E3-ubiquitin ligase; acts as a negative regulator of cell proliferation through mechanisms involving G2/M arrest and cell death (PubMed:10500182, PubMed:12032852, PubMed:17016439). Required for MHC class I ubiquitination in cells expressing the cytomegalovirus protein US2 before dislocation from the endoplasmic reticulum (ER) (PubMed:19720873). Affects SREBP processing by hindering the SREBP-SCAP complex translocation from the ER to the Golgi, thereby reducing SREBF2 target gene expression (PubMed:19706601, PubMed:20068067). Involved in the sterol-accelerated degradation of HMGCR (PubMed:22143767, PubMed:23223569). This is achieved through binding of RNF139 to INSIG1 and/or INSIG2 at the ER membrane (PubMed:22143767). In addition, interaction of RNF139 with AUP1 facilitates interaction of RNF139 with ubiquitin-conjugating enzyme UBE2G2 and ubiquitin ligase AMFR, leading to ubiquitination of HMGCR (PubMed:23223569). The ubiquitinated HMGCR is then released from the ER into the cytosol for subsequent destruction (PubMed:22143767, PubMed:23223569). Required for INSIG1 ubiquitination (PubMed:20068067). May be required for EIF3 complex ubiquitination (PubMed:20068067). {ECO:0000269|PubMed:10500182, ECO:0000269|PubMed:12032852, ECO:0000269|PubMed:17016439, ECO:0000269|PubMed:19706601, ECO:0000269|PubMed:19720873, ECO:0000269|PubMed:20068067, ECO:0000269|PubMed:22143767, ECO:0000269|PubMed:23223569}. This is the function of ENSG00000170881 (RNF139, ring finger protein 139).