A quantitative map of human Condensins provides new insights into mitotic chromosome architecture

Abstract

Walther et al. systematically fluorescently tag endogenous Condensin subunits and map their abundance, physical spacing, and mitotic dynamics by fluorescence correlation spectroscopy–calibrated live-cell imaging and superresolution microscopy. They propose a three-step hierarchical loop model of mitotic chromosome compaction.

Condensin controls mitotic chromosome stiffness and stability without forming a structurally contiguous scaffold

“…This suggests that the elastic modulus of the highly labeled condensin centers is much larger than that of the inter-condensin-cluster chromatin, and that the condensin-antibody-stained puncta and the unstained regions have different structures. Our results are consistent with a very recent super-resolution imaging study (Walther et al, 2018) where condensins are observed to be separated from one another but with modulation of their density on longer length scales, consistent with the structure seen in our experiment where we have ~250 nm diffraction-limited wide-field imaging resolution.…”

“…, axial scaffold (Adolphs et al, 1977, Earnshaw and Laemmli, 1983), helical loop (Bak et al, 1977), and chromatin network (Poirier and Marko, 2002) models], experiments have not indicated clearly which of these, if any, is most correct, although recent Hi-C studies do suggest a helical loop-array organization (Gibcus et al, 2018). Prior studies have also not clearly established whether condensin is organized into a continuous or discontinuous distribution along chromosome arms, although a recent super-resolution study does suggest the latter (Walther et al, 2018). …”

Chromosome disentanglement driven via optimal compaction of loop-extruded brush structures

“…Loop extrusion activity by SMC complexes maximizes compaction by minimizing the axial length of brush chromosomes. Condensin II is a likely candidate that drives the prophase compaction, suggesting an important role of condensin II in determining the axial length of chromosomes, in accord with the observation of an increase in the axial length for condensin II-depleted chromosomes [17,23,[67][68][69]. Note, due to the optimal loop architecture of chromosomes in interphase, we do not expect the axial length to significantly change during the course of the cell cycle -which is central to our conclusion that inter-chromosomal entanglements are suppressed by SMC activity during the cell cycle.…”

“…The core enhances the mechanical rigidity of the chromosomes; the width of the core r0, which is proportional to the average loop valency, is plotted in (d); the lines lying in the shaded region in (d) do not have a core, since a minimal core must be at least one monomer thick. The experimental data for human chromosome core corresponds to the thickness of the axial region where condensin-colocalizes in metaphase [23]. The thermal persistence length of the brush ρ (e), and the force associated with stretching the chromosome to twice its native length or the doubling force f0 (e), show increased stiffness and mechanical rigidity for smaller backbone and higher valency.…”

“…Structural Maintenance of Chromosomes (SMC) complexes are thought to be responsible for organizing chromosome structure [17][18][19][20][21][22][23][24][25], and recently have been directly observed to processively translocate and loop-extrude DNA [26,27]. The three-dimensional conformation of the interphase genome has been observed to be organized into loops that are associated with regulation of gene expression, and these loops appear to be actively driven by a concerted action of proteins like SMC complexes and other architectural proteins [20,24,25,[28][29][30][31][32][33].…”