BAF / pbaf complexes are members of the human chromatin regulatory factor family. They act like "gatekeepers" and affect the final expression of human genes. In recent years, scientists have found that the mutation of the complex is related to more than 20% of cancers and a variety of neurological development defects. Chen Zhucheng, a professor of Tsinghua University, used the advanced method of freeze electron microscopy to reveal the assembly mode of pbaf complex and its mechanism of recognizing nucleosomes, which provided a theoretical framework for studying the pathogenic mechanism of mutations related to human diseases. Relevant results were recently published in nature. Structural analysis "problem solving" Chromatin is the carrier of eukaryotic genetic material. It is a dense structure formed by DNA winding on histone octamer to form nucleosome and highly compressed. On the one hand, this form and structure ensures the stability of the genome, on the other hand, it hinders the replication of genetic information, transcription, DNA damage repair and other life activities. So, when do chromosomes "keep calm" and when do they "open their hearts"? This depends on a series of regulatory mechanisms developed by eukaryotic cells. For example, in the process of the regulation mechanism of "chromatin remodeling", some proteins called "remodeling complexes" use the energy of ATP to slide, exchange or depolymerize nucleosomes, and realize the dynamic regulation of chromatin by changing the binding position between nucleosomes and DNA. The complexes named BAF and pbaf are two typical chromatin remodeling complexes belonging to mammals. They regulate chromatin structure and gene expression and widely participate in the development and differentiation of animal cells. As "gatekeepers" of genes, these two complexes are also considered as potential drug targets for the treatment of diseases such as cancer. According to researchers, the two complexes were identified as early as 1996. Moreover, thanks to the development of freeze electron microscopy technology, the high-resolution structure of one of the BAF complexes has also been released. However, the assembly mode of pbaf complex, the mechanism of recognizing nucleosomes and the difference between pbaf complex and BAF complex are still the scientific frontier. The research team, which has long been committed to the study of chromosome remodeling mechanism, believes that understanding the structure of pbaf complex will help to "see" its appearance and understand its working process. Three dimensional structure discharge The premise to solve these problems is to obtain the complete pbaf complex for research. However, the complex is a super large complex composed of 12 subunits with a molecular weight of more than 1 megadalton, which brings the team the first technical difficulty to be solved urgently - how to obtain a high-quality complete pbaf complex? Chen Zhucheng led his team to work hard in the laboratory to find a way out to solve problems. After five years of research, they made a series of explorations in protein expression system, protein component boundary conditions and purification methods, explored a new scheme for biochemical recombination experiment of pbaf complex, and finally obtained high-quality recombinant pbaf complex. Then, the team used the frozen electron microscope of Tsinghua University to image and reconstruct the complex in three dimensions. However, the second difficulty arises again - how to build a three-dimensional model of the composite? The team first thought of "fuzzy recognition algorithm of amino acid residue characteristics". Based on the high-resolution electron microscope density map, combined with the prediction of protein secondary structure and the chemical properties of amino acid residues, they finally successfully analyzed the structure of pbaf binding nucleosomes and revealed the assembly mode of pbaf complex. The three-dimensional structure shows that the 12 subunits of pbaf can be divided into three modules according to different functions, including the "motor module" responsible for catalytic activity, the "ARP module" with regulatory function and the "SRM module" with chromatin targeting function. Interestingly, the "SRM module" is composed of nine auxiliary subunits shuttle and intertwined to form a trilobal sheet. The research team also named these leaves according to their main biological functions, including nucleosome binding leaves (NBL), histone tail binding leaves (HBL) and DNA binding leaves (DBL). Chen Kangjing, co-author of the paper and doctoral student of Tsinghua University, introduced to China Science Daily that compared with BAF, pbaf has a special HBL and contains some specific subunits. "This is equivalent to a super histone recognition sub module." The researchers believe that it is this special domain that enables pbaf complex to sense chromatin signals more efficiently in the body. Help precision medicine After clarifying the mechanism of pbaf complex assembly and chromatin remodeling, the team began to think about the next step: how can this study serve the research of human diseases? In previous studies, because the core "motor subunit" of BAF complex is inactive, it is difficult to provide pathogenic mechanism for disease-related mutations. In the study around pbaf, the researchers found that the motor subunit in the pbaf complex was active. "We clearly see that a large number of disease-related mutations are distributed at two key active interfaces, and mutations at these sites will also significantly reduce chromatin remodeling activity, indicating that the occurrence of disease may be related to the loss of BAF / pbaf complex function." Introduction of researchers. In addition, they also revealed for the first time the mechanism by which motor subunits recognize nucleosomes. "Our findings provide a basis for precision medicine in the future." Yuan Junjie, co-author of the paper and doctoral student of Tsinghua University, told China Science Daily that "the research on pbaf complex and its active subunits provides a structural basis for the development of new drugs targeting BAF / pbaf complex." Industry experts believe that this work not only clarifies the mechanism of pbaf complex assembly, nucleosome recognition and chromatin remodeling, but also provides a theoretical basis for understanding the pathogenic mechanism of BAF / pbaf related mutations. This discovery will help to understand the mechanism of chromatin remodeling at the chromatin level and promote the research and development of targeted drugs for related diseases. (Xinhua News Agency)