Basic mechanism of eukaryotic chromosome segregation

Abstract

We now have firm evidence that the basic mechanism of chromosome segregation is similar among diverse eukaryotes as the same genes are employed. Even in prokaryotes, the very basic feature of chromosome segregation has similarities to that of eukaryotes. Many aspects of chromosome segregation are closely related to a cell cycle control that includes stage-specific protein modification and proteolysis. Destruction of mitotic cyclin and securin leads to mitotic exit and separase activation, respectively. Key players in chromosome segregation are SMC-containing cohesin and condensin, DNA topoisomerase II, APC/C ubiquitin ligase, securin-separase complex, aurora passengers, and kinetochore microtubule destabilizers or regulators. In addition, the formation of mitotic kinetochore and spindle apparatus is absolutely essential. The roles of principal players in basic chromosome segregation are discussed: most players have interphase as well as mitotic functions. A view on how the centromere/kinetochore is formed is described.

Figure 1

Fission yeast proteins required for chromosome segregation. See text for explanation. Most of them are conserved in higher eukaryotes. Gene products are grouped by different roles in chromosome segregation. Ub, uqhiquitin; MT, microtubule; M-cyclin, mitotic cyclin.

Figure 2

Three principal chromosome segregation defects in fission yeast mutants at the restrictive temperature. Arrest: β-tubulin nda3 mutant; Cut phenotype: top2 mutant; unequal segregation: mis6 mutant. DNA is stained by DAPI.

Figure 3

Two-domain centromere structures in fission yeast. (a) Fission yeast has three chromosomes, and their centromeres (variable in length from 35 to 110 kb) are designated cen1, cen2 and cen3 (Takahashi et al. 1992). The central domain (cnt, imr) has approximately constant (approximately 15 kb) length (red line), while the outer region (blue) line is variable in length. The central domains have specialized chromatin, revealing the smeared micrococcal digestion patterns. The central sequences, if inserted into plasmid pYC310 or pYC306, no longer show the smeared patterns. The minichromosome Ch16 can be visualized in the mitotically arrested cells of nda3 mutant. The central cnt and imr domain is associated with CENP-A, Mis6, Mis12 and Bub1, etc., while the outer otr domain contains HP-1/Swi6 and cohesin (see text). (b) The centromeric organization of fission yeast resembles that of human. The fission yeast central domain associates with CENP-A and Mis12 and can form the base for the protruded sister kinetochores. The outer repeats correspond to the heterochromatin that is enriched in cohesin Rad21. G. Goshima prepared this figure for his doctoral thesis.

Figure 4

Chromosome dynamics in cell cycle that leads to sister chromatid separation in anaphase. Interphase centromere is altered, upon the entry of mitosis into the kinetochore structure, which can associate with kinetochore microtubules. Sister chromatid cohesion is made during the S phase and maintained in the G2/M until anaphase. Two protein complexes, cohesin and condensin, which both contain structural maintenance of chromosome (SMC) and non-SMC subunits, are required for sister chromatid cohesion and mitotic chromosome condensation. Resulting condensed chromosomes interact with the spindle in prophase and prometaphase under the control of Aurora-passenger proteins and Bub, Mad spindle checkpoint proteins. The metaphase chromosomes are thus bi-oriented and their sister kinetochores are end-on connected with kinetochore microtubules. Chromosomes are congressed in the middle of the spindle prior to segregation. The onset of anaphase A is thought to be triggered by activation of APC/C, resulting in ubiquitin-mediated destruction of mitotic cyclin and securin by 26S proteasome. Exit of mitosis is accomplished by mitotic cyclin destruction, while sister chromatid separation requires the destruction of securin, followed by the activation of separase that can cleave the Scc1/Rad21 subunit of cohesin. Classes of proteins MCAK/KinI and Tog/Dis1/Stu2 are implicated in destabilization of kinetochore microtubules (e.g. microtubule shortening) in anaphase. The kinetochore microtubules are shortened in anaphase A and the pole-to-pole microtubules are fully elongated in anaphase B. The late anaphase and telophase events include the disassembly of the spindle, chromosome de-condensation, nuclear envelope reformation and the coordination with cytokinesis.

Figure 5

The basic chromosome segregation mechanism is ensured by quality control mechanisms.

Figure 6

Principal players of chromosome segregation.

Figure 7

A hypothesis to explain how condensin and cohesin affect the organization of centromere/kinetochore (see text for explanation).