Metaphase

About: Metaphase is a research topic. Over the lifetime, 6925 publications have been published within this topic receiving 291590 citations. The topic is also known as: GO:0007091 & mitotic metaphase/anaphase transition.

Aurora B and Cdk1 mediate Wapl activation and release of acetylated cohesin from chromosomes by phosphorylating Sororin.

Abstract: Sister chromatid cohesion depends on Sororin, a protein that stabilizes acetylated cohesin complexes on DNA by antagonizing the cohesin release factor Wings-apart like protein (Wapl). Cohesion is essential for chromosome biorientation but has to be dissolved to enable sister chromatid separation. To achieve this, the majority of cohesin is removed from chromosome arms in prophase and prometaphase in a manner that depends on Wapl and phosphorylation of cohesin’s subunit stromal antigen 2 (SA2), whereas centromeric cohesin is cleaved in metaphase by the protease separase. Here we show that the mitotic kinases Aurora B and Cyclin-dependent kinase 1 (Cdk1) destabilize interactions between Sororin and the cohesin subunit precocious dissociation of sisters protein 5 (Pds5) by phosphorylating Sororin, leading to release of acetylated cohesin from chromosome arms and loss of cohesion. At centromeres, the cohesin protector shugoshin (Sgo1)-protein phosphatase 2A (PP2A) antagonizes Aurora B and Cdk1 partly by dephosphorylating Sororin and thus maintains cohesion until metaphase. We propose that the stepwise loss of cohesion between chromosome arms and centromeres is caused by local regulation of Wapl activity, which is controlled by the phosphorylation state of Sororin.

MAST/Orbit has a role in microtubule–kinetochore attachment and is essential for chromosome alignment and maintenance of spindle bipolarity

Abstract: Multiple asters (MAST)/Orbit is a member of a new family of nonmotor microtubule-associated proteins that has been previously shown to be required for the organization of the mitotic spindle. Here we provide evidence that MAST/Orbit is required for functional kinetochore attachment, chromosome congression, and the maintenance of spindle bipolarity. In vivo analysis of Drosophila mast mutant embryos undergoing early mitotic divisions revealed that chromosomes are unable to reach a stable metaphase alignment and that bipolar spindles collapse as centrosomes move progressively closer toward the cell center and eventually organize into a monopolar configuration. Similarly, soon after depletion of MAST/Orbit in Drosophila S2 cells by double-stranded RNA interference, cells are unable to form a metaphase plate and instead assemble monopolar spindles with chromosomes localized close to the center of the aster. In these cells, kinetochores either fail to achieve end-on attachment or are associated with short microtubules. Remarkably, when microtubule dynamics is suppressed in MAST-depleted cells, chromosomes localize at the periphery of the monopolar aster associated with the plus ends of well-defined microtubule bundles. Furthermore, in these cells, dynein and ZW10 accumulate at kinetochores and fail to transfer to microtubules. However, loss of MAST/Orbit does not affect the kinetochore localization of D-CLIP-190. Together, these results strongly support the conclusion that MAST/Orbit is required for microtubules to form functional attachments to kinetochores and to maintain spindle bipolarity.

High-mobility group protein HMG-I localizes to G/Q- and C-bands of human and mouse chromosomes.

Abstract: Mammalian metaphase chromosomes can be identified by their characteristic banding pattern when stained with Giemsa dye after brief proteolytic digestion. The resulting G-bands are known to contain regions of DNA enriched in A/T residues and to be the principal location for the L1 (or Kpn 1) family of long interspersed repetitive sequences in human chromosomes. Here we report that antibodies raised against a highly purified and biochemically well characterized nonhistone "High-Mobility Group" protein, HMG-I, specifically localize this protein to the G-bands in mammalian metaphase chromosomes. In some preparations in which chromosomes are highly condensed, HMG-I appears to be located at the centromere and/or telomere regions of mammalian chromosomes as well. To our knowledge, this is the first well-characterized mammalian protein that localizes primarily to G-band regions of chromosomes.