Trends in Cell Biology
Volume 8, Issue 4, 1 April 1998, Pages 144-149
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Fission yeast cut mutations revisited: control of anaphase

https://doi.org/10.1016/S0962-8924(98)01236-7Get rights and content

Abstract

Studies of anaphase are approaching a golden age. Several different disciplines have contributed immensely to advances in our understanding of cell-cycle control and chromosome and spindle dynamics during mitosis. This article describes control of anaphase based on results obtained from Schizosaccharomyces pombe cut (cell untimely torn) mutants. These temperature-sensitive mutants were isolated by selection for uncoordinated mitosis with aberrant sister-chromatid separation and post-anaphase events. Characterization of some of the cut gene products has led to identification of novel molecular events related to chromosome condensation, sister-chromatid separation, anaphase-promoting proteolysis, fatty-acid metabolism, and cell-cycle arrest induced by stress or a replication block.

Section snippets

Chromosome condensation

Studies of three cut mutant phenotypes (top2, cut3 and cut14) strongly support the hypothesis that proper condensation is a prerequisite for normal sister-chromatid separation8, 9, 10. S. pombe cells show clearly distinguishable chromosome condensation during wild-type mitosis10, 11, and this property was used to identify by FISH (fluorescence in situ hybridization) the top2, cut3 and cut14 mutants, which are defective in condensation. In wild-type metaphase, the chromosomes become compact. By

SMCs are required for chromosome condensation

Fission yeast Cut3 and Cut14 (Ref. [10]) are very similar to Xenopus XCAP-C and XCAP-E proteins[13]and are also similar to budding yeast Smc2p and Smc4p[14], respectively. These proteins belong to the SMC family, members of which are found in species ranging from yeast to mammals14, 15. SMCs contain N- and C-terminal globular domains, and central putative coiled-coil regions. The N-terminal domain has the potential to bind to ATP, and the C-terminus contains the DA box, implicated in ATPase

Top2 is required for condensation and segregation

The first cut mutants isolated, top2 mutants, were defective in topo II activity[8]. Phenotypic analysis of S. pombe top2 mutants established that topo II is required for condensation and also for sister-chromatid separation. Mutant chromosomes defective in condensation are seemingly entangled, and only the centromeric portions of the chromatids can be pulled out by the spindle. Topo II is required even after chromosomes are condensed[9]: when top2 mutant chromosomes, condensed at the

The Cut2–Cut1 complex is regulated by proteolysis

Overexpression of a nondestructible form of cyclin B blocks exit from mitosis as it inhibits inactivation of mitotic CDK, but sister-chromatid separation can proceed while mitotic CDK is active23, 24, 25. However, ubiquitin-mediated proteolysis is known to be a crucial event for metaphase–anaphase progression23, 26, 27, 28, 29, 30, suggesting that a protein or proteins other than cyclin B needs to be degraded for the onset of anaphase. Recent studies have shown that fission yeast Cut2, like

Control of cyclosome assembly by Cut9, Cut4 and Nuc2

The fission yeast mutants cut4, cut9 and nuc2 show a block or a significant delay at metaphase and are defective in anaphase-promoting proteolysis. The nuc2 mutant, originally classified as a metaphase arrest mutant from its phenotype at 36°C, shows a cut phenotype at 33°C (Ref. [37]). Condensed chromosomes and a short metaphase spindle are observed commonly in cut4, cut9 and nuc2 mutants before septation37, 38, 39, 40(Fig. 1b). The Cut4, Cut9 and Nuc2 proteins are essential subunits of the

PKA and stress inhibit anaphase proteolysis in the cut4 mutant

An interesting feature of the cut4 mutant is its culture-medium dependency: the ts phenotype is lost in minimal medium. cut4 is also hypersensitive to cyclic AMP (cAMP) and addition of low concentrations of heavy metals such as Co2+, Ni2+ or Cd2+ and canavanine, an amino-acid analogue[41]. cut4 mutant cells show a long delay in exit from mitosis when these compounds are present. Under such conditions, the 20S cyclosome structure is disrupted, and Cdc13/cyclin B is not polyubiquitinated. These

The Cut5 protein is essential for the replication checkpoint

The Cut5 protein plays a unique role in genome maintenance as it is required for DNA replication, the replication checkpoint and normal UV sensitivity46, 47. Loss of Cut5 function induces anaphase-like events and cytokinesis without replication. When cells are blocked by a replication inhibitor such as hydroxyurea or have mutations in replication enzymes, Cut5 is essential for restraining entry into mitosis and cytokinesis. The Cut5 protein contains the BRCT sequence motif present in a large

The Cut6 protein—fatty acid synthesis and nuclear division

A surprising discovery made from the cut6 mutant is that fatty acid synthesis is involved in inheritance of nuclear material. Aberrant nuclear division occurs in cut6 mutant cells, producing daughter nuclei strikingly distinct in size[51]. The cut6+ gene encodes acetyl CoA carboxylase. Another mutation, lsd1 (large and small daughter), defective in the catalytic subunit of fatty acid synthase, has a basically identical phenotype. The lsd1 mutation can be suppressed by addition of palmitate to

A kinesin-like motor protein, Cut7, is required for spindle formation

Two half-spindles forming a V-shape structure defective in the separation of the two spindle pole bodies (SPBs) are characteristic of the cut7 phenotype[53]. The cut7+ gene encodes a kinesin-like protein, a member of the BimC family[54], showing a remarkable localization during mitosis: it is highly enriched in the mitotic SPBs and also along the spindle microtubules[55]. The defective phenotype of cut7 mutants clearly indicates that Cut7 plays an essential role in spindle formation. It is not

Lessons and prospects

One lesson learnt from characterizing cut mutations is that septation and cytokinesis can be induced in some circumstances without prior chromosome condensation, spindle formation, cyclin destruction, sister-chromatid separation or, even, replication. Some cut gene products are involved in checkpoints or surveillance systems in cell-cycle progression. Defects in other cut gene product activities might not be detected, and the cells move on to initiate post-anaphase events. Another insight

Acknowledgements

I apologize to colleagues for not citing a number of relevant references owing to space limitation. Members of the author's laboratory are gratefully acknowledged for comments on the manuscript. The work in the author's laboratory is supported by a CREST grant from the Japan Science Technology Corporation and grants from the Science and Technology Agency of Japan and the Human Frontier Science Program Organization.

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