LOCALIZATION OF DYNEIN IN SEA URCHIN EGGS DURING CLEAVAGE*

Shape-motion relationships of centering microtubule asters

Although mechanisms that contribute to microtubule (MT) aster positioning have been extensively studied, still little is known on how asters move inside cells to faithfully target a cellular location. Here, we study sperm aster centration in sea urchin eggs, as a stereotypical large-scale aster movement with extreme constraints on centering speed and precision. By tracking three-dimensional aster centration dynamics in eggs with manipulated shapes, we show that aster geometry resulting from MT growth and interaction with cell boundaries dictates aster instantaneous directionality, yielding cell shape-dependent centering trajectories. Aster laser surgery and modeling suggest that dynein-dependent MT cytoplasmic pulling forces that scale to MT length function to convert aster geometry into directionality. In contrast, aster speed remains largely independent of aster size, shape, or absolute dynein activity, which suggests it may be predominantly determined by aster growth rate rather than MT force amplitude. These studies begin to define the geometrical principles that control aster movements.

Microtubule motors in the early sea urchin embryo

Sea urchin gametes and early embryos have proven to be a useful system for studying the roles of microtubule (MT)-associated motors in axonemal motility and cytoplasmic MT-based movements in dividing cells. In this brief article, known and potential sea urchin MT motors are listed and their possible biological functions are discussed.

Cytoplasmic dynein is localized to kinetochores during mitosis

Recent evidence suggests that the force for poleward movement of chromosomes during mitosis is generated at or close to the kinetochores. Chromosome movement depends on motion relative to microtubules, but the identities of the motors remain uncertain. One candidate for a mitotic motor is dynein, a large multimeric enzyme which can move along microtubules toward their slow growing end. Dyneins were originally found in axonemes of cilia and flagella where they power microtubule sliding. Recently, cytoplasmic dyneins have also been found, and specific antibodies have been raised against them. The cellular localization of dynein has previously been studied with several antibodies raised against flagellar dynein, but the relevance of these data to the distribution of cytoplasmic dynein is not known. Antibodies raised against cytoplasmic dyneins have shown localization of dynein antigens to the mitotic spindles in Caenorhabditis elegans embryos (Lye et al., personal communication) and punctate cytoplasmic structures in Dictyostelium amoebae. Using antibodies that recognize subunits of cytoplasmic dyneins, we show here that during mitosis, cytoplasmic dynein antigens concentrate near the kinetochores, centrosomes and spindle fibres of HeLa and PtK1 cells, whereas at interphase they are distributed throughout the cytoplasm. This is consistent with the hypothesis that cytoplasmic dynein is a mitotic motor.

Localization of cytoplasmic dynein to mitotic spindles and kinetochores

What is the origin of the forces generating chromosome and spindle movements in mitosis? Both microtubule dynamics and microtubule-dependent motors have been proposed as the source of these motor forces. Cytoplasmic dynein and kinesin are two soluble proteins that power membranous organelle movements on microtubules. Kinesin directs movement of organelles to the 'plus' end of microtubules, and is found at the mitotic spindle in sea urchin embryos, but not in mammalian cells. Cytoplasmic dynein translocates organelles to the 'minus' end of microtubules, and is composed of two heavy chains and several light chains. We report here that monoclonal antibodies to two of these subunits and to another polypeptide that associates with dynein localize the protein to the mitotic spindle and to the kinetochores of isolated chromosomes, suggesting that cytoplasmic dynein is important in powering movements of the spindle and chromosomes in dividing cells.

Substructure of sea urchin egg cytoplasmic dynein

The substructure of the cytoplasmic dynein molecule was studied using the quick-freeze, deep-etch technique. Cytoplasmic dynein purified as a 12 S form from the eggs of the sea urchin Hemicentrotus pulcherrimus was composed of a single high molecular weight polypeptide. Rotary shadowing images of cytoplasmic dynein either sprayed on to a mica surface or quick-frozen on mica flakes demonstrated a single-headed molecule, in contrast to the two-headed molecule of sea urchin sperm flagellar 21 S dynein. More detailed substructure was visualized by rotary shadowing after quick-freeze deep-etching. Cytoplasmic dynein consisted of a head and a stem. The head was pear-shaped (16 nm X 11 nm) and a little smaller than the pear-shaped head of 21 S dynein (18 nm X 14 nm). The form of the stem was irregular, and its apparent length varied from 0 to 32 nm. Binding of cytoplasmic dynein to brain microtubule in the solution was observed by negative staining, and that in the precipitate was examined by the quick-freeze, deep-etch method as well. Both methods revealed the presence of two kinds of microtubules, one a fully decorated microtubule and the other a non-decorated microtubule. Cytoplasmic dynein bound to microtubule also appeared as a globular particle. Neither the periodic binding nor the crossbridges that were observed with 21 S dynein were formed by cytoplasmic dynein, although cytoplasmic dynein appeared to bind to microtubules co-operatively.

The clam embryo protein cyclin A induces entry into M phase and the resumption of meiosis in Xenopus oocytes

Fertilized clam embryos synthesize several new cell-cycle-related proteins. The cloned cDNA and derived amino acid sequences of one of these, cyclin A, are presented here. Immunoblots with an anti-cyclin A antibody reveal that cyclin A is undetectable in oocytes, appears within 15 min of fertilization, and is destroyed near the end of each meiosis and mitosis. We directly tested the ability of cyclin A to induce M phase by injecting SP6 cyclin A mRNA into Xenopus oocytes, which are arrested at the G2/M border of first meiosis. The injected mRNA was translated, with the result that the Xenopus oocytes entered meiosis. These findings indicate that the rise in cyclin A plays a direct and natural role in driving cells into M phase.

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.

Tubulin composition and microtubule nucleation of a griseofulvin-resistant Chinese hamster ovary cell mutant with abnormal spindles

A griseofulvin-resistant Chinese hamster ovary (CHO) mutant (Grs-2) which has an altered beta-tubulin subunit as well as wild-type beta-tubulin is temperature-sensitive (ts) for growth at 40.5 degrees C. This growth defect appears to result from the formation of abnormal mitotic spindles at the non-permissive temperature (Abraham, I et al., J cell biol 97 (1983) 1055) [19]. Light microscopy of spindles isolated from mutant cells cultured at the permissive temperature showed a typical bipolar morphology, whereas spindles isolated at the non-permissive temperature were multipolar. In order to study the role of tubulin in spindle formation, we analyzed the tubulin composition of the multipolar spindles. Two-dimensional gels and immunoblotting analysis of one-dimensional electrophoretic gels stained with monoclonal anti-Chinese hamster brain beta-tubulin antibody revealed that both mutant and wild-type beta-tubulins were present in similar proportions in both bipolar spindles at 37 degrees C and multipolar spindles at 40.5 degrees C. The ratio between wild-type and mutant tubulin in spindles was also found to be the same as in the cytoplasmic microtubule network in interphase cells, providing evidence that the mutant beta-tubulin appeared to be incorporated in a similar manner into both interphase and mitotic microtubule structures. In vitro microtubule polymerization onto centrosomes prepared from mutant Grs-2 demonstrated that 80% of the sites for microtubule nucleation were without centrioles, suggesting fragmentation of pericentriolar material away from centrioles. This may be one of the causes of multipolar spindle formation in the mutant cells. These results, therefore, suggest that abnormal formation of spindles in mutant cells is due not to the presence of the mutant tubulin per se, but to the abnormal behavior of this mutant tubulin in the cellular environment during mitosis or abnormal interaction with other components in the spindle at 40.5 degrees C.

The immotile-cilia syndrome: a microtubule-associated defect

The immotile-cilia syndrome is a congenital disorder characterized by all the cilia in the body being either immotile or showing an abnormal and inefficient beating pattern. Most symptoms come from the ciliated airways (nose, paranasal sinuses, and bronchs) and from the middle ear. Two further symptoms are situs inversus and male sterility. Situs inversus occurs in 50% of the cases and this subgroup is termed the Kartagener's syndrome; it might be due to an inability of the embryonic cilia to shift the heart to the left side and situs laterality seems to be a random process in the immotile-cilia syndrome. Male sterility is caused by the spermatozoa being unable to swim progressively; the sperm tail has the same structure as a cilium. In a few cases only the sperm tail or only the cilia of the body are affected. Female patients have a decreased fertility; most are involuntarily childless. The immotile-cilia syndrome is a heterogeneous disorder in that one out of many different genes may be involved. The different subtypes can be distinguished by an electron microscopic examination which will show defects in either one of a number of the ciliary components.

Regulation of the microtubule steady state in vitro by ATP

ATP increases microtubule steady state assembly and disassembly rates in vitro in a concentration-dependent manner. Bovine brain microtubules, composed of 75% tubulin and 25% high molecular weight microtubule-associated proteins (MAPs), were purified by three cycles of assembly and disassembly in the absence of ATP. When assembled to steady state, these microtubules add dimers at one end and lose them at the other in a unidirectional assembly-disassembly process. In the presence of 1.0 mM ATP the unidirectional flow of tubulin from one end of the microtubules to the other increases as much as 20 fold, as revealed by loss of 3H-GTP from uniformly labeled microtubules under GTP chase conditions and by the rate of disassembly following addition of 50 microM podophyllotoxin. UTP, CTP and 5' adenylylimidodiphosphate (AMP-PNP) cannot substitute for ATP in producing this effect. Furthermore, the increase in steady state flow rate persists afer ATP is removed. Thus microtubules assembled in ATP and centrifuged through sucrose cushions to separate them from nucleotides continue to exhibit increased rates in the next assembly cycle in the absence of ATP. It is possible that an ATP-dependent microtubule protein kinase is responsible for the observed increase in tubulin flow rate. A kinase activity associated with brain MAPs has been reported to be cAMP-dependent (Sloboda et al., 1975). We have found an adenylate cyclase activity associated with these microtubules. Whether the adenylate cyclase is a contaminant or due to a specific microtubules-associated protein, and whether its activity is functionally linked to the increased rate of assembly and disassembly in the presence of ATP, remain to be determined.