Frozen‐hydrated chromatin from metaphase chromosomes has an interdigitated multilayer structure

Abstract

Cryo‐electron tomography and small‐angle X‐ray scattering were used to investigate the chromatin folding in metaphase chromosomes. The tomographic 3D reconstructions show that frozen‐hydrated chromatin emanated from chromosomes is planar and forms multilayered plates. The layer thickness was measured accounting for the contrast transfer function fringes at the plate edges, yielding a width of ~ 7.5 nm, which is compatible with the dimensions of a monolayer of nucleosomes slightly tilted with respect to the layer surface. Individual nucleosomes are visible decorating distorted plates, but typical plates are very dense and nucleosomes are not identifiable as individual units, indicating that they are tightly packed. Two layers in contact are ~ 13 nm thick, which is thinner than the sum of two independent layers, suggesting that nucleosomes in the layers interdigitate. X‐ray scattering of whole chromosomes shows a main scattering peak at ~ 6 nm, which can be correlated with the distance between layers and between interdigitating nucleosomes interacting through their faces. These observations support a model where compact chromosomes are composed of many chromatin layers stacked along the chromosome axis.

Chromatin plates in the interphase nucleus

“…As indicated in the Introduction, metaphase chromosomes are formed by many stacked layers of chromatin . The results presented above indicate that interphase nuclei also contain plates.…”

“…The results obtained indicated that plates are formed by mononucleosome layers, which are interdigitated with each other in stacked multilayers within metaphase chromosomes. Furthermore, X‐ray scattering of whole chromosomes under metaphase ionic conditions showed a dominant peak at ~ 6 nm that can be correlated with the distance between nucleosomes interacting face‐to‐face and with the distance between interdigitating layers . Interdigitation leading to stabilizing face‐to‐face nucleosome interactions were proposed in previous experimental and modeling studies of diverse chromatin higher order structures .…”

“…More recently, cryoelectron tomography was used to study the three‐dimensional structure of frozen‐hydrated chromatin plates . The results obtained indicated that plates are formed by mononucleosome layers, which are interdigitated with each other in stacked multilayers within metaphase chromosomes.…”

Revealing chromatin organization in metaphase chromosomes

“…A new cryo-electron tomography study from the Daban laboratory (Chicano et al, 2019) reveals that mitotic chromatin isolated from human cells can assume a plate-like fine structure, containing layers of interdigitated nucleosomes. A new cryo-electron tomography study from the Daban laboratory (Chicano et al, 2019) reveals that mitotic chromatin isolated from human cells can assume a plate-like fine structure, containing layers of interdigitated nucleosomes.…”

“…The question therefore remains whether the observed structures are also prevalent in vivo. Third, the in vitro investigation by Chicano et al (2019) requires sample isolation and preparation. In that case, the authors were not able to detect higher-order organization, but instead found that many nucleosomes were partially unwrapped.…”