E3 ubiquitin-protein ligase that mediates ubiquitination and subsequent proteasomal degradation of target proteins (PubMed:11786535). E3 ubiquitin ligases accept ubiquitin from an E2 ubiquitin-conjugating enzyme in the form of a thioester and then directly transfers the ubiquitin to targeted substrates (PubMed:11786535). Mediates E3 ubiquitin ligase activity either through direct binding to substrates or by functioning as the essential RING domain subunit of larger E3 complexes (By similarity). Mediates ubiquitination and proteasomal degradation of DYRK2 in response to hypoxia (By similarity). Promotes monoubiquitination of SNCA (By similarity). Triggers the ubiquitin-mediated degradation of many substrates, including proteins involved in transcription regulation (GPS2, POU2AF1, PML, NCOR1), a cell surface receptor (DCC), an antiapoptotic protein (BAG1), and a protein involved in synaptic vesicle function in neurons (SYP) (PubMed:11786535). It is thereby involved in apoptosis, tumor suppression, cell cycle, transcription and signaling processes (By similarity). Has some overlapping function with SIAH1 (By similarity). Triggers the ubiquitin-mediated degradation of TRAF2, whereas SIAH1 does not (By similarity). Regulates cellular clock function via ubiquitination of circadian transcriptional repressors NR1D1 and NR1D2 leading to their proteasomal degradation. Plays an important role in mediating the rhythmic degradation/clearance of NR1D1 and NR1D2 contributing to their circadian profile of protein abundance (By similarity). Mediates ubiquitination and degradation of EGLN2 and EGLN3 in response to the unfolded protein response (UPR), leading to their degradation and subsequent stabilization of ATF4 (By similarity). Also part of the Wnt signaling pathway in which it mediates the Wnt-induced ubiquitin-mediated proteasomal degradation of AXIN1 (By similarity). S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine + [acceptor protein]-L-lysine = [E2 ubiquitin-conjugating enzyme]-L-cysteine + N(6)-ubiquitinyl-[acceptor protein]-L-lysine. Protein modification; protein ubiquitination. Homodimer (By similarity). Interacts with VAV1, without mediating its ubiquitin-mediated degradation (By similarity). Probable component of some large E3 complex possibly composed of UBE2D1, SIAH2, CACYBP/SIP, SKP1, APC and TBL1X (By similarity). Interacts with UBE2I (By similarity). Interacts with UBE2E2 (PubMed:11786535). Interacts with PEG10, which may inhibit its activity (By similarity). Interacts with PEG3 and EGLN2 (By similarity). Interacts with DYRK2 (By similarity). Interacts with SNCAIP (PubMed:15064394, PubMed:19224863). Interacts with NR1D1 and NR1D2 (By similarity). Interacts with DCC (By similarity). Interacts with AXIN1. Predominantly cytoplasmic. Partially nuclear. Detected in brain (at protein level). The RING-type zinc finger domain is essential for ubiquitin ligase activity. The SBD domain (substrate-binding domain) mediates the homodimerization and the interaction with substrate proteins. It is related to the TRAF family. Phosphorylated at Thr-24 and Ser-29 by MAPK14, which mediates the degradation by the proteasome of EGLN3. Phosphorylated at Ser-29 by DYRK2; this increases the ubiquitin ligase activity and promotes degradation of EGLN3 (By similarity). Belongs to the SINA (Seven in absentia) family.