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Licensing centrosome duplication
HEADLINES
Engulfed by CED-5 WU, Y-C. and HORVITZ, H. R. (1998) C. elegans phagocytosis and cell-migration protein CED-5 is similar to human DOCK180 Nature 392, 501–504 Studies of proteins in nematodes have contributed significantly to the understanding of complex events, such as apoptosis, in vertebrates. In this elegant paper, Wu and Horwitz study the process of cell-corpse engulfment following apoptosis. Previous studies have indicated the involvement of at least six genes, named ced (for cell death abnormal), in this process. Nematodes deficient in ced-5 cannot engulf cell corpses following apoptosis and, in addition, are defective in the migration of a subset of cells. Cloning of the ced-5 gene and cDNA revealed that the deduced protein shows identity with human protein DOCK180, implicated in integrinmediated cell signalling and cell movement. Furthermore, both proteins show similarity to the Drosophila protein myoblast city (MBC), involved in myoblast fusion and migration of epithelial cells. The authors name this new family of proteins ‘CDM’ (for CED-5, DOCK180 and MBC) and propose that these proteins all function to mediate extension of cell surfaces. To test whether CED-5 and DOCK180 are related functionally, DOCK180 was expressed under the control of a heatshock promoter in ced-5 nematode mutants, and this indeed rescued part of the phenotype. These results are highly interesting and shed new light on a step in the apoptotic response as well as in the complex mechanism of cell migration. Since only the migration of a subset of cells is affected in ced-5 mutants, and syncytia formation is left unaffected, it will be interesting to determine whether similar proteins are involved in the migration of other cell types as well as in cell-fusion events.
Licensing centrosome duplication HINCHCLIFFE, E. H., CASSELS, G. O., RIEDER, C. L. and SLUDER, G. (1998) The coordination of centrosome reproduction with nuclear events of the cell cycle in the sea urchin zygote J. Cell Biol. 140, 1417–1426
This month’s headlines were contributed by Søren Andersen, Donald Gullberg and Fiona Townsley.
222
The centrioles are the fundamental building blocks of the centrosome and are delivered to the egg upon fertilization. Two centrosomes are present in normal mitotic spindles. Failure to regulate centrosome number leads to multipolar spindles and, consequently, lethal erratic distribution of the chromosomes at cell division. Previous studies had indicated that centrosome number is regulated independently from the nuclear cycle. But what determines that centrosomes only duplicate once per cell cycle? Hinchcliffe et al. fertilized sea urchin eggs (zygotes) to address this question. Continuous centrosome duplication was observed when S phase was prolonged by inhibition of DNA polymerase. Duplication was not affected by Cdk1–cyclin-B kinase activity (that, in this system, stably rises to supra-mitotic
levels when DNA duplication is inhibited). By contrast, mitotic arrest caused failure to duplicate the centrosome, although anaphase still occurred. In accord with previous findings, multiple centrosome duplication occurred if protein translation was blocked at fertilization, a condition that also blocks Cdk1–cyclin-B activation. Thus, regulation of centrosome duplication occurs independently of Cdk1–cyclin-B activity and of the proteolytic events occurring during anaphase. Remarkably, if inhibitors of protein translation were applied during prophase of the first cell cycle (rather than at fertilization), the zygotes arrested in the second cell cycle and subsequently duplicated their centrosome only once over an eight-hour period (five cell cycles). Analysis of the arrests showed that zygotes treated with inhibitors of translation at fertilization were arrested in active S phase (verified by re-fertilizing zygotes to monitor active DNA replication by BrdU incorporation), whereas no DNA replication occurred in those arrested during the second cell cycle (G1 equivalent). Thus, genuine S-phase arrest is apparently necessary and sufficient to support continuous centrosome duplication. Interestingly, during S-phase arrest, the time for centrosome re-duplication was up to three times longer than one complete cell cycle (this also holds true for somatic cells). A slow S-phase-regulated clock might assure that centrosomes duplicate only once during the faster cell cycle. Moreover, during a normal cell cycle, the previous S phase might prime the centrioles/centrosome, ‘licensing’ them to duplicate only once during the next cell cycle.
A destructive role for the Polo-like kinases DESCOMBES, P. and NIGG, E. A. (1998) The polo-like kinase Plx1 is required for M phase exit and destruction of mitotic regulators in Xenopus egg extracts EMBO J. 17, 1328–1335 KOTANI, S. et al. (1998) PKA and MPF-activated Polo-like kinase regulate Anaphase-Promoting Complex activity and mitosis progression Mol. Cell 1, 371–380 SHIRAYAMA, M., ZACHARIAE, W., CIOSK, R. and NASMYTH, K. (1998) The Polo-like kinase Cdc5p and the WD-repeat protein Cdc20p/fizzy are regulators and substrates of the anaphase promoting complex in Saccharomyces cerevisiae EMBO J. 17, 1336–1349 The anaphase-promoting complex/cyclosome (APC/C) is a ubiquitin–protein ligase complex required for the destruction of anaphase inhibitors and the mitotic cyclins. The APC/C is activated in mitosis and inactivated during interphase, suggesting that it plays a major role in determining the timing of substrate degradation, but little is known about how the activity of the APC/C is regulated. These papers provide evidence that the Polo-like kinases (PLKs), a family of cell-cycle regulators implicated in multiple aspects of mitotic progression, activate the APC/C to promote the destruction of cyclin B and exit from mitosis. trends in CELL BIOLOGY (Vol. 8) June 1998
Engulfed by CED-5 WU, Y-C. and HORVITZ, H. R. (1998) C. elegans phagocytosis and cell-migration protein CED-5 is similar to human DOCK180 Nature 392, 501–504 Studies of proteins in nematodes have contributed significantly to the understanding of complex events, such as apoptosis, in vertebrates. In this elegant paper, Wu and Horwitz study the process of cell-corpse engulfment following apoptosis. Previous studies have indicated the involvement of at least six genes, named ced (for cell death abnormal), in this process. Nematodes deficient in ced-5 cannot engulf cell corpses following apoptosis and, in addition, are defective in the migration of a subset of cells. Cloning of the ced-5 gene and cDNA revealed that the deduced protein shows identity with human protein DOCK180, implicated in integrinmediated cell signalling and cell movement. Furthermore, both proteins show similarity to the Drosophila protein myoblast city (MBC), involved in myoblast fusion and migration of epithelial cells. The authors name this new family of proteins ‘CDM’ (for CED-5, DOCK180 and MBC) and propose that these proteins all function to mediate extension of cell surfaces. To test whether CED-5 and DOCK180 are related functionally, DOCK180 was expressed under the control of a heatshock promoter in ced-5 nematode mutants, and this indeed rescued part of the phenotype. These results are highly interesting and shed new light on a step in the apoptotic response as well as in the complex mechanism of cell migration. Since only the migration of a subset of cells is affected in ced-5 mutants, and syncytia formation is left unaffected, it will be interesting to determine whether similar proteins are involved in the migration of other cell types as well as in cell-fusion events.
Licensing centrosome duplication HINCHCLIFFE, E. H., CASSELS, G. O., RIEDER, C. L. and SLUDER, G. (1998) The coordination of centrosome reproduction with nuclear events of the cell cycle in the sea urchin zygote J. Cell Biol. 140, 1417–1426
This month’s headlines were contributed by Søren Andersen, Donald Gullberg and Fiona Townsley.
222
The centrioles are the fundamental building blocks of the centrosome and are delivered to the egg upon fertilization. Two centrosomes are present in normal mitotic spindles. Failure to regulate centrosome number leads to multipolar spindles and, consequently, lethal erratic distribution of the chromosomes at cell division. Previous studies had indicated that centrosome number is regulated independently from the nuclear cycle. But what determines that centrosomes only duplicate once per cell cycle? Hinchcliffe et al. fertilized sea urchin eggs (zygotes) to address this question. Continuous centrosome duplication was observed when S phase was prolonged by inhibition of DNA polymerase. Duplication was not affected by Cdk1–cyclin-B kinase activity (that, in this system, stably rises to supra-mitotic
levels when DNA duplication is inhibited). By contrast, mitotic arrest caused failure to duplicate the centrosome, although anaphase still occurred. In accord with previous findings, multiple centrosome duplication occurred if protein translation was blocked at fertilization, a condition that also blocks Cdk1–cyclin-B activation. Thus, regulation of centrosome duplication occurs independently of Cdk1–cyclin-B activity and of the proteolytic events occurring during anaphase. Remarkably, if inhibitors of protein translation were applied during prophase of the first cell cycle (rather than at fertilization), the zygotes arrested in the second cell cycle and subsequently duplicated their centrosome only once over an eight-hour period (five cell cycles). Analysis of the arrests showed that zygotes treated with inhibitors of translation at fertilization were arrested in active S phase (verified by re-fertilizing zygotes to monitor active DNA replication by BrdU incorporation), whereas no DNA replication occurred in those arrested during the second cell cycle (G1 equivalent). Thus, genuine S-phase arrest is apparently necessary and sufficient to support continuous centrosome duplication. Interestingly, during S-phase arrest, the time for centrosome re-duplication was up to three times longer than one complete cell cycle (this also holds true for somatic cells). A slow S-phase-regulated clock might assure that centrosomes duplicate only once during the faster cell cycle. Moreover, during a normal cell cycle, the previous S phase might prime the centrioles/centrosome, ‘licensing’ them to duplicate only once during the next cell cycle.
A destructive role for the Polo-like kinases DESCOMBES, P. and NIGG, E. A. (1998) The polo-like kinase Plx1 is required for M phase exit and destruction of mitotic regulators in Xenopus egg extracts EMBO J. 17, 1328–1335 KOTANI, S. et al. (1998) PKA and MPF-activated Polo-like kinase regulate Anaphase-Promoting Complex activity and mitosis progression Mol. Cell 1, 371–380 SHIRAYAMA, M., ZACHARIAE, W., CIOSK, R. and NASMYTH, K. (1998) The Polo-like kinase Cdc5p and the WD-repeat protein Cdc20p/fizzy are regulators and substrates of the anaphase promoting complex in Saccharomyces cerevisiae EMBO J. 17, 1336–1349 The anaphase-promoting complex/cyclosome (APC/C) is a ubiquitin–protein ligase complex required for the destruction of anaphase inhibitors and the mitotic cyclins. The APC/C is activated in mitosis and inactivated during interphase, suggesting that it plays a major role in determining the timing of substrate degradation, but little is known about how the activity of the APC/C is regulated. These papers provide evidence that the Polo-like kinases (PLKs), a family of cell-cycle regulators implicated in multiple aspects of mitotic progression, activate the APC/C to promote the destruction of cyclin B and exit from mitosis. trends in CELL BIOLOGY (Vol. 8) June 1998