LGP2 acts as a natural negative regulator of dsRNA signaling. Several mechanisms may account for the LGP2 inhibitory effects. As a homolog of RIG-I, one attractive model is that LGP2 can sequester RNA ligands from recognition by RIG-I/MDA5. LGP2 has also been demonstrated to associate with RIG-I to inhibit its auto-oligomerizaton via the LGP2 C-terminal region comparable to the RIG-I repressor domain. In this model, dimerization of RIG-I by viral RNA, proposed to be an active form of RIG-I, is replaced by a RIG-I:LGP2 hetero-oligomer.
Probable ATP-dependent RNA helicase DDX58 (DDX58, RIG-I, RIG-1) has two caspase recruitment domains (CARD) in its N-terminus, a DExD/H helicase domain with an ATP binding motif in the middle and a repressor domain (RD) in the C-terminus. In the absence of appropriate stimulation, DDX58 is in a 'closed' conformation in which the repressor domain phyically interacts with the helicase domain masking CARD. Upon viral infection, the free triphosphate structure at the 5' end of viral RNAs activates DDX58 by binding to its RNA helicase domain. This provokes a change in DDX58 conformation exposing the CARD leading to DDX58 dimerization, allowing it to interact with Mitochondrial antiviral-signaling protein (MAVS, IPS-1).
ISG15 is an ubiquitin (Ub)-like protein which is conjugated to intracellular proteins via an isopeptide bond. Similar to ubiquitination, the conjugation of ISG15 (ISGylation) requires a three-step process, involving an E1 activating enzyme (UBE1L), an E2 conjugating enzyme (UbcM8/H8), and HERC5/Ceb1 an IFN-inducible ISG15-specific E3 ligase. ISG15 conjugation may play an important regulatory role in IFN-mediated antiviral responses. IFN induces ISG15 conjugation to DDX58 negatively regulating DDX58-mediated antiviral signaling. ISGylated DDX58 becomes subject to an irreversible biochemical process, such as proteolysis or proteasomeal degradation.