The relative ratio of condensin I to II determines chromosome shapes

Abstract

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.

Contrasting roles of condensin I and II in mitotic chromosome formation

“…3), the two KO cell lines displayed distinct chromosome morphologies, with CAP-H OFF (condensin I OFF ) chromosomes becoming short and fuzzy, whereas CAP-D3 OFF (condensin II OFF ) chromosomes were long, twisted and bent over. These morphologies are broadly consistent with other chromosomespread analyses of condensin I and II knockdown in vertebrates (Abe et al, 2011;Ono et al, 2003) and also reflect the conclusions of the recent in vitro analysis on assembled chromosomes using varying ratios of condensin I and II from Xenopus egg extracts (Shintomi and Hirano, 2011).…”

“…However, the bridges seen in the absence of condensin II are much more robust and obvious, whereas those seen in the absence of condensin I are highly attenuated and only seen after careful examination. In Xenopus egg extracts, the ratio of condensins I to II has been calculated to be 5:1 (Shintomi and Hirano, 2011) and 1:1 from HeLa nuclear extracts (Ono et al, 2003). Our measurements are based on quantitative proteomics analysis of condensins I and II in DT40 mitotic chromosomes and not from total condensin, although it is also possible there exists some species variation (Ohta et al, 2010).…”

HEAT repeats – versatile arrays of amphiphilic helices working in crowded environments?

“…Moreover, condensin II is located more internally than condensin I, which is found along the axial core of chromatids (Ono et al, 2003). On the basis of these and other data, condensin II has been proposed to contribute to lengthwise shortening, especially of large and thick chromatids (Shintomi and Hirano, 2011), and to confer their physical rigidity (Houlard et al, 2015). Thus, if condensins indeed act as crosslinkers for chromatin networks as has been predicted (Marko, 2008), condensin II would be a more robust crosslinker than condensin I.…”

“…In vertebrates, embryonic (middle) and somatic (right) chromatids have different ratios of condensins I and II, and display different shapes; the embryonic chromatids are thin and long, whereas the somatic ones are thick and short. Experiments using Xenopus cellfree egg extracts have provided evidence that the relative ratio between condensin I and II determines chromosome shape (Shintomi and Hirano, 2011). Moreover, condensin II is located more internally than condensin I, which is found along the axial core of chromatids (Ono et al, 2003).…”

“Breaking Up Is Hard to Do”: The Formation and Resolution of Sister Chromatid Intertwines

“…However, it has been known for some time that it is required for the initiation (but not maintenance) of mitotic rDNA compaction in budding yeast [78]. In Xenopus the dosage of cohesin complexes on chromosomes influences the final shape of the condensed chromosome, probably by antagonizing the activity of the condensin II complex [79] (metazoans have multiple condensin type complexes -reviewed by Hirano [66]). It has now being shown in mammalian cells that deletion of the cohesin regulator Wapl causes premature chromosome condensation in interphase and extensive chromosome bridging in anaphase [80].…”