E3 ubiquitin-protein ligase which is a component of the N-end rule pathway (PubMed:26383956, PubMed:27195754). Recognizes and binds to proteins bearing specific N-terminal residues, leading to their ubiquitination and subsequent degradation (PubMed:26383956, PubMed:27195754). Binds to the E3 ubiquitin-protein ligase Diap1 and enhances its ubiquitination and anti-apoptotic functions (PubMed:25146930). Essential during trichome development for the ubiquitination of the N-terminus of ovo isoform B (svb), converting it from a transcriptional inhibitor to an activator (PubMed:26383956). Positively regulates a hh-signaling pathway which functions in photoreceptor differentiation (PubMed:27195754). Activation of hh up-regulates transcription of Ubr3, which in turn promotes hh signaling by mediating the ubiquitination and degradation of cos (PubMed:27195754). Necessary for auditory transduction: plays a role in Johnston's organ organization by acting in the regulation of zip and ck function in scolopidial apical attachment (PubMed:27331610). Likely to function by acting in a pathway that negatively regulates the ubiquitination of zip, consequently affecting its interaction with ck (PubMed:27331610). May also negatively regulate a component of the SCF (SKP1-CUL1-F-box protein) E3 ubiquitin-protein ligase complex Cul1, which also appears to function in the negative regulation of the zip-ck interaction and scolopidial apical attachment (PubMed:27331610). 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. Selectively interacts (via UBR-type zinc finger) with the cleaved form of Diap1; this interaction is enhanced by tal (PubMed:25146930, PubMed:26383956). Interacts with tal and Rrp1 (PubMed:26383956). Interacts with ovo isoform B (via N-terminus) (PubMed:26383956). Interacts with Cad99C (via the cytoplasmic domain) (PubMed:27331610). Interacts with ck and Sans (PubMed:27331610). Interacts with cos (via Kinesin motor domain) (PubMed:27195754). In Johnston's organ, high expression at the apical tips of neurons and scolopale cells (PubMed:27331610). In the cytoplasm expressed in puncta (PubMed:27195754). Ubiquitously expressed in the second and third segments of the pupal antenna (at protein level) (PubMed:27331610). In the eye imaginal disks of 3rd instar larvae, expression is highest in the morphogenetic furrow (PubMed:27195754). The UBR-type zinc finger domain is necessary for interaction with Diap1 and anti-apoptotic activity (PubMed:25146930). Domain is also sufficient for interaction with cos (PubMed:27195754). The RING-type zinc finger domain is not necessary for interaction with Diap1 and anti-apoptotic activity. In vitro, self-ubiquitination in the presence of E1, E2 and ubiquitin. RNAi-mediated knockdown in the eye results in small rough eyes. Belongs to the UBR1 family.