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Condensin restructures chromosomes in preparation for meiotic divisions.

Chan RC, Severson AF, Meyer BJ - J. Cell Biol. (2004)

Bottom Line: We showed that condensin, the protein complex needed for mitotic chromosome compaction, restructures chromosomes during meiosis in Caenorhabditis elegans.Condensin helps resolve cohesin-independent linkages between sister chromatids and alleviates recombination-independent linkages between homologues.The safeguarding of chromosome resolution by condensin permits chromosome segregation and is crucial for the formation of discrete, individualized bivalent chromosomes.

View Article: PubMed Central - PubMed

Affiliation: Howard Hughes Medical Institute, Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720, USA.

ABSTRACT
The production of haploid gametes from diploid germ cells requires two rounds of meiotic chromosome segregation after one round of replication. Accurate meiotic chromosome segregation involves the remodeling of each pair of homologous chromosomes around the site of crossover into a highly condensed and ordered structure. We showed that condensin, the protein complex needed for mitotic chromosome compaction, restructures chromosomes during meiosis in Caenorhabditis elegans. In particular, condensin promotes both meiotic chromosome condensation after crossover recombination and the remodeling of sister chromatids. Condensin helps resolve cohesin-independent linkages between sister chromatids and alleviates recombination-independent linkages between homologues. The safeguarding of chromosome resolution by condensin permits chromosome segregation and is crucial for the formation of discrete, individualized bivalent chromosomes.

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HCP-6 and MIX-1 associate independently with mitotic chromosomes and are required for chromosome condensation and segregation. (A) HCP-6 was undetectable in hcp-6(mr17, RNAi) mutant embryos, yet MIX-1 and CENP-A still associated with chromosomes at the disorganized metaphase plate. Similarly, MIX-1 was undetectable in mix-1(b285, RNAi) embryos, but HCP-6 and CENP-A still accumulated on chromosomes. HCP-6 did not associate with chromosomes of embryos depleted for two CENP-A paralogues, but MIX-1 did. (B) Mitotic chromosomes tagged with GFP::H2B histone condensed in pronuclei of wild-type embryos (red arrowhead) at least 2 min before NEBD, but decondensed chromosomes persisted until NEBD in pronuclei of hcp-6(mr17) and CENP-A–depleted embryos (red arrows). Defective mitotic chromosome segregation resulted in the formation of anaphase bridges (green arrows). Two polar bodies were extruded during meiosis in wild-type embryos (yellow arrowheads), but extra pronuclei formed in CENP-A–depleted embryos (yellow arrow) due to meiotic defects. Bars, 5 μm.
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fig2: HCP-6 and MIX-1 associate independently with mitotic chromosomes and are required for chromosome condensation and segregation. (A) HCP-6 was undetectable in hcp-6(mr17, RNAi) mutant embryos, yet MIX-1 and CENP-A still associated with chromosomes at the disorganized metaphase plate. Similarly, MIX-1 was undetectable in mix-1(b285, RNAi) embryos, but HCP-6 and CENP-A still accumulated on chromosomes. HCP-6 did not associate with chromosomes of embryos depleted for two CENP-A paralogues, but MIX-1 did. (B) Mitotic chromosomes tagged with GFP::H2B histone condensed in pronuclei of wild-type embryos (red arrowhead) at least 2 min before NEBD, but decondensed chromosomes persisted until NEBD in pronuclei of hcp-6(mr17) and CENP-A–depleted embryos (red arrows). Defective mitotic chromosome segregation resulted in the formation of anaphase bridges (green arrows). Two polar bodies were extruded during meiosis in wild-type embryos (yellow arrowheads), but extra pronuclei formed in CENP-A–depleted embryos (yellow arrow) due to meiotic defects. Bars, 5 μm.

Mentions: HCP-6 and MIX-1 colocalize with the centromeric histone variant CENP-A on the poleward faces of metaphase chromosomes during mitotic divisions in embryos and in the germline (Fig. 1 E and Fig. 2 A; Hagstrom et al., 2002; Stear and Roth, 2002). This pattern of localization and the biochemistry above indicate that HCP-6 and MIX-1 form a complex that associates with centromeres of mitotic chromosomes. To further define the roles for condensin in mitosis, we identified conditions that severely reduce HCP-6 function. The hcp-6(mr17) allele has a missense mutation (Stear and Roth, 2002) that results in temperature-sensitive embryonic lethality (Table S1, available at http://www.jcb.org/cgi/content/full/jcb.200408061/DC1); however, HCP-6 protein levels are not reduced (Fig. 1 C, lanes 1 and 2). Therefore, we treated hcp-6(mr17) mutants with hcp-6 RNA interference (RNAi) to deplete HCP-6 to levels undetectable by Western blot analysis (Fig. 1 C, lane 3) and immunostaining (Fig. 2 A, Fig. 5 E). Any residual protein would be compromised by the hcp-6(mr17) mutation. MIX-1 function was similarly reduced by mix-1 RNAi in worms homozygous for the maternal-effect embryonic lethal allele mix-1(b285) (Fig. 2 A, Fig. 5 E).


Condensin restructures chromosomes in preparation for meiotic divisions.

Chan RC, Severson AF, Meyer BJ - J. Cell Biol. (2004)

HCP-6 and MIX-1 associate independently with mitotic chromosomes and are required for chromosome condensation and segregation. (A) HCP-6 was undetectable in hcp-6(mr17, RNAi) mutant embryos, yet MIX-1 and CENP-A still associated with chromosomes at the disorganized metaphase plate. Similarly, MIX-1 was undetectable in mix-1(b285, RNAi) embryos, but HCP-6 and CENP-A still accumulated on chromosomes. HCP-6 did not associate with chromosomes of embryos depleted for two CENP-A paralogues, but MIX-1 did. (B) Mitotic chromosomes tagged with GFP::H2B histone condensed in pronuclei of wild-type embryos (red arrowhead) at least 2 min before NEBD, but decondensed chromosomes persisted until NEBD in pronuclei of hcp-6(mr17) and CENP-A–depleted embryos (red arrows). Defective mitotic chromosome segregation resulted in the formation of anaphase bridges (green arrows). Two polar bodies were extruded during meiosis in wild-type embryos (yellow arrowheads), but extra pronuclei formed in CENP-A–depleted embryos (yellow arrow) due to meiotic defects. Bars, 5 μm.
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Related In: Results  -  Collection

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fig2: HCP-6 and MIX-1 associate independently with mitotic chromosomes and are required for chromosome condensation and segregation. (A) HCP-6 was undetectable in hcp-6(mr17, RNAi) mutant embryos, yet MIX-1 and CENP-A still associated with chromosomes at the disorganized metaphase plate. Similarly, MIX-1 was undetectable in mix-1(b285, RNAi) embryos, but HCP-6 and CENP-A still accumulated on chromosomes. HCP-6 did not associate with chromosomes of embryos depleted for two CENP-A paralogues, but MIX-1 did. (B) Mitotic chromosomes tagged with GFP::H2B histone condensed in pronuclei of wild-type embryos (red arrowhead) at least 2 min before NEBD, but decondensed chromosomes persisted until NEBD in pronuclei of hcp-6(mr17) and CENP-A–depleted embryos (red arrows). Defective mitotic chromosome segregation resulted in the formation of anaphase bridges (green arrows). Two polar bodies were extruded during meiosis in wild-type embryos (yellow arrowheads), but extra pronuclei formed in CENP-A–depleted embryos (yellow arrow) due to meiotic defects. Bars, 5 μm.
Mentions: HCP-6 and MIX-1 colocalize with the centromeric histone variant CENP-A on the poleward faces of metaphase chromosomes during mitotic divisions in embryos and in the germline (Fig. 1 E and Fig. 2 A; Hagstrom et al., 2002; Stear and Roth, 2002). This pattern of localization and the biochemistry above indicate that HCP-6 and MIX-1 form a complex that associates with centromeres of mitotic chromosomes. To further define the roles for condensin in mitosis, we identified conditions that severely reduce HCP-6 function. The hcp-6(mr17) allele has a missense mutation (Stear and Roth, 2002) that results in temperature-sensitive embryonic lethality (Table S1, available at http://www.jcb.org/cgi/content/full/jcb.200408061/DC1); however, HCP-6 protein levels are not reduced (Fig. 1 C, lanes 1 and 2). Therefore, we treated hcp-6(mr17) mutants with hcp-6 RNA interference (RNAi) to deplete HCP-6 to levels undetectable by Western blot analysis (Fig. 1 C, lane 3) and immunostaining (Fig. 2 A, Fig. 5 E). Any residual protein would be compromised by the hcp-6(mr17) mutation. MIX-1 function was similarly reduced by mix-1 RNAi in worms homozygous for the maternal-effect embryonic lethal allele mix-1(b285) (Fig. 2 A, Fig. 5 E).

Bottom Line: We showed that condensin, the protein complex needed for mitotic chromosome compaction, restructures chromosomes during meiosis in Caenorhabditis elegans.Condensin helps resolve cohesin-independent linkages between sister chromatids and alleviates recombination-independent linkages between homologues.The safeguarding of chromosome resolution by condensin permits chromosome segregation and is crucial for the formation of discrete, individualized bivalent chromosomes.

View Article: PubMed Central - PubMed

Affiliation: Howard Hughes Medical Institute, Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720, USA.

ABSTRACT
The production of haploid gametes from diploid germ cells requires two rounds of meiotic chromosome segregation after one round of replication. Accurate meiotic chromosome segregation involves the remodeling of each pair of homologous chromosomes around the site of crossover into a highly condensed and ordered structure. We showed that condensin, the protein complex needed for mitotic chromosome compaction, restructures chromosomes during meiosis in Caenorhabditis elegans. In particular, condensin promotes both meiotic chromosome condensation after crossover recombination and the remodeling of sister chromatids. Condensin helps resolve cohesin-independent linkages between sister chromatids and alleviates recombination-independent linkages between homologues. The safeguarding of chromosome resolution by condensin permits chromosome segregation and is crucial for the formation of discrete, individualized bivalent chromosomes.

Show MeSH
Related in: MedlinePlus