STING functions as both an adaptor protein signaling cytoplasmic double-stranded DNA and a direct immunosensor of cyclic diguanylate monophosphate (c-di-GMP). The crystal structures of the C-terminal domain of human STING (STING(CTD)) and its complex with c-di-GMP reveal how STING recognizes c-di-GMP. In response to c-di-GMP binding, two surface loops, which serve as a gate and latch of the cleft formed by the dimeric STING(CTD), undergo rearrangements to interact with the ligand.
"These results indicate that PLpro- TM interacts with the STING-TRAF3-TBK1 complex. Both PLpro-TM and STING are membrane-bound proteins that are anchored to the endoplasmic reticulum (ER) membrane with the transmembrane (TM) domain (Huang et al., 2012; Ouyang et al., 2012; Shang et al., 2012; Shu et al., 2012; Yin et al., 2012), and the TM domains at the N-terminus of STING were found to be indispensable for IRF3 activation in vitro (Tanaka and Chen, 2012). Our recent study demonstrated that coronavirus PLpros negatively regulate antiviral innate immune response by disruption of STING-mediated signaling (Sun et al., 2012). "
[Show abstract][Hide abstract] ABSTRACT: SARS coronavirus (SARS-CoV) develops an antagonistic mechanism by which to evade the antiviral activities of interferon (IFN). Previous studies suggested that SARS-CoV papain-like protease (PLpro) inhibits activation of the IRF3 pathway, which would normally elicit a robust IFN response, but the mechanism(s) used by SARS PLpro to inhibit activation of the IRF3 pathway is not fully known. In this study, we uncovered a novel mechanism that may explain how SARS PLpro efficiently inhibits activation of the IRF3 pathway. We found that expression of the membrane-anchored PLpro domain (PLpro-TM) from SARS-CoV inhibits STING/TBK1/IKKε-mediated activation of type I IFNs and disrupts the phosphorylation and dimerization of IRF3, which are activated by STING and TBK1. Meanwhile, we showed that PLpro-TM physically interacts with TRAF3, TBK1, IKKε, STING, and IRF3, the key components that assemble the STING-TRAF3-TBK1 complex for activation of IFN expression. However, the interaction between the components in STING-TRAF3-TBK1 complex is disrupted by PLpro-TM. Furthermore, SARS PLpro-TM reduces the levels of ubiquitinated forms of RIG-I, STING, TRAF3, TBK1, and IRF3 in the STING-TRAF3-TBK1 complex. These results collectively point to a new mechanism used by SARS-CoV through which PLpro negatively regulates IRF3 activation by interaction with STING-TRAF3-TBK1 complex, yielding a SARS-CoV countermeasure against host innate immunity.
Protein & Cell 03/2014; 5(5). DOI:10.1007/s13238-014-0026-3 · 3.25 Impact Factor
"Further biochemical and biophysical characterization of 2 0 ,5 0 cGAMP binding to STING should provide insight into these questions. Moreover, although crystal structures of STING bound to c-di-GMP have been reported (Huang et al., 2012; Ouyang et al., 2012; Shang et al., 2012; Shu et al., 2012; Yin et al., 2012), crystallographic analysis of STING bound to 2 0 ,5 0 cGAMP is technically challenging because STING forms a homodimer but the ligand is asymmetrical . Even though two structures of 2 0 ,5 0 cGAMP bound to STING have been reported (Gao et al., 2013b; Zhang et al., 2013), the electron density observed in these studies corresponds to the average electron density of 2 0 ,5 0 cGAMP in two distinct orientations. "
[Show abstract][Hide abstract] ABSTRACT: Cyclic GMP-AMP synthase (cGAS) is a cytosolic DNA sensor mediating innate antimicrobial immunity. It catalyzes the synthesis of a noncanonical cyclic dinucleotide, 2',5' cGAMP, that binds to STING and mediates the activation of TBK1 and IRF-3. Activated IRF-3 translocates to the nucleus and initiates the transcription of the IFN-β gene. The structure of mouse cGAS bound to an 18 bp dsDNA revealed that cGAS interacts with dsDNA through two binding sites, forming a 2:2 complex. Enzyme assays and IFN-β reporter assays of cGAS mutants demonstrated that interactions at both DNA binding sites are essential for cGAS activation. Mutagenesis and DNA binding studies showed that the two sites bind dsDNA cooperatively and that site B plays a critical role in DNA binding. The structure of mouse cGAS bound to dsDNA and 2',5' cGAMP provided insight into the catalytic mechanism of cGAS. These results demonstrated that cGAS is activated by dsDNA-induced oligomerization.
"Previous structural studies demonstrated that STING binds c-di- GMP through a dimeric configuration (Huang et al., 2012; Ouyang et al., 2012; Shang et al., 2012; Shu et al., 2012; Yin et al., 2012). Surprisingly, most of the reports did not show structural changes in STING upon ligand binding. "
[Show abstract][Hide abstract] ABSTRACT: The nucleotidyl transferase cGAS, its second-messenger product cGAMP, and the cGAMP sensor STING form the basic mechanism of DNA sensing in the cytoplasm of mammalian cells. Several new reports now uncover key structural features associated with DNA recognition by cGAS and the catalytic mechanisms of cGAMP generation. Concurrent studies also reveal unique phosphodiester linkages in endogenous cGAMP that distinguish it from microbial cGAMP and other cyclic dinucleotides. Together, these studies provide a new perspective on DNA recognition in the innate immune system.
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