E2-dependent E3 ubiquitin-protein ligase that functions as a RIGI coreceptor in the sensing of viral RNAs in cell cytoplasm and the activation of the antiviral innate immune response (PubMed:19017631, PubMed:19484123, PubMed:21147464, PubMed:23950712, PubMed:28469175, PubMed:31006531). Together with the UBE2D3, UBE2N and UB2V1 E2 ligases, catalyzes the 'Lys-63'-linked polyubiquitination of RIGI oligomerized on viral RNAs, an essential step in the activation of the RIG-I signaling pathway (PubMed:19017631, PubMed:21147464, PubMed:28469175, PubMed:31006531). Through a ubiquitin-independent parallel mechanism, which consists in bridging RIGI filaments forming on longer viral RNAs, further activates the RIG-I signaling pathway (PubMed:31006531). This second mechanism that synergizes with the ubiquitin-dependent one would thereby allow an RNA length-dependent regulation of the RIG-I signaling pathway (Probable). Associated with the E2 ligase UBE2N, also constitutively synthesizes unanchored 'Lys-63'-linked polyubiquitin chains that may also activate the RIG-I signaling pathway (PubMed:28469175, PubMed:31006531). {ECO:0000269|PubMed:19017631, ECO:0000269|PubMed:19484123, ECO:0000269|PubMed:21147464, ECO:0000269|PubMed:23950712, ECO:0000269|PubMed:28469175, ECO:0000269|PubMed:31006531, ECO:0000305|PubMed:31006531}. This is the function of RNF135 (ring finger protein 135, Ensembl gene identifier ENSG00000181481).