Biology subjects

Samejima, I.

Publications and source records attributed to Samejima, I..

3 recordsLinked to original sources

bioRxiv2025

A time-resolved atlas of histone modifications during mitotic entry

Mitotic chromosome formation is essential for faithful chromosome segregation in metazoans. While condensin complexes are critical for the formation of rod-shaped mitotic chromosomes, additional mechanisms--particularly those involving phosphorylation and deacetylation of specific histone residues--have been proposed to contribute a further 2- to 4-fold reduction in mitotic chromatin volume. In this study, we employ high-resolution mass spectrometry to determine the kinetics of histone modifications in cell cultures undergoing a highly synchronous mitotic entry at 2.5-minute resolution. Our analysis reveals three different programmes of histone H3 phosphorylation on T3, S10 and S28. These modifications are consistent with methyl-phos switches regulating the association of readers with chromatin other than at promoters. Mass spectrometry and quantitative ChIP-Seq reveal that H3 T3 phosphorylation is a general marker of heterochromatin and not specifically centromeres as previously suggested. Finally, we show that histone acetylation undergoes only modest changes as rod-shaped chromosomes form during unperturbed mitotic entry. Thus, previously reported reductions in acetylation associated with chromosome formation were apparently attributable to delays in mitotic exit used as part of mitotic synchronisation protocols. The mechanism of condensin-independent chromatin compaction in mitosis remains unexplained.

cell biology↗

bioRxiv2024

Rules of engagement for condensins and cohesins guide mitotic chromosome formation

During mitosis, interphase chromatin is rapidly converted into rod-shaped mitotic chromosomes. Using Hi-C, imaging, proteomics and polymer modeling, we determine how the activity and interplay between loop-extruding SMC motors accomplishes this dramatic transition. Our work reveals rules of engagement for SMC complexes that are critical for allowing cells to refold interphase chromatin into mitotic chromosomes. We find that condensin disassembles interphase chromatin loop organization by evicting or displacing extrusive cohesin. In contrast, condensin bypasses cohesive cohesins, thereby maintaining sister chromatid cohesion while separating the sisters. Studies of mitotic chromosomes formed by cohesin, condensin II and condensin I alone or in combination allow us to develop new models of mitotic chromosome conformation. In these models, loops are consecutive and not overlapping, implying that condensins do not freely pass one another but stall upon encountering each other. The dynamics of Hi-C interactions and chromosome morphology reveal that during prophase loops are extruded in vivo at [~]1-3 kb/sec by condensins as they form a disordered discontinuous helical scaffold within individual chromatids.

molecular biology↗

bioRxiv2017

Mitotic chromosomes fold by condensin-dependent helical winding of chromatin loop arrays

During mitosis, chromosomes fold into compacted rod shaped structures. We combined imaging and Hi-C of synchronous DT40 cell cultures with polymer simulations to determine how interphase chromosomes are converted into compressed arrays of loops characteristic of mitotic chromosomes. We found that the interphase organization is disassembled within minutes of prophase entry and by late prophase chromosomes are already folded as arrays of consecutive loops. During prometaphase, this array reorganizes to form a helical arrangement of nested loops. Polymer simulations reveal that Hi-C data are inconsistent with solenoidal coiling of the entire chromatid, but instead suggest a centrally located helically twisted axis from which consecutive loops emanate as in a spiral staircase. Chromosomes subsequently shorten through progressive helical winding, with the numbers of loops per turn increasing so that the size of a helical turn grows from around 3 Mb (~40 loops) to ~12 Mb (~150 loops) in fully condensed metaphase chromosomes. Condensin is essential to disassemble the interphase chromatin conformation. Analysis of mutants revealed differing roles for condensin I and II during these processes. Either condensin can mediate formation of loop arrays. However, condensin II was required for helical winding during prometaphase, whereas condensin I modulated the size and arrangement of loops inside the helical turns. These observations identify a mitotic chromosome morphogenesis pathway in which folding of linear loop arrays produces long thin chromosomes during prophase that then shorten by progressive growth of loops and helical winding during prometaphase.\n\nOne Sentence SummaryMitotic chromosome morphogenesis occurs through condensin-mediated disassembly of the interphase conformation and formation of extended prophase loop arrays that then shorten by loop growth and condensin-dependent helical winding.

genomics↗