RESUMO
Base excision repair (BER) is initialized by DNA glycosylases, which recognize and flip damaged bases out of the DNA duplex into the enzymes active site, followed by cleavage of the glycosidic bond. Recent studies have revealed that all types of DNA glycosylases repair base lesions less efficiently within nucleosomes, and their repair activity is highly depended on the lesion's location within the nucleosome. To reveal the underlying molecular mechanism of this phenomenon, we determine the 3.1 Å cryo-EM structure of human 8-oxoguanine-DNA glycosylase 1 (hOGG1) bound to a nucleosome core particle (NCP) containing a common oxidative base lesion, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo). Our structural analysis shows that hOGG1 can recognize and flip 8-oxodGuo even within NCPs; however, the interaction between 8-oxodGuo and hOGG1 in a NCP context is weaker than in free DNA due to competition for nucleosomal DNA by the histones. Binding of OGG1 and the flipping of 8-oxodGuo by hOGG1 leads to a partial detachment of DNA from the histone core and a ratchet-like inward movement of nucleosomal DNA. Our findings provide insights into how the dynamic structure of nucleosomes modulate the activity of repair enzymes within chromatin.
Assuntos
8-Hidroxi-2'-Desoxiguanosina , Microscopia Crioeletrônica , DNA Glicosilases , Reparo do DNA , Nucleossomos , DNA Glicosilases/metabolismo , DNA Glicosilases/química , Nucleossomos/metabolismo , Nucleossomos/ultraestrutura , Humanos , 8-Hidroxi-2'-Desoxiguanosina/metabolismo , 8-Hidroxi-2'-Desoxiguanosina/química , DNA/metabolismo , DNA/química , Modelos Moleculares , Ligação Proteica , Histonas/metabolismo , Histonas/química , Dano ao DNARESUMO
DNA damage and apoptosis lead to the release of free nucleosomes-the basic structural repeating units of chromatin-into the blood circulation system. We recently reported that free nucleosomes that enter the cytoplasm of mammalian cells trigger immune responses by activating cGMP-AMP synthase (cGAS). In the present study, we designed experiments to reveal the mechanism of nucleosome uptake by human cells. We showed that nucleosomes are first absorbed on the cell membrane through nonspecific electrostatic interactions between positively charged histone N-terminal tails and ligands on the cell surface, followed by internalization via clathrin- or caveolae-dependent endocytosis. After cellular internalization, endosomal escape occurs rapidly, and nucleosomes are released into the cytosol, maintaining structural integrity for an extended period. The efficient endocytosis of extracellular nucleosomes suggests that circulating nucleosomes may lead to cellular disorders as well as immunostimulation, and thus, the biological effects exerted by endocytic nucleosomes should be addressed in the future.