Mechanism of Cytosolic DNA Sensing Through the cGAS-STING Pathway
题目：Mechanism of Cytosolic DNA Sensing Through the cGAS-STING Pathway
报告人：Prof. Pingwei Li 李平伟
Dept. of Biochemistry & Biophysics,
Texas A&M University
联系人： 苏晓东 010-62759743
Microbial nucleic acids induce potent immune responses by stimulating the expression type I interferons (IFN-I). Cyclic GMP-AMP synthase (cGAS) is a cytosolic DNA sensor mediating the innate immunity to microbial DNA. cGAS is activated by DNA and catalyzes the synthesis of a cyclic dinucleotide cGAMP, which binds to the adaptor STING that mediates the recruitment and activation of protein kinase TBK1 and transcription factor IRF-3. Activated IRF-3 then translocates to the nucleus and initiates the transcription of the IFN-I genes. To elucidate the mechanism of cGAS activation, we determined the structures of cGAS in isolation and in complex with an 18 bp dsDNA. The cGAS/DNA complex structure reveals that cGAS interacts with DNA through two binding sites, forming a 2:2 complex. Enzyme assays and IFN-β reporter assays demonstrate that interactions at both DNA binding sites are essential for cGAS activation. These results demonstrate that cGAS is activated by DNA-induced oligomerization. To investigate how cGAMP activates STING, we determined the structures of STING in isolation and in complex with cGAMP. These structures reveal that STING forms a V-shaped dimer and binds cGAMP at the dimer interface. Ligand binding induces a conformational change of STING that is likely involved in the activation of STING. We have also determined the structures of TBK1 in complex with two inhibitors. These structures show that TBK1 exhibits an IκB kinase fold with distinct domain arrangement. STING mediates the recruitment of IRF-3 through a pLxIS motif within it C-terminal 37 residues. To elucidate the mechanism of IRF-3 recruitment by STING, we determined the structure of a phosphorylated STING peptide bound to IRF-3. To understand how phosphorylation activates IRF-3, we solved the structure of IRF-3 phosphomimetic mutant S386/396E bound to CREB binding protein (CBP), which reveals how an internal pLxIS motif of IRF-3 mediates its activation. These comprehensive structural and functional studies provide critical insights into the mechanism of dsDNA sensing through the cGAS-STING pathway. Moreover, our recent studies reveal potent antiviral and antitumor activities of cGAMP, suggesting a novel approach to treat viral diseases and cancer.