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Genes Dev
2011 Jul 15;2514:1464-9. doi: 10.1101/gad.2060311.
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The relative ratio of condensin I to II determines chromosome shapes.
Shintomi K, Hirano T.
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To understand how chromosome shapes are determined by actions of condensins and cohesin, we devised a series of protocols in which their levels are precisely changed in Xenopus egg extracts. When the relative ratio of condensin I to II is forced to be smaller, embryonic chromosomes become shorter and thicker, being reminiscent of somatic chromosomes. Further depletion of condensin II unveils its contribution to axial shortening of chromosomes. Cohesin helps juxtapose sister chromatid arms by collaborating with condensin I and counteracting condensin II. Thus, chromosome shaping is achieved by an exquisite balance among condensin I and II and cohesin.
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Figure 2. The relative ratio of condensin I to II determines the shape of duplicated chromosomes in Xenopus egg extracts. (A) Duplicated chromosomes were assembled in a control extract (I:II = 5:1) or a reconstituted extract (I:II = 1:1) (for details, see Supplemental Fig. S2A,B), and processed for immunofluorescence. The third panels on each row are galleries of CAP-G-labeled images of individualized chromosomes. Magnified DAPI images of the selected regions of chromosomes (indicated by the broken boxes) are shown at the right. Bars, 5 μm. (B) The lengths of chromosomes assembled in A are shown in the box plot. The middle line of each box is the median. The top and bottom lines are the third and first quartiles, and the whiskers indicate the 90th and 10th percentiles. The numbers of chromosomes measured are shown in parentheses (P = 1.2 × 10−51, t-test). (C,D) The lengths of chromosomes assembled in different extracts are compared pairwise in the box plots. A statistically significant difference was seen only between the right pair (P = 0.22, left; P = 2.2 × 10−31, right). The distribution of chromosome lengths in each condition examined in B–D is also shown in histograms (Supplemental Fig. S6).
Figure 3. Cohesin collaborates with condensins to shape chromosomes. (A) Chromosomes were assembled in extracts containing various levels of cohesin and condensin I (for details, see Supplemental Fig. S4A), and subjected to immunofluorescence analysis with anti-topoisomerase II (topo II). Blowup images of selected regions (indicated by the rectangles) are shown in the bottom row. Bar, 5 μm. The average distance between sister chromatids, together with standard deviation, is provided below each image (n = 22, 28, 21, 20, from left to right). (B) Chromosomes were assembled in a control extract (δmock), an extract depleted of cohesin (δcohesin), or Wapl (δWapl). To visualize cohesin left on these chromosomes, immunofluorescence was performed with an antibody against its SA1 subunit. Bar, 5 μm. (C) Chromosomes were assembled as described in B, and labeled with anti-CAP-G and anti-CAP-H2 antibodies. Bar, 5 μm.
Figure 4. Balancing actions of two condensins are also important for single-chromatid assembly. (A) Single chromatids (yellow arrows) and duplicated chromosomes (blue arrowheads) were assembled separately, fixed on a single slide, and subjected to immunofluorescence. Bar, 5 μm. (B) Sperm chromatin was incubated with metaphase-arrested extracts containing condensin I and II at the indicated ratios. After an 80-min incubation, chromatids were analyzed by immunofluorescence. Bar, 5 μm. (C) A model of how balancing actions of condensins and cohesin determine the shape of chromosomes is depicted in the cartoon.
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