Structural and functional characterization of hMEX‐3C Ring finger domain as an E3 ubiquitin ligase

Abstract

MEX‐3C, a novel RNA binding E3 ubiquitin ligases, contains two N‐terminal heterogeneous nuclear ribonucleoprotein K homology (KH) domains and C‐terminal Ring finger domain. Recent evidence has suggested that human MEX‐3C has a strong bondage with carcinogenesis and the MEX‐3C‐mediated ubiquitination of RIG‐I is essential for the antiviral innate immune response. Moreover, the Ring finger domain of MEX‐3C could regulate the degradation of HLA‐A2 (an MHC‐I allotype) mRNA with a novel mechanism. However, the structural basis for the ubiquitination catalyzed by hMEX‐3C Ring finger domain remains evasive. In this study, we solved the crystal structure of dimeric Ring finger domain of hMEX‐3C and compared it with the complex structure of MDM2/MDMX–UbcH5b–Ub. Our ubiquitination assay demonstrated that the Ring finger domain of hMEX‐3C acts as a ubiquitin E3 ligase in vitro, cooperating with specific E2 to mediate ubiquitination. Then, we identified several key residues in Ring finger domain of hMEX‐3C possibly involved in the interaction with E2–Ub conjugate and analyzed the E3 ligase activities of wild type and mutants at key sites. Additionally, zinc chelation experiments indicated that the intact structural stability is essential for the self‐ubiquitination activity of the Ring finger domain of hMEX‐3C. Taken together, our studies provided new insight into the mechanism of the Ring finger domain of hMEX‐3C that may play an important role in eliciting antiviral immune responses and therapeutic interventions.

RNA-Binding Proteins as Regulators of Migration, Invasion and Metastasis in Oral Squamous Cell Carcinoma

“…Another aspect that should be considered in the design of future screening and targeting strategies is the dual functioning of some RBPs. For example, RBPs might not only act by modulating RNA metabolism, but might also bind to DNA and other proteins, or contain (pseudo)kinase and RING domains, which suggest non-canonical, potentially RNA-independent functions and novel options to modulate the activity of RBPs [ 231 , 232 , 233 , 234 ]. Furthermore, several RNA-dependent protein complexes that might comprise of direct and indirect RNA binders have been recently identified [ 235 ].…”

The RNA-Binding Ubiquitin Ligase MEX3A Affects Glioblastoma Tumorigenesis by Inducing Ubiquitylation and Degradation of RIG-I

“…In particular, alterations of MEX3A activity have been described in gastric, colorectal and bladder cancers, although its mechanism of action and its potential substrates remain still elusive [21][22][23][24]26]. To date, only the RING finger domain of human MEX3C has been functionally and structurally characterized, and Retinoic acid-inducible gene I (RIG-I) is the unique substrate described for this E3 ligase [27]. Interestingly, the ubiquitylation of RIG-I mediated by MEX3C leads to RIG-I activation, without interfering with its degradation and protein stability [28].RIG-I is a critical cytosolic pattern recognition receptor (PRR) that acts as RNA sensor to activate innate antiviral immunity and interferon (IFN) production [29].…”

Investigation on Potential Correlation Between the RNA-Binding Protein of Evolutionarily Conserved MEX3 Family and Non–Small-Cell Lung Cancer

“…In melanoma 41 , under-expression of MEX3B is associated with antibodies against the programmed cell death 1 (PD-1) receptor, while over-expression can inhibit T cell-mediated tumor elimination. MEX3C is involved in the regulation of proteins, degradation and ubiquitination, and it has been identi ed as a new type of RNA-binding E3 ubiquitin ligase, which is responsible for posttranscriptional regulation 42 . In present study, our results showed that MEX3B and MEX3C expression was positively related to tumor purity, and CD8 + T cell and CD4 + T cell in ltrating levels.…”