BAF复合物 z-bo 2020-10-30 623

　　A key molecular "machine" called the BAF complex can change the structure of DNA, and mutations often occur in cancer and other diseases. In a new study, researchers from the Dana-Farber Cancer Institute, Rockefeller University and Washington University constructed an unprecedented three-dimensional structural model of this complex. For the first time, they reported the first batch of "pictures" of the three-dimensional structure of the BAF complex in its natural state purified directly from human cells, which gave people the opportunity to spatially map thousands of cancer-related mutations to this complex. Specific location of the object.

　　Kadoch said, "Prior to this, the three-dimensional structural model or'picture' of the actual appearance of this complex in the nucleus of our cells was still unclear." These researchers said that these newly obtained models "representatives" This is the most complete picture of the human BAF complex analyzed so far."

　　These new findings "provide a key basis for understanding the human disease-related mutations in the components of the BAF complex, which are present in more than 20% of human cancers, some intellectual disabilities, and neurodevelopmental disorders." These new insights may It is helpful for scientists to understand how mutations in the proteins that make up this complex cause damage to the normal regulation of DNA, which has the potential to destroy gene expression in cells, which may lead to cell cancer and tumor formation. For example, mutations in the BAF complex are the only cause of rare childhood cancers such as synovial sarcoma and malignant rhabdomyomas, as well as one of the causes of common cancers such as ovarian cancer and lung cancer.

　　As a "molecular machine", BAF complex is essentially a set of proteins that reshape the way DNA is wrapped in cells. It consists of 12 protein subunits. Kadoch said that previous attempts to obtain a three-dimensional structural model of the BAF complex were based on genetically modified protein molecules in the laboratory. "It is impossible to reproduce the entire complex." She said that since 2014, she and her colleagues have been trying to analyze the three-dimensional structure of BAF. Their main goal is to build a structural model to help them understand the impact of mutations, and ultimately help in BAF structure based on mutations. The position on the "picture" is classified. She said that extracting BAF complexes from human cells is a huge challenge: "We designed a new method to purify this complex --- it took several years."

　　BAF is one of several molecular "machines" that modulate chromatin (made of DNA and protein) to regulate gene expression. Chromatin is formed by compacting long DNA strands containing genes more tightly. A cell contains hundreds of thousands of chromatin modification complexes, and BAF is one of them. Although the mutations in BAF will not change the DNA gene code and cause cancer, it will destroy the topological structure and accessibility of DNA, leading to abnormal gene expression and the growth of malignant tumors.

　　These researchers combined several powerful new analysis tools to construct a structural model of the BAF complex as a single isolated complex and when combined with nucleosomes. Nucleosomes are scroll-like units of chromatin, and DNA fragments are wrapped around the nucleosomes. Binding to nucleosomes is a necessary condition for BAF complex to reshape chromatin and affect gene expression. One of the newest and most useful tools they use to obtain the BAF structure combined with nucleosomes is cryo-electron microscopy (cryo-EM). As a form of electron microscopy, it can build high-resolution models of molecules in the natural environment and is triggering changes in structural biology. In this case, the human BAF complex is too complex and flexible to produce high-resolution structures by this method alone, but when combined with the other two methods (cross-linked mass spectrometry and homology modeling) , The structural connection between subunits becomes clearer.

　　Kadoch and her colleagues report that the BAF composite is composed of three modules that form a "C"-shaped structure and clamp the nucleosomes like a carpenter uses a C-clamp to hold wood blocks together. They found that the two regions sandwiching the nucleosome in the BAF structure are "hot spots" where oncogenic mutations often occur, and they experimented to show how mutations can disrupt the normal regulation of chromatin by the BAF complex. They also found other sites of known cancer mutations in this complex. Among them, these known cancer mutations are contained in a database called COSMIC (Catalogue of Somatic Mutations in Cancer). , "Clustered" together in structure and reveal their functions.

　　Kadoch said, "Drawing such a mutation map on the structure of BAF complexes and understanding their functional effects has been a major unmet goal in this field for decades. This marks the beginning of an era in In this era, we will be able to functionally classify mutations that determine specific tumor characteristics and provide treatment opportunities."