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  • Y-27632 In the absence of ligand RNA CARD


    In the absence of ligand RNA, 2CARD of RIG-I is masked by the intramolecular interaction with the helicase domain, showing auto-repressed state [33], [34]. But upon viral RNA binding, 2CARD of RIG-I is exposed to interact with the CARD domain of MAVS (also known as Cardif, IPS-1 or VISA) on mitochondria [4], [35], [36], [37]. The CARD-CARD interaction between RLRs and MAVS leads to the conformational change of MAVS, converting MAVS on the outer membrane of mitochondria to form a prion-like structure and activate the downstream signaling [38]. The prion-like structure of MAVS recruits the TRAF2, 5, and 6 to activate kinases TBK1 and IKK complex [39]. Besides these TRAFs, the previous study also demonstrated that TRAF3 is essential for type I IFN production by interacting with MAVS [40]. These TRAF proteins play a redundant role, but deletion of TRAF2, 3, 5, and 6 at the same time absolutely abolishes MAVS-mediated signaling [41]. MAVS-TRAFs complex associates with TBK1/IKKε for their activation. The activated TBK1/IKKε phosphorylate and activate IRF3 to trigger transcriptional activation of type I IFN. In addition, MAVS Y-27632 also activate the classical IKK complex, ultimately eliciting the production of proinflammatory cytokines as aforementioned.
    cGAS-STING signaling In the past years, several proteins such as AIM2, DAI, RNA polymerase III, IFI16, DDX41 have been reported to recognize microbial DNA and are regarded as CDSs [68]. However, these proteins are found to be important for sensing of various DNA pathogens in specific cell type or mouse models. Different from the DNA sensors above, cGAS has been identified to detect cytosolic microbial or endogenous aberrant DNA in various cell types [69]. cGAS contains an N-terminal regulatory domain (RD), a central nucleotidyltransferase (NTase) domain, and a C-terminal domain (CTD) [70]. In the absence of ligand DNA, cGAS exists in an autoinhibited state [71]. The binding of cGAS with DNA results in a conformational change, allowing access of the nucleotide substrates including GTP and ATP into the active site and subsequent synthesis of cGAMP [70]. The cGAMP acts as a second messenger and binds to the endoplasmic reticulum (ER)-membrane adaptor protein STING [72]. The interaction with cGAMP leads to a conformational change and activation of STING [73]. STING then traffics from ER to an ER-Golgi intermediate compartment and the Golgi apparatus [74]. During this process, STING recruits and activates the kinase TBK1 and IKK complex, eventually resulting in the induction of IFN and NF-κB response, respectively [5].
    Common downstream molecules in the antiviral signaling pathway
    Conclusions and perspectives
    Conflicts of interest None declared.
    Introduction Protein ubiquitination and degradation through the ubiquitin proteasome system (UPS) are conserved mechanisms in prokaryotic and eukaryotic organisms, and disruption of these processes is associated with the occurrence of many diseases, including cancer.1, 2, 3 Protein ubiquitination requires three critical enzymes: ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s), and ubiquitin ligases (E3s).4, 5 The human genome encodes at least 2 E1s, 38 E2s, and nearly 1,000 E3s, which further recognize thousands of substrates for modification in different biological processes.6, 7 Currently, four major classes of E3 ligases, including homologs to E6-AP carboxyl terminus (HECT), RING-finger, U-box, and plant homeodomain (PHD) finger, have been identified.4, 8 Of these, the RING-finger E3 ligases represent the largest family, which can be further classified into different subgroups, such as the anaphase-promoting complex (APC), cullin-RING ubiquitin ligases (CRLs), and the Skp1-cullin-F-box protein (SCF) complex, depending on the presence of different adapters.10, 11 CRLs, the largest class of RING-type E3 ligases, conservatively utilize seven cullins, including CUL1, CUL2, CUL3, CUL4A, CUL4B, CUL5, and CUL7, as scaffolds to facilitate the assembly of E3 ligase complexes and to transfer ubiquitin from E2 to the substrates.10, 12 Dysregulation of cullin members has been broadly reported to contribute to tumorigenesis through diverse mechanisms such as involvement in DNA damage and repair, cell-cycle progression, as well as participation in the ubiquitination of oncoproteins or tumor suppressors.13, 14 For example, CUL4A and CUL4B, two paralogous genes in the human genome that share 82% amino acid sequence identity, are highly expressed in different cancers, such as breast cancer, hepatocellular carcinoma, and osteosarcoma.16, 17 Both CUL4A and CUL4B have been shown to form CRL complexes with RING-box protein 1 (RBX1), DNA damage binding protein 1 (DDB1), and DDB1- and CUL4-associated factors (DCAFs).18, 19 CUL4-RBX is responsible for E3 ligase activities, whereas DCAFs are in charge of the specificity of substrates.18, 19 In the human genome, more than 100 DCAFs have been characterized according to the WD40 repeats that they contain. Through these DCAFs, the CRL4A-based E3 ligases are capable of recognizing a number of substrates, including DDB2, p21,22, 23 p27, CDT1 (chromatin licensing and DNA replication factor 1), Chk1 (checkpoint kinase 1), and STAT1 (signal transducer and activation of transcription 1), in different cancers. By contrast, although CUL4B is highly expressed in several cancer types, such as esophageal cancer and osteosarcoma,17, 27 the underlying mechanism of CUL4B overexpression and the specific substrates in these cancers are generally unknown.