(1/1931) Cell cycle-dependent expression and centrosome localization of a third human aurora/Ipl1-related protein kinase, AIK3.

We earlier isolated cDNAs encoding novel human protein kinases AIK and AIK2 sharing high amino acid sequence identities with Drosophila Aurora and Saccharomyces cerevisiae Ipl1 kinases whose mutations cause abnormal chromosome segregation. In the present study, a third human cDNA (AIK3) highly homologous to aurora/IPL1 was isolated, and the nucleotide sequence was determined. This cDNA encodes 309 amino acids with a predicted molecular mass of 35.9 kDa. C-terminal kinase domain of AIK3 protein shares high amino acid sequence identities with those of Aurora/Ipl1 family protein kinases including human AIK, human AIK2, Xenopus pEg2, Drosophila Aurora, and yeast Ipl1, whereas the N-terminal domain of AIK3 protein shares little homology with any other Aurora/Ipl1 family members. AIK3 gene was assigned to human chromosome 19q13.43, which is a frequently deleted or rearranged region in several tumor tissues, by fluorescence in situ hybridization, somatic cell hybrid panel, and radiation hybrid cell panel. Northern blot analyses revealed that AIK3 expression was limited to testis. The expression levels of AIK3 in several cancer cell lines were elevated severalfold compared with normal fibroblasts. In HeLa cells, the endogenous AIK3 protein level is low in G1/S, accumulates during G2/M, and reduces after mitosis. Immunofluorescence studies using a specific antibody have shown that AIK3 is localized to centrosome during mitosis from anaphase to cytokinesis. These results suggest that AIK3 may play a role(s) in centrosome function at later stages of mitosis.  (+info)

(2/1931) Abnormal spindle protein, Asp, and the integrity of mitotic centrosomal microtubule organizing centers.

The product of the abnormal spindle (asp) gene was found to be an asymmetrically localized component of the centrosome during mitosis, required to focus the poles of the mitotic spindle in vivo. Removing Asp protein function from Drosophila melanogaster embryo extracts, either by mutation or immunodepletion, resulted in loss of their ability to restore microtubule-organizing center activity to salt-stripped centrosome preparations. This was corrected by addition of purified Asp protein. Thus, Asp appears to hold together the microtubule-nucleating gamma-tubulin ring complexes that organize the mitotic centrosome.  (+info)

(3/1931) Influence of centriole behavior on the first spindle formation in zygotes of the brown alga Fucus distichus (Fucales, Phaeophyceae).

The influence of centrioles, derived from the sperm flagellar basal bodies, and the centrosomal material (MTOCs) on spindle formation in the brown alga Fucus distichus (oogamous) was studied by immunofluorescence microscopy using anti-centrin and anti-beta-tubulin antibodies. In contrast to a bipolar spindle, which is formed after normal fertilization, a multipolar spindle was formed in polyspermic zygote. The number of mitotic poles in polyspermic zygotes was double the number of sperm involved in fertilization. As an anti-centrin staining spot (centrioles) was located at these poles, the multipolar spindles in polyspermic zygotes were produced by the supplementary centrioles. When anucleate egg fragments were fertilized, chromosome condensation and mitosis did not occur in the sperm nucleus. Two anti-centrin staining spots could be detected, microtubules (MTs) radiated from nearby, but the mitotic spindle was never produced. When a single sperm fertilized multinucleate eggs (polygyny), abnormal spindles were also observed. In addition to two mitotic poles containing anti-centrin staining spots, extra mitotic poles without anti-centrin staining spots were also formed, and as a result multipolar spindles were formed. When karyogamy was blocked with colchicine, it became clear that the egg nucleus proceeded independently into mitosis accompanying chromosome condensation. A monoastral spindle could be frequently observed, and in rare cases a barrel-shaped spindle was formed. However, when a sperm nucleus was located near an egg nucleus, the two anti-centrin staining spots shifted to the egg nucleus from the sperm nucleus. In this case, a normal spindle was formed, the egg chromosomes arranged at the equator, and the associated MTs elongated from one pole of the egg spindle toward the sperm chromosomes which were scattered. From these results, it became clear that paternal centrioles derived from the sperm have a crucial role in spindle formation in the brown algae, such as they do during animal fertilization. However, paternal centrioles were not adequate for the functional centrosome during spindle formation. We speculated that centrosomal materials from the egg cytoplasm aggregate around the sperm centrioles and are needed for centrosomal activation.  (+info)

(4/1931) Tobacco BY-2 cell-free extracts induce the recovery of microtubule nucleating activity of inactivated mammalian centrosomes.

The structure and the molecular composition of the microtubule-organizing centers in acentriolar higher plant cells remain unknown. We developed an in vitro complementation assay where tobacco BY-2 extracts can restore the microtubule-nucleating activity of urea-inactivated mammalian centrosomes. Our results provide first evidence that soluble microtubule-nucleating factors are present in the plant cytosolic fraction. The implication for microtubule nucleation in higher plants is discussed.  (+info)

(5/1931) Cyclin-dependent kinase control of centrosome duplication.

Centrosomes nucleate microtubules and duplicate once per cell cycle. This duplication and subsequent segregation in mitosis results in maintenance of the one centrosome/cell ratio. Centrosome duplication occurs during the G1/S transition in somatic cells and must be coupled to the events of the nuclear cell cycle; failure to coordinate duplication and mitosis results in abnormal numbers of centrosomes and aberrant mitoses. Using both in vivo and in vitro assays, we show that centrosome duplication in Xenopus laevis embryos requires cyclin/cdk2 kinase activity. Injection of the cdk (cyclin-dependent kinase) inhibitor p21 into one blastomere of a dividing embryo blocks centrosome duplication in that blastomere; the related cdk inhibitor p27 has a similar effect. An in vitro system using Xenopus extracts carries out separation of the paired centrioles within the centrosome. This centriole separation activity is dependent on cyclin/cdk2 activity; depletion of either cdk2 or of the two activating cyclins, cyclin A and cyclin E, eliminates centriole separation activity. In addition, centriole separation is inhibited by the mitotic state, suggesting a mechanism of linking the cell cycle to periodic duplication of the centrosome.  (+info)

(6/1931) GMAP-210, A cis-Golgi network-associated protein, is a minus end microtubule-binding protein.

We report that a peripheral Golgi protein with a molecular mass of 210 kD localized at the cis-Golgi network (Rios, R.M., A.M. Tassin, C. Celati, C. Antony, M.C. Boissier, J.C. Homberg, and M. Bornens. 1994. J. Cell Biol. 125:997-1013) is a microtubule-binding protein that associates in situ with a subpopulation of stable microtubules. Interaction of this protein, now called GMAP-210, for Golgi microtubule-associated protein 210, with microtubules in vitro is direct, tight and nucleotide-independent. Biochemical analysis further suggests that GMAP-210 specifically binds to microtubule ends. The full-length cDNA encoding GMAP-210 predicts a protein of 1, 979 amino acids with a very long central coiled-coil domain. Deletion analyses in vitro show that the COOH terminus of GMAP-210 binds to microtubules whereas the NH2 terminus binds to Golgi membranes. Overexpression of GMAP-210-encoding cDNA induced a dramatic enlargement of the Golgi apparatus and perturbations in the microtubule network. These effects did not occur when a mutant lacking the COOH-terminal domain was expressed. When transfected in fusion with the green fluorescent protein, the NH2-terminal domain associated with the cis-Golgi network whereas the COOH-terminal microtubule-binding domain localized at the centrosome. Altogether these data support the view that GMAP-210 serves to link the cis-Golgi network to the minus ends of centrosome-nucleated microtubules. In addition, this interaction appears essential for ensuring the proper morphology and size of the Golgi apparatus.  (+info)

(7/1931) HP33: hepatocellular carcinoma-enriched 33-kDa protein with similarity to mitochondrial N-acyltransferase but localized in a microtubule-dependent manner at the centrosome.

Using a new subtraction method and chemically induced rat hepatocellular carcinomas, we identified a hepatocellular carcinogenesis and hepatocyte proliferation-related gene designated hp33 that encoded a 33-kDa protein. The predicted protein was similar to the bovine aralkyl N-acyltransferase and arylacetyl N-acyltransferase. HP33 was restrictively expressed in the liver and kidney, and its gene expression was stimulated in the regenerating liver as well as in hepatocellular carcinoma. Interestingly, it was demonstrated in various hepatic cells that HP33 was localized in regions surrounding the centrosome, where mitochondria were not concentrated. Moreover, its centrosomal localization was evident in the interphase but not in the mitotic phase of the cell cycle. The centrosomal localization of HP33 was dependent on microtubules, and ectopically expressed HP33 was seen at centrosomes even in fibroblasts, which do not exhibit a typical staining pattern of HP33. The centrosomal localization of HP33 became invisible by nocodazole treatment, whereas the mitochondrial staining pattern was not affected by it. In vitro cosedimentation experiments using purified microtubules indicated that HP33 bound to MTs directly and that its MT-binding ability was dependent on the C-terminal basic domain of the protein. These results suggest that, different from early predictions based on its primary structure, HP33 has a growth- and carcinogenesis-related function that may be independent of mitochondrial function.  (+info)

(8/1931) Centrosome amplification and a defective G2-M cell cycle checkpoint induce genetic instability in BRCA1 exon 11 isoform-deficient cells.

Germline mutations of the Brca1 tumor suppressor gene predispose women to breast and ovarian cancers. To study mechanisms underlying BRCA1-related tumorigenesis, we derived mouse embryonic fibroblast cells carrying a targeted deletion of exon 11 of the Brca1 gene. We show that the mutant cells maintain an intact G1-S cell cycle checkpoint and proliferate poorly. However, a defective G2-M checkpoint in these cells is accompanied by extensive chromosomal abnormalities. Mutant fibroblasts contain multiple, functional centrosomes, which lead to unequal chromosome segregation, abnormal nuclear division, and aneuploidy. These data uncover an essential role of BRCA1 in maintaining genetic stability through the regulation of centrosome duplication and the G2-M checkpoint and provide a molecular basis for the role of BRCA1 in tumorigenesis.  (+info)