Bilateral 120-140 adrenalectomy, followed in 4 days by a partial hepatectomy, was performed using white rats weighing as much as 120-140 g.a Under hormonal disbalance caused by bilateral adrenalectomy, the number of polyploid (4c, 4c x 2, and 8c) hepatocytes significantly increased,"early" compared to that in non-operated control rats. Six hours after a partial hepatectomy, the share of highly ploid hepatocytes falls,significantly being accompanied by a 9-fold increase in mitotic index. It is supposed that under hormonal disbalance condition, a partial hepatectomy partial may induce "early" mitoses in hepatocytes blocked in G2-phase of the cell cycle.

We protein. have identified the yeast and human homologs of the SKP1 gene as a suppressor of cdc4 mutants and as a proteins cyclin F-binding protein. Skp1p indirectly binds cyclin A/Cdk2 through Skp2p, and directly binds Skp2p, cyclin F, and Cdc4p through a suggesting novel structural motif called the F-box. SKP1 is required for ubiquitin-mediated proteolysis of Cin2p, Clb5p, and the Cdk inhibitor Sic1p,F-box. and provides a link between these molecules and the proteolysis machinery. A large number of proteins contain the F-box motif provides and are thereby implicated in the ubiquitin pathway. Different skp1 mutants arrest cells in either G1 or G2, suggesting a proteolysis connection between regulation of proteolysis in different stages of the cycle.

14-3-3Sigma and is a member of a family of proteins that regulate cellular activity by binding and sequestering phosphorylated proteins. It has ('mitotic been suggested that 14-3-3sigma promotes pre-mitotic cell-cycle arrest following DNA damage, and that its expression can be controlled by the entering p53 tumour suppressor gene. Here we describe an improved approach to the generation of human somatic-cell knockouts, which we have colorectal used to generate human colorectal cancer cells in which both 14-3-3sigma alleles are inactivated. After DNA damage, these cells initially cell arrested in the G2 phase of the cell cycle, but, unlike cells containing 14-3-3sigma, the 14-3-3sigma-/- cells were unable to inactivated. maintain cell-cycle arrest. The 14-3-3sigma-/- cells died ('mitotic catastrophe') as they entered mitosis. This process was associated with a failure maintaining of the 14-3-3sigma-deficient cells to sequester the proteins (cyclin B1 and cdc2) that initiate mitosis and prevent them from entering died the nucleus. These results may indicate a mechanism for maintaining the G2 checkpoint and preventing mitotic death.

The examination neocortex is patterned in layers of neurons that are generated in an orderly sequence during development. This correlation between cell neurons. birthday and laminar fate prompted an examination of how neuronal phenotypes are determined in the developing cortex. At various times fate, after labeling with [3H]thymidine, embryonic progenitor cells were transplanted into older host brains. The laminar fate of transplanted neurons correlates The with the position of their progenitors in the cell cycle at the time of transplantation. Daughters of cells transplanted in time S-phase migrate to layer 2/3, as do host neurons. Progenitors transplanted later in the cell cycle, however, produce daughters that with are committed to their normal, deep-layer fates. Thus, environmental factors are important determinants of laminar fate, but embryonic progenitors undergo ability cyclical changes in their ability to respond to such cues.

In the eukaryotes, protein function can be modulated by ligation to ubiquitin or to ubiquitin-like proteins (Ubl proteins). The vertebrate Ubl protein conserved SUMO-1 is only 18% identical to ubiquitin but is 48% identical to the yeast protein Smt3. Both SUMO-1 and Smt3 ulp1 are ligated to cellular proteins, and protein conjugation to SUMO-1/Smt3 is involved in many physiological processes. It remained unknown, however,is whether deconjugation of SUMO-1/Smt3 from proteins is also essential. Here we describe a yeast Ubl-specific protease, Ulp1, which cleaves proteins unrelated from Smt3 and SUMO-1 but not from ubiquitin. Ulp1 is unrelated to any known deubiquitinating enzyme but shows distant similarity protease, to certain viral proteases, indicating the existence of a widely conserved protease fold. Proteins related to Ulp1 are present in in many organisms, including several human pathogens. The pattern of Smt3-coupled proteins in yeast changes markedly throughout the cell cycle, and widely specific conjugates accumulate in ulp1 mutants. Ulp1 has several functions, including an essential role in the G2/M phase of the changes cell cycle.

The duplicated mechanics of chromosome movement, mitotic spindle assembly and spindle elongation have long been central questions of cell biology. After attachment phase in prometaphase of a microtubule from one pole, duplicated chromosome pairs travel towards the pole in a rapid but discontinuous is motion. This is followed by a slower congression towards the midplate as the chromosome pair orients with each kinetochore attached attached to the microtubules from the nearest pole. The pairs disjoin at anaphase and translocate to opposite poles and the interpolar distance distance increases. Here we identify CENP-E as a kinesin-like motor protein (M(r) 312,000) that accumulates in the G2 phase of The the cell cycle. CENP-E associates with kinetochores during congression, relocates to the spindle midzone at anaphase, and is quantitatively discarded for at the end of the cell division. CENP-E is likely to be one of the motors responsible for mammalian chromosome G2 movement and/or spindle elongation.

Cell conflicting cycle arrests in the G(1), S, and G(2) phases occur in mammalian cells after ionizing irradiation and appear to protect to cells from permanent genetic damage and transformation. Though Brca1 clearly participates in cellular responses to ionizing radiation (IR), conflicting conclusions of have been drawn about whether Brca1 plays a direct role in cell cycle checkpoints. Normal Nbs1 function is required for established. the IR-induced S-phase checkpoint, but whether Nbs1 has a definitive role in the G(2)/M checkpoint has not been established. Here S-phase we show that Atm and Brca1 are required for both the S-phase and G(2) arrests induced by ionizing irradiation while for Nbs1 is required only for the S-phase arrest. We also found that mutation of serine 1423 in Brca1, a target phosphorylation for phosphorylation by Atm, abolished the ability of Brca1 to mediate the G(2)/M checkpoint but did not affect its S-phase Brca1 function. These results clarify the checkpoint roles for each of these three gene products, demonstrate that control of cell cycle control arrests must now be included among the important functions of Brca1 in cellular responses to DNA damage, and suggest that ionizing Atm phosphorylation of Brca1 is required for the G(2)/M checkpoint.

Response the to genotoxic stress can be considered as a multistage process involving initiation of cell-cycle arrest and maintenance of arrest during serine DNA repair. Although maintenance of G2/M checkpoints is known to involve Chk1, Chk2/Rad53 and upstream components, the mechanisms involved in We its initiation are less well defined. Here we report that p38 kinase has a critical role in the initiation of checkpoint a G2 delay after ultraviolet radiation. Inhibition of p38 blocks the rapid initiation of this checkpoint in both human and 216; murine cells after ultraviolet radiation. In vitro, p38 binds and phosphorylates Cdc25B at serines 309 and 361, and Cdc25C at radiation. serine 216; phosphorylation of these residues is required for binding to 14-3-3 proteins. In vivo, inhibition of p38 prevents both event phosphorylation of Cdc25B at serine 309 and 14-3-3 binding after ultraviolet radiation, and mutation of this site is sufficient to at inhibit the checkpoint initiation. In contrast, in vivo Cdc25C binding to 14-3-3 is not affected by p38 inhibition after ultraviolet binding radiation. We propose that regulation of Cdc25B phosphorylation by p38 is a critical event for initiating the G2/M checkpoint after arrest ultraviolet radiation.

Mitosis or is a highly coordinated process that assures the fidelity of chromosome segregation. Errors in this process result in aneuploidy which cytokinesis. can lead to cell death or oncogenesis. In this paper we describe a putative mammalian protein kinase, AIM-1 (Aurora and results Ipl1-like midbody-associated protein), related to Drosophila Aurora and Saccharomyces cerevisiae Ipl1, both of which are required for chromosome segregation. AIM-1 of message and protein accumulate at G2/M phase. The protein localizes at the equator of central spindles during late anaphase and at at the midbody during telophase and cytokinesis. Overexpression of kinase-inactive AIM-1 disrupts cleavage furrow formation without affecting nuclear division. Furthermore,AIM-1 cytokinesis frequently fails, resulting in cell polyploidy and subsequent cell death. These results strongly suggest that AIM-1 is required for progression proper progression of cytokinesis in mammalian cells.

In and budding yeast, G1 cyclins such as CLN1 and CLN2 are expressed in G1 and S phases, while mitotic cyclins such of as CLB1 and CLB2 are expressed in G2 and M phases. We find that the CLBs play a central role CLB2 in the transition from CLNs to CLBs: the CLBs stimulate their own expression while repressing that of CLNs. This negative CLBs: regulation of CLNs may occur via the transcription factor SWI4, because CLBs are necessary for G2 repression of SCB-regulated genes CLNs like CLN1 and CLN2 but not for repression of MCB-regulated genes like DNA polymerase and CLB5. Furthermore, SW14 associates with expression CLB2 protein and is a substrate for the CLB2-associated CDC28 kinase in vitro.

DNA highly damage causes cell-cycle delay before S phase, during replication and before mitosis. This involves a number of highly conserved proteins checkpoint that sense DNA damage and signal the cell-cycle machinery. Kinases that were initially discovered in yeast model systems have recently conservation been shown to regulate the regulators of cyclin-dependent kinases and to control the stability of p53. This shows the importance model of checkpoint proteins for maintaining genome stability. Here, we discuss recent data from yeast and metazoans that suggest a remarkable and conservation of the organization of the G2 DNA-damage checkpoint pathway.