Reactivation of isolated mitotic apparatus: metaphase versus anaphase spindles

Anaphase B

Anaphase B spindle elongation is characterized by the sliding apart of overlapping antiparallel interpolar (ip) microtubules (MTs) as the two opposite spindle poles separate, pulling along disjoined sister chromatids, thereby contributing to chromosome segregation and the propagation of all cellular life. The major biochemical “modules” that cooperate to mediate pole–pole separation include: (i) midzone pushing or (ii) braking by MT crosslinkers, such as kinesin-5 motors, which facilitate or restrict the outward sliding of antiparallel interpolar MTs (ipMTs); (iii) cortical pulling by disassembling astral MTs (aMTs) and/or dynein motors that pull aMTs outwards; (iv) ipMT plus end dynamics, notably net polymerization; and (v) ipMT minus end depolymerization manifest as poleward flux. The differential combination of these modules in different cell types produces diversity in the anaphase B mechanism. Combinations of antagonist modules can create a force balance that maintains the dynamic pre-anaphase B spindle at constant length. Tipping such a force balance at anaphase B onset can initiate and control the rate of spindle elongation. The activities of the basic motor filament components of the anaphase B machinery are controlled by a network of non-motor MT-associated proteins (MAPs), for example the key MT cross-linker, Ase1p/PRC1, and various cell-cycle kinases, phosphatases, and proteases. This review focuses on the molecular mechanisms of anaphase B spindle elongation in eukaryotic cells and briefly mentions bacterial DNA segregation systems that operate by spindle elongation.

Novel localization and possible functions of cyclin E in early sea urchin development

In somatic cells, cyclin E-cdk2 activity oscillates during the cell cycle and is required for the regulation of the G1/S transition. Cyclin E and its associated kinase activity remain constant throughout early sea urchin embryogenesis, consistent with reports from studies using several other embryonic systems. Here we have expanded these studies and show that cyclin E rapidly and selectively enters the sperm head after fertilization and remains concentrated in the male pronucleus until pronuclear fusion, at which time it disperses throughout the zygotic nucleus. We also show that cyclin E is not concentrated at the centrosomes but is associated with condensed chromosomes throughout mitosis for at least the first four cell cycles. Isolated mitotic spindles are enriched for cyclin E and cdk2, which are localized to the chromosomes. The chromosomal cyclin E is associated with active kinase during mitosis. We propose that cyclin E may play a role in the remodeling of the sperm head and re-licensing of the paternal genome after fertilization. Furthermore, cyclin E does not need to be degraded or dissociated from the chromosomes during mitosis; instead, it may be required on chromosomes during mitosis to immediately initiate the next round of DNA replication.

Isolation of centrosomes from Spisula solidissima oocytes

We have described methods for the preparation of lysates and isolation of centrosomes from parthenogenetically activated oocytes of the surf clam, S. solidissima. Although oocyte availability is seasonal, between June and August as much as 2 liters of lysate can be generated by a single person. Since lysate can be stored frozen at -80 degrees C with no apparent loss in centrosome-dependent microtubule nucleation, this is a convenient system for year-round experimentation. On average, per milliliter of frozen-stored lysate, 2 or 3 x 10(6) centrosomes can be obtained at 3000- or 4000-fold purification by sucrose-density gradient centrifugation. Centrosome fractions typically contain 6.0 x 10(-12) g of protein per centrosome (Vogel et al., 1997) and 140-200 micrograms of protein is usually obtained from a single run involving six sucrose-density gradients (12 ml of lysate). One person can easily run three preparations in a day, and thus 420-600 micrograms of centrosome protein could be prepared daily. Therefore, based on the effort of one individual, as much as 20-40 mg of centrosome protein could be prepared per year. Another convenient feature of the system is that once centrosomes are isolated, they can be stored in high sucrose media at -80 degrees C for years with little or no loss in microtubule nucleation potential. Once isolated, centrosomes can be used for protein analysis, ultrastructural studies, or in functional reconstitution assays (Vogel, 1997). In addition, these preparations offer the isolation of sufficient quantities of centrosome proteins to be used as antigens for generating centrosome-specific antibodies or for obtaining protein sequence for the purpose of antibody production or the design of oligonucleotide primers for isolating cDNA fragments coding for centrosome proteins. Thus, the preparations described offer a biochemical approach for defining centrosome composition. The methods described for immunofluorescence analysis of asters assembled in lysates offer rapid and convenient preparations for screening antibodies for centrosome localization and specificity. Finally, the ability to prepare large quantities of homogeneous centrosomes should enhance ultrastructural studies since many centrosomes can be sectioned and analyzed simultaneously by EM, avoiding the problem of having to hunt through sections of single cells to find a single centrosome for analysis. In addition, colloidal gold localization studies, using antibodies and EM to pinpoint the relative location of individual proteins, could be carried out on populations of centrosomes in the same preparation simultaneously, thus drastically expanding the quantity of data gathered. In conclusion, the clam oocyte system described here offers the potential for a combined structural and biochemical approach for identification of novel centrosome proteins and elucidation of the molecular basis of microtubule nucleation, centrosome assembly, and centrosome function.

Anaphase in vitro

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DSK1, a novel kinesin-related protein from the diatom Cylindrotheca fusiformis that is involved in anaphase spindle elongation

We have identified an 80-kD protein that is involved in mitotic spindle elongation in the diatom Cylindrotheca fusiformis. DSK1 (Diatom Spindle Kinesin 1) was isolated using a peptide antibody raised against a conserved region in the motor domain of the kinesin superfamily. By sequence homology, DSK1 belongs to the central motor family of kinesin-related proteins. Immunoblots using an antibody raised against a non-conserved region of DSK1 show that DSK1 is greatly enriched in mitotic spindle preparations. Anti-DSK1 stains in diatom central spindle with a bias toward the midzone, and staining is retained in the spindle midzone during spindle elongation in vitro. Furthermore, preincubation with anti-DSK1 blocks function in an in vitro spindle elongation assay. This inhibition of spindle elongation can be rescued by preincubating concurrently with the fusion protein against which anti-DSK1 was raised. We conclude that DSK1 is involved in spindle elongation and is likely to be responsible for pushing hal-spindles apart in the spindle midzone.

The force-producing mechanism for centrosome separation during spindle formation in vertebrates is intrinsic to each aster

A popular hypothesis for centrosome separation during spindle formation and anaphase is that pushing forces are generated between interacting microtubules (MTs) of opposite polarity, derived from opposing centrosomes. However, this mechanism is not consistent with the observation that centrosomes in vertebrate cells continue to separate during prometaphase when their MT arrays no longer overlap (i.e., during anaphase-like prometaphase). To evaluate whether centrosome separation during prophase/prometaphase, anaphase-like prometaphase and anaphase is mediated by a common mechanism we compared their behavior in vivo at a high spatial and temporal resolution. We found that the two centrosomes possess a considerable degree of independence throughout all stages of separation, i.e., the direction and migration rate of one centrosome does not impart a predictable behavior to the other, and both exhibit frequent and rapid (4-6 microns/min) displacements toward random points within the cell including the other centrosome. The kinetic behavior of individual centrosomes as they separate to form the spindle is the same whether or not their MT arrays overlap. The characteristics examined include, e.g., total displacement per minute, the vectorial rate of motion toward and away from the other centrosome, the frequency of toward and away motion as well as motion not contributing to separation, and the rate contributed by each centrosome to the separation process. By contrast, when compared with prometaphase, anaphase centrosomes separated at significantly faster rates even though the average vectorial rate of motion away from the other centrosome was the same as in prophase/prometaphase. The difference in separation rates arises because anaphase centrosomes spend less time moving toward one another than in prophase/prometaphase, and at a significantly slower rate. From our data we conclude that the force for centrosome separation during vertebrate spindle formation is not produced by MT-MT interactions between opposing asters, i.e., that the mechanism is intrinsic to each aster. Our results also strongly support the contention that forces generated independently by each aster also contribute substantially to centrosome separation during anaphase, but that the process is modified by interactions between opposing astral MTs in the interzone.