Structural And Mechanistic Investigation Of SWI/SNF Chromatin Remodeling Complex

In eukaryotic cells,DNA organized into chromatin serves to store the genetic materials.However,this creates a repressive barrier to many DNA-mediated fundamental nuclear processes.Chromatin remodelers are ATP-dependent DNA translocases altering the accessibility of nucleosomal DNA.A key question in the field is how remodelers work coupling ATP hydrolysis to substrate remodeling.This project focuses on SWI/SNF2 remodeler.We use a combination of crystallography,cryo-EM and biochemistry technique to elucidate the mechanism of chromatin remodeling.At first,we solved the crystal structure of Snf2 from the yeast Myceliophthora thermophile,at the resolution of 2.33 ?.The two core domains of MtSnf2 are trapped together through hydrophobic interactions with each other.We identified several conserved DNA binding sites of Snf2.These elements are exposed to solvent.In addition,ATP-binding elements(motifs I and VI)twisted towards different orientations.The results suggest MtSnf2 rests in an inactive conformation and is poised for activation.To elucidate the chromatin remodeling mechanism,we solved the Snf2-nucleosome structure by cryo-EM.The structure shows the two core domains of Snf2 are rearranged upon nucleosome binding.The new core1-core2 interface is generated between SuppH helix and Brace helices.Moreover,ATP-binding elements(motifs I and VI)are brought into close proximity,explaining the activation of the enzyme.Snf2 binds to the nucleosome mainly through its helicase motifs contacting the phosphate backbones of one DNA gyre,which suggests a conserved substrate engagement mechanism.The core1 domain of Snf2 contacts the other DNA gyre as the secondary binding site to anchor the remodeler at a fixed positon.The nucleosomal DNA at the protein binding site are locally distorted.Our findings describe how Snf2 engages with the nucleosome providing the structural basis for chromatin remodeling.Our structure of Snf2 in complex with the nucleosome substrate achieved a much higher resolution(4.69 ?)than the previous work(~20 ?).Our work reveals many insights that are not available based on the low resolution data.Besides,it may shed light on the treatment of Brg1 related disease.