitotic apparatus from sea urchin eggs using quick-freeze, deep-etch electron microscopy. This method revealed the existence of an extensive three- dimensional network of straight and branching crossbridges between spindle microtubules. The surface of the spindle microtubules was almost entirely covered with hexagonally packed, small, round button- like structures which were very uniform in shape and size (approximately 8 nm in diameter), and these microtubule buttons frequently provided bases for crossbridges between adjacent microtubules. These structures were removed from the surface of microtubules by high salt (0.6 M NaCl) extraction. Microtubule- associated proteins (MAPs) and microtubules isolated from mitotic spindles which were mainly composed of a large amount of 75-kD protein and some high molecular mass (250 kD, 245 kD) proteins were polymerized in vitro and examined by quick-freeze, deep-etch electron microscopy. The surfaces of microtubules were entirely covered with the same hexagonally packed round buttons, the arrangement of which is intimately related to that of tubulin dimers. Short crossbridges and some longer crossbridges were also observed. High salt treatment (0.6 M NaCl) extracted both 75-kD protein and high molecular weight proteins and removed microtubule buttons and most of crossbridges from the surface of microtubules. Considering the relatively high amount of 75- kD protein among MAPs isolated from mitotic spindles, it is concluded that these microtubule buttons probably consist of 75-kD MAP and that some of the crossbridges in vivo could belong to MAPs. Another kind of granule, larger in size (11-26 nm in diameter), was also on occasion associated with the surface of microtubules of mitotic spindles. A fine sidearm sometimes connected the larger granule to adjacent microtubules. Localization of cytoplasmic dynein ATPase in the mitotic spindle was investigated by electron microscopic immunocytochemistry with a monoclonal antibody (D57) against sea urchin sperm flagellar 21S dynein and colloidal gold-labeled second antibody. Immunogold particles were closely associated with spindle microtubules. 76% of these were within 50 nm and 55% were within 20 nm from the surface of the microtubules. These gold particles were sporadically found on both polar and kinetochore microtubules of half-spindles at both metaphase and anaphase. They localized also on the microtubules between sister chromatids in late anaphase. These data indicate that cytoplasmic dynein is attached to the microtubules in sea urchin mitotic spindles.(ABSTRACT TRUNCATED AT 400 WORDS)     

A novel 24-kDa microtubule-associated protein purified from sea urchin eggs.

Chromatographic fractionation of a crude extract of sea urchin eggs on a hydrophobic column enabled us to find a new 24-kDa microtubule-associated protein (SU-MAP24) that bound tightly to the column and was eluted under alkaline conditions. Biochemical studies using the purified protein showed its direct binding to microtubules reconstituted from tubulin purified from starfish sperm outer fibers. SU-MAP24 promoted tubulin polymerization in a dose-dependent manner. Immunoblotting analysis showed that SU-MAP24 is present in a microtubule protein fraction obtained from a crude extract using taxol, and immunostaining of paraffin-sectioned metaphase eggs showed its localization in the mitotic apparatus. These results show that SU-MAP24 is a newly identified microtubule-associated protein.     

THE MECHANISM OF ACTION OF COLCHICINE : Colchicine Binding to Sea Urchin Eggs and the Mitotic Apparatus

Colchicine forms a complex in vivo with a protein present in fertilized or unfertilized sea urchin eggs; similar binding was obtained in vitro with the soluble fraction from egg homogenates. Kinetic parameters and binding equilibrium constant were essentially the same in vivo and in vitro. The binding site protein was shown to have a sedimentation constant of 6S by zone centrifugation. The protein was present in extracts of the isolated mitotic apparatus at a concentration which was several times higher than in whole-egg homogenates. It was extracted from the mitotic apparatus at low ionic strength under conditions which lead to the disappearance of microtubules. No binding could be detected to the 27S protein, previously described by Kane, which is a major protein component of the isolated mitotic apparatus. The properties of the colchicine-bindinG protein, (binding constant, sedimentation constant, Sephadex elution volume) are similar to those obtained with the protein from mammalian cells, sea-urchin sperm tails, and brain tissue, and thus support the conclusion that the protein is a subunit of microtubules.     

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.     

Monoclonal antibodies specific for an acetylated form of alpha-tubulin recognize the antigen in cilia and flagella from a variety of organisms

Seven monoclonal antibodies raised against tubulin from the axonemes of sea urchin sperm flagella recognize an acetylated form of alpha-tubulin present in the axoneme of a variety of organisms. The antigen was not detected among soluble, cytoplasmic alpha-tubulin isoforms from a variety of cells. The specificity of the antibodies was determined by in vitro acetylation of sea urchin and Chlamydomonas cytoplasmic tubulins in crude extracts. Of all the acetylated polypeptides in the extracts, only alpha-tubulin became antigenic. Among Chlamydomonas tubulin isoforms, the antibodies recognize only the axonemal alpha-tubulin isoform acetylated in vivo on the epsilon-amino group of lysine(s) (L'Hernault, S.W., and J.L. Rosenbaum, 1985, Biochemistry, 24:473-478). The antibodies do not recognize unmodified axonemal alpha-tubulin, unassembled alpha-tubulin present in a flagellar matrix-plus-membrane fraction, or soluble, cytoplasmic alpha-tubulin from Chlamydomonas cell bodies. The antigen was found in protein fractions that contained axonemal microtubules from a variety of sources, including cilia from sea urchin blastulae and Tetrahymena, sperm and testis from Drosophila, and human sperm. In contrast, the antigen was not detected in preparations of soluble, cytoplasmic tubulin, which would not have contained tubulin from stable microtubule arrays such as centrioles, from unfertilized sea urchin eggs, Drosophila embryos, and HeLa cells. Although the acetylated alpha-tubulin recognized by the antibodies is present in axonemes from a variety of sources and may be necessary for axoneme formation, it is not found exclusively in any one subset of morphologically distinct axonemal microtubules. The antigen was found in similar proportions in fractions from sea urchin sperm axonemes enriched for central pair or outer doublet B or outer doublet A microtubules. Therefore the acetylation of alpha-tubulin does not provide the mechanism that specifies the structure of any one class of axonemal microtubules. Preliminary evidence indicates that acetylated alpha-tubulin is not restricted to the axoneme. The antibodies described in this report may allow us to deduce the role of tubulin acetylation in the structure and function of microtubules in vivo.     

Ap58: a novel in situ outer dynein arm-binding protein

Outer arm dynein is a molecular motor that is positioned at 24 nm intervals on outer doublet microtubules in cilia and flagella. In the present paper, we report identification of a 58 kDa novel protein with a tetratricopeptide repeat (TPR), referred to as ap58 (for 58 kDa axonemal protein) in sea urchin sperm axonemes. Ap58 is extracted along with the outer arm dynein by a high salt solution from axonemes. Sucrose density gradient centrifugation or gel filtration of the extract separates the outer arm dynein core from ap58. Most ap58 sediments to the lower density fraction or elutes in fractions of smaller molecules. However, immunogold localization reveals that ap58 is distributed at approximately 25 nm intervals on doublet microtubules, suggesting that in situ it is associated with the outer dynein arm. Thus, ap58 with the TPR motif is a new member of outer dynein arm-binding proteins distinct from the outer dynein arm-docking complex.     

Localization of tektin filaments in microtubules of sea urchin sperm flagella by immunoelectron microscopy

Extraction of doublet microtubules from the sperm flagella of the sea urchin Strongylocentrotus purpuratus with sarkosyl (0.5%)-urea (2.5 M) yields a highly pure preparation of "tektin" filaments that we have previously shown to resemble intermediate filament proteins. They form filaments 2-3 nm in diameter as seen by negative stain electron microscopy and are composed of approximately equal amounts of three polypeptide bands with apparent molecular weights of 47,000, 51,000, and 55,000, as determined by SDS PAGE. We prepared antibodies to this set of proteins to localize them in the doublet microtubules of S. purpuratus and other species. Tektins and tubulin were antigenically distinct when tested by immunoblotting with affinity-purified antitektin and antitubulin antibodies. Fixed sperm or axonemes from several different species of sea urchin showed immunofluorescent staining with antitektin antibodies. We also used antibodies coupled to gold spheres to localize the proteins by electron microscopy. Whereas a monoclonal antitubulin (Kilmartin, J.V., B. Wright, and C. Milstein, 1982, J. Cell Biol. 93:576-582) decorates intact microtubules along their lengths, antitektins labeled only the ends of intact microtubules and sarkosyl-insoluble ribbons. However, if microtubules and ribbons attached to electron microscope grids were first extracted with sarkosyl-urea, the tektin filaments that remain were decorated by antitektin antibodies throughout their length. These results suggest that tektins form integral filaments of flagellar microtubule walls, whose antigenic sites are normally masked, perhaps by the presence of tubulin around them.     

The mechanism of action of vinblastine. Binding of [acetyl-3H]vinblastine to embryonic chick brain tubulin and tubulin from sea urchin sperm tail outer doublet microtubules.

Tritium-labeled viblastine, specific activity 107 Ci/mol, was prepared by acetylation of desacetylvinblastine with [3H]acetic anhydride, and has been employed in a study of vinblastine binding to tubulin. There are two high affinity vinblastine-binding sites per mole of embryonic chick brain tubulin (KA = 3-5 X 10(5) l./mol). Binding to these sites was rapid, and relatively independent of temperature between 37 and 0degreeC. Vincristin sulfate and desacetylvinblastine sulfate, two other active vinca alkaloid derivatives, competitively inhibited the binding of vinblastine. The inhibition constant for vincristine was 1.7 X 10(-5) M; and for desacetylvinblastine, 2 X 10(-5) M. The vinblastine binding activity of tubulin decayed upon aging, but this property was not studied in detail. Vinblastine did not depolymerize stable sea urchin sperm tail outer doublet microtubules, nor did it bind to these microtubules. However, tubulin solubilized from the B subfiber of the outer doublet microtubules possessed the two high affinity binding sites (KA = 1-3 X 105 l./mol). These data suggest that vinblastine destroys microtubules in cells primarily by inhibition of microtubule polymerization, and does not directly destroy preformed microtubules.     

Microinjection of antibodies to a 62 kd mitotic apparatus protein arrests mitosis in dividing sea urchin embryos

Previously, we described a 62 kd protein that is a component of the isolated sea urchin mitotic apparatus. This protein is a substrate for an endogenous, calcium/calmodulin-dependent protein kinase also associated with the mitotic apparatus. Phosphorylation of the 62 kd protein directly correlates with the depolymerization of microtubules in isolated mitotic apparatuses. Here we report a test of the function of the 62 kd protein in vivo. Double labeling studies using a monoclonal antibody to tubulin and an affinity purified antibody specific for the 62 kd protein reveal that the 62 kd protein co-localizes with mitotic apparatus microtubules. When affinity purified antibodies to the 62 kd protein were microinjected into dividing sea urchin embryos, mitosis was blocked in a stage-specific manner. The results are discussed with respect to the role of the 62 kd protein in the metaphase-anaphase transition.     

A tektin homologue is decreased in chlamydomonas mutants lacking an axonemal inner-arm dynein

In ciliary and flagellar axonemes, various discrete structures such as inner and outer dynein arms are regularly arranged on the outer doublet microtubules. Little is known about the basis for their regular arrangement. In this study, proteins involved in the attachment of inner-arm dyneins were searched by a microtubule overlay assay on Chlamydomonas mutant axonemes. A 58-kDa protein (p58) was found approximately 80% diminished in the mutants ida6 and pf3, both lacking one (species e) of the seven inner-arm species (a-g). Analysis of its cDNA indicated that p58 is homologous to tektin, a protein that was originally found in sea urchin and thought to be crucial for the longitudinal periodicity of the doublet microtubule. Unlike sea urchin tektin, which is a component of protofilament ribbons that occur after Sarkosyl treatment of axonemes, p58 was not contained in similar Sarkosyl-resistant ribbons from Chlamydomonas axonemes. Immunofluorescence microscopy showed that p58 was localized uniformly along the axoneme and on the basal body. The p58 signal was reduced in ida6 and pf3. These results suggest that a reduced amount of p58 is sufficient for the production of outer doublets, whereas an additional amount of it is involved in inner-arm dynein attachment.     

Heterogeneity of the alpha subunit of tubulin and the variability of tubulin within a single organism

When tubulins obtained from particular microtubules of the sea urchin (ciliary doublet A tubules, flagellar doublet microtubules, and mitotic microtubules) are analyzed by electrophoresis in a polyacrylamide gel system containing sodium dodecyl sulfate and urea, heterogeneity of the alpha subunit, and differences between the tubulins are revealed. The alpha subunit of tubulin from mitotic apparatus and from A microtubules of ciliary doublets is resolved into two bands, while the alpha subunit of flagellar doublet tubulin gives a single band. The mitotic and ciliary tubulins differ in the mobilities of their two alpha species, or in the relative amounts present, or both. The existence of differences between the tubulins has been confirmed by a preliminary analysis of their cyanogen bromide peptides.     

The Mitotic Apparatus with Unusually Many Microtubules from Sea Urchin Eggs Treated by Hexyleneglycol

The effect of hexyleneglycol on the structure of the mitotic apparatus was studied by light and electron microscopies. By treating sea urchin eggs at prometaphase and metaphase with hexyleneglycol, the mitotic apparatus was found to become remarkably decorated with unusually many astral microtubules which were conspicuously uniform in length. These microtubules appeared to be associated with the granular materials which are most likely microtubule initiating sites or microtubule-organizing centers.     

Sea urchin sperm creatine kinase: The flagellar isozyme is a microtubule-associated protein

Sea urchin sperm contain two isozymes of creatine kinase (CrK) in the sperm head and tail, as termini of a phosphocreatine shuttle to transport energy. The head isozyme is located at the mitochondrion. By using an antibody prepared against denatured flagellar CrK, we now show that the tail isozyme exists along the entire flagellum. This unusual CrK isozyme, of Mr 145 kDa, is a component of the flagellar axoneme as indicated by electron microscopic immunolocalization and cell fractionation. Flagellar CrK specifically reassociated with extracted sperm axonemes as well as with in vitro polymerized sea urchin egg microtubules. Neither sperm mitochondrial CrK nor mammalian muscle CrK bound to axonemes under similar conditions. Thus, although the two sperm isozymes have similar kinetic properties, they differ in affinity for microtubules, a characteristic that may determine the regional differentiation needed for establishing a phosphocreatine shuttle.     

Change in the heterogeneous distribution of tubulin isotypes in mitotic microtubules of the sea urchin egg by treatment with microtubule depolymerizing or stabilizing drugs.

In the mitotic sea urchin egg, the spindle microtubules were composed of different tubulin isotypes from those of astral microtubules using monoclonal antibodies [Oka et al. (1990) Cell Motil. Cytoskeleton, 16, 239-250]. Three of the antibodies, D2D6, DM1B, and YL1/2, were specific for spindle microtubules, astral microtubules and reactive with both microtubules, respectively. The mitotic sea urchin egg was treated with microtubule depolymerizing (colcemid and nocodazole) and stabilizing (hexylene glycol) drugs and change in the heterogeneous distribution of the tubulin isotypes was investigated by the immunofluorescence procedure using these three monoclonal anti-tubulin antibodies. We observed that: (1) the microtubule depolymerizing drugs caused quick depolymerization of most mitotic microtubules, and a small number of spindle microtubules remaining were stained with all three antibodies; (2) hexylene glycol induced many microtubules in the mitotic apparatus, which was stained with D2D6 but was not stained with DM1B; (3) hexylene glycol also induced a great number of miniasters in the cytoplasm, and they were stained with three antibodies. These results suggest that these drugs altered the distribution of tubulin isotypes in the mitotic microtubules during depolymerization or polymerization within a short time.     

A 62-kD protein required for mitotic progression is associated with the mitotic apparatus during M-phase and with the nucleus during interphase

A protein of 62 kD is a substrate of a calcium/calmodulin-dependent protein kinase, and both proteins copurify with isolated mitotic apparatuses (Dinsmore, J. H., and R. D. Sloboda. 1988. Cell. 53:769-780). Phosphorylation of the 62-kD protein increases after fertilization; maximum incorporation of phosphate occurs during late metaphase and anaphase and correlates directly with microtubule disassembly as determined by in vitro experiments with isolated mitotic apparatuses. Because 62-kD protein phosphorylation occurs in a pattern similar to the accumulation of the mitotic cyclin proteins, experiments were performed to determine the relationship between cyclin and the 62-kD protein. Continuous labeling of marine embryos with [35S]methionine, as well as immunoblots of marine embryo proteins using specific antibodies, were used to identify both cyclin and the 62-kD protein. These results clearly demonstrate that the 62-kD protein is distinct from cyclin and, unlike cyclin, is a constant member of the cellular protein pool during the first two cell cycles in sea urchin and surf clam embryos. Similar results were obtained using immunofluorescence microscopy of intact eggs and embryos. In addition, immunogold electron microscopy reveals that the 62-kD protein associates with the microtubules of the mitotic apparatus in dividing cells. Interestingly, the protein changes its subcellular distribution with respect to microtubules during the cell cycle. Specifically, during mitosis the 62-kD protein associates with the mitotic apparatus; before nuclear envelope breakdown, however, the 62-kD protein is confined to the nucleus. After anaphase, the 62-kD protein returns to the nucleus, where it resides until nuclear envelope disassembly of the next cell cycle.     

The velocity of microtubule sliding: its stability and load dependency

It is now well understood that ATP-driven active sliding between the doublet microtubules in the sperm axoneme generates flagellar movement. However, much remains to be learned about how this movement is controlled. Detailed analyses of the flagellar beating of the mammalian spermatozoa revealed that there were two beating modes at a constant rate of microtubule sliding: that is, a nearly constant-curvature beating in nonhyperactivated spermatozoa and a nearly constant-frequency beating in hyperactivated spermatozoa. The constant rate of microtubule sliding suggests that the beat frequency and waveform of the flagellar beating are dependently regulated. Comparison of the sliding velocity of several mammalian and sea urchin sperm flagella with their mechanical property clarified that the sliding velocity of the microtubule was determined by the stiffness of the flagellum at its base, and that its relationship was expressed by a logarithmic equation that is similar to the classical force-velocity equation of the muscle contraction. Data from sea urchin spermatozoa also satisfied the equation, suggesting that the same microtubule sliding system functions in both the mammalian and echinoderm spermatozoa.     

An antiserum to the sea urchin 20 S egg dynein reacts with embryonic ciliary dynein but it does not react with the mitotic apparatus

Unfertilized sea urchin eggs contain one or more dynein-like enzymes which may be able to serve as microtubule translocators during embryonic development. There are at least two interesting possibilities for the function of the egg dynein: the enzyme may be involved in cytoplasmic microtubule movement such as mitotic spindle anaphase motion; or the enzyme may be a stored precursor for the dynein that functions in embryonic cilia, which are expressed and highly motile at the blastula stage of development. In order to determine directly the distribution and possible function of one of the previously described egg dyneins, the latent-activity 20 S egg dynein (Asai and Wilson, 1985), an antiserum was produced which was highly reactive with the important high Mr polypeptides of 20 S dynein. This antiserum reacted in "Western" immunoblots and in dot-blotting experiments with egg dynein and with embryonic ciliary dynein, but it did not react with any component of sperm flagella. Indirect double immunofluorescence microscopy demonstrated that the anti-20 S antiserum could brightly stain embryonic cilia but it did not stain the sperm flagella from the same sea urchin species. Under the same conditions that the antiserum stained cilia, anti-20 S did not stain the mitotic apparatus but it did appear to stain the cortical region of the dividing egg. In a time-course experiment, the antigen reactive with the anti-20 S antiserum gradually accumulated in the developing early sea urchin embryo. The most significant increase in the apparent concentration of the 20 S dynein occurred just prior to embryonic ciliation and during a period when the mitotic activity of the embryo was in decline. These results lead to two conclusions. First, ciliary dynein and sperm flagellar dynein, although derived from very similar organelles and from the same species of sea urchin, are immunologically distinct. Second, the 20 S egg dynein may be a stored precursor of embryonic ciliary dynein and does not appear to be a component of the mitotic apparatus.