Regulation of RIG-I Activation by K63-Linked Polyubiquitination

Abstract

RIG-I is a pattern recognition receptor and recognizes cytoplasmic viral double-stranded RNA (dsRNA). Influenza A virus, hepatitis C virus, and several other pathogenic viruses are mainly recognized by RIG-I, resulting in the activation of the innate immune responses. The protein comprises N-terminal two caspase activation and recruitment domains (2CARDs), an RNA helicase domain, and the C-terminal domain (CTD). The CTD recognizes 5′-triphosphate viral dsRNA. After recognition of viral dsRNA, the protein harbors K63-linked polyubiquitination essential for RIG-I activation. First, it was reported that TRIM25 ubiquitin ligase delivered K63-linked polyubiquitin moiety to the 2CARDs. The polyubiquitin chain stabilizes a structure called the 2CARD tetramer, in which four 2CARDs assemble and make a core that promotes the aggregation of the mitochondrial antiviral-signaling (MAVS) protein on mitochondria. MAVS aggregation then triggers the signal to induce the innate immune responses. However, subsequent studies have reported that Riplet, MEX3C, and TRIM4 ubiquitin ligases are also involved in K63-linked polyubiquitination and the activation of RIG-I. MEX3C and TRIM4 mediate polyubiquitination of the 2CARDs. By contrast, Riplet ubiquitinates the CTD. The physiological significance of each ubiquitin ligases has been shown by knockout and knockdown studies, but there appears to be contradictory to evidence reported in the literature. In this review, we summarize recent findings related to K63-linked polyubiquitination and propose a model that could reconcile current contradictory theories. We also discuss the physiological significance of the ubiquitin ligases in the immune system against viral infection.

Mechanistic insights into RNA binding and RNA-regulated RIG-I ubiquitination by TRIM25

“…Several E3 ligases other than TRIM25 promote ubiquitination of RIG-I leading to controversial discussions about their relative importance (64)(65)(66)(67)(68)(69). Among these are several TRIM proteins (TRIM4, 15,40) and Riplet, a close relative of TRIM25, that lost the B-Box domains and parts of the CC (70)(71)(72).…”

Innate Immune Evasion by Human Respiratory RNA Viruses

“…About a decade ago, influenza virus NS1 was shown to bind E3 ligase TRIM25, thereby interfering with K63-linked ubiquitination of RIG-I, and therefore uniquely inhibiting innate immune signaling in the type-I IFN pathway [127]. There has been a recent debate as to whether these chains are actually conjugated to RIG-I or other factors within the cascade or whether they are free ubiquitin chains that provide a scaffold for activating the aggregation of RIG-I and MAVS, which in turn enables downstream signaling [128]. Influenza B virus-encoded NS1 additionally inhibits ISG15 antiviral activity by binding the N-terminus of human ISG15 (and not mouse ISG15) [129].…”

To TRIM or not to TRIM: the balance of host–virus interactions mediated by the ubiquitin system

“…To date, most studies have implicated TRIM25 as a critical mediator of innate immune signalling through ubiquitination of RIG-I [2,11,[101][102][103][104][105][106][107][108][109][110][111][112][113][114][115][116][117][118], while only a handful of studies have proposed RIG-I-independent mechanisms [46,119]. Recent years have brought both exciting discoveries and rousing controversies regarding how ubiquitin and E3 ligases precisely function to activate RIG-I for antiviral signalling, with some groups suggesting a single factor operates as the critical component while others have put forth a cooperative model based on trends gleaned from numerous studies.…”

“…Recent years have brought both exciting discoveries and rousing controversies regarding how ubiquitin and E3 ligases precisely function to activate RIG-I for antiviral signalling, with some groups suggesting a single factor operates as the critical component while others have put forth a cooperative model based on trends gleaned from numerous studies. For viruses that are sensed by the RIG-I PRR, initial recognition of vRNA and binding of the viral 5′ tri-and diphosphate PAMPs occurs via the RIG-I helicase and C-terminal domains (CTDs) [108,120,121]. Identification of vRNA triggers a conformational change in RIG-I, freeing up the N-terminal 2CARD domain which would otherwise be suppressed by the CTD and a linker region between the helicase and CTD.…”

“…The K63-linked ubiquitination of RIG-I by TRIM25 is well established, but additional E3 ubiquitin ligases such as Riplet, MEX3C and TRIM4 have been identified as playing critical roles in fully activating the RIG-I pathway (Fig. 5) [101,108]. Therefore, the fact that TRIM25 may be involved in different aspects of antiviral signalling does not exclude the possibility that TRIM25 and Riplet have redundant functions in the activation of RIG-I, and that one factor may be dominant over the other.…”