开放核小体,染色质松散 z-bo 2020-10-30 353

　　The structure of a conventional nucleosome includes a protein core assembled from four histones H2A, H2B, H3, and H4, and a double-stranded DNA with a length of 147 bp wrapped around the histone core in 1.6 circles. Nucleosomes have a stable structure and are insensitive to changes in DNA composition and histone modifications. Histone variants can change the nucleosome and chromatin structure to regulate gene transcription. Among all the mononucleosomes determined so far, the histone H3 variant nucleosome is the CENP-A nucleosome with the largest conformational change. It is shown that the DNA contained in the CENP-A nucleosome is 121 bp, but the protein core structure has little change. The histone H2A variants H2A.B and H2A.Z.2.2 are specifically expressed in spermatogonia and human brain tissue, respectively, and play important functions in spermatogenesis, transcription initiation, RNA shearing and other processes. H2A.B and H2A.Z.2.2 form an open nucleosome structure and "destroy" the chromatin structure, resulting in loose chromatin. However, such open nucleosomes are extremely unstable, and it is difficult to obtain a high-precision structure.

　　Recently, the research group of researcher Zhou Zheng and researcher Zhu Ping of the Institute of Biophysics of the Chinese Academy of Sciences has collaborated with the research group of researcher Zhu Ping to publish online research titled Structural basis of nucleosome dynamics modulation by histone variants H2A.B and H2A.Z.2.2 on THE EMBO JOURNAL. paper. In this study, cryo-electron microscopy was used to analyze the nucleosome structures containing histone variants H2A.B and H2A.Z.2.2, respectively, and to clarify the molecular mechanism of H2A variant proteins regulating nucleosome dynamics and chromatin structure opening.

　　To stabilize the H2A.B nucleosome, the researchers recombinantly expressed the DNA binding domain peptide PARP1-DBD of PARP1 and added it to the H2A.B nucleosome to prepare cryo-EM samples. Biochemical and electron microscopy results show that PARP1-DBD reduces the volatility of nucleosomal DNA ends, but does not affect the structure of nucleosomes. On this basis, the researchers analyzed the resolution of 2.8? The H2A.B nucleosome structure.

　　Compared with conventional nucleosomes, the protein core of H2A.B nucleosomes has obvious conformational changes, and some structural elements are missing. The distance between H2A.B-H2B dimers increases, which leads to an increase in the helical pitch of the winding DNA and thicker nucleosomes. The length of the DNA wound on the histone core is reduced by a third to only 103 bp, resulting in only 1.2 turns of the protein core. In addition, the researchers also analyzed the 3.9? of H2A.Z.2.2 nucleosomes. The structure of the electron microscope showed that the conformational change of the H2A.Z.2.2 protein core was similar to that of the H2A.B nucleosome, but the DNA wrapped around the core was about 125 bp. Mnase digestion and octamer assembly results show that there is an octamer assembly regulatory sequence (ROF, regulating-octamer-folding) composed of 6 amino acid residues at the C-terminus of H2A.B and H2A.Z.2.2, H2A .Z.2.2 The unique ROF sequence can improve the efficiency of the H2A.Z enzymatic exchange reaction. This study analyzes the most open nucleosome structure discovered so far and reveals the molecular mechanism of H2A.B and H2A.Z.2.2 regulating the changes in nucleosome structure and chromatin dynamics.

　　Zhu Ping group assistant researcher Zhou Min and Zhou Zheng group postdoctoral and special research assistant Dai Linchang are the co-first authors of the paper, and Zhou Zheng and Zhu Ping are the co-corresponding authors of the paper. Doctor Li Chengmin, doctoral student Shi Liu Xin and doctor Huang Yan participated in the research. The Bioimaging Center of the Institute of Biophysics provided cryo-electron microscopy research equipment and technical support for the research. The research work was funded by the National Natural Science Foundation of China, the National Key R&D Program of the Ministry of Science and Technology, and the Strategic Leading Science and Technology Special Project (Class B) of the Chinese Academy of Sciences.