Functions as an E3-type small ubiquitin-like modifier (SUMO) ligase, stabilizing the interaction between UBE2I and the substrate, and as a SUMO-tethering factor (PubMed:12511558, PubMed:12631292, PubMed:12727872, PubMed:15831457, PubMed:15976810, PubMed:22508508, PubMed:32832608). Mediates sumoylation of CEBPA, PARK7, HERC2, MYB, TCF4 and RNF168 (PubMed:12511558, PubMed:12631292, PubMed:12727872, PubMed:15831457, PubMed:15976810, PubMed:22508508). Plays a crucial role as a transcriptional coregulation in various cellular pathways, including the STAT pathway, the p53/TP53 pathway, the Wnt pathway and the steroid hormone signaling pathway (PubMed:11388671). Involved in gene silencing (PubMed:11248056). In Wnt signaling, represses LEF1 and enhances TCF4 transcriptional activities through promoting their sumoylations (PubMed:12727872, PubMed:15831457). Enhances the sumoylation of MTA1 and may participate in its paralog-selective sumoylation (PubMed:21965678). Binds to AT-rich DNA sequences, known as matrix or scaffold attachment regions (MARs/SARs) (By similarity). Catalyzes conjugation of SUMO2 to KAT5 in response to DNA damage, facilitating repair of DNA double-strand breaks (DSBs) via homologous recombination (HR) (PubMed:32832608). 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 sumoylation. Interacts with AR, AXIN1, GATA2, LEF1, TP53 and STAT1 (IFNG-induced) (PubMed:11248056, PubMed:11388671, PubMed:11439351, PubMed:12223491, PubMed:12750312). Interacts with TICAM1 (PubMed:15251447). Interacts with KLF8; the interaction results in SUMO ligation and repression of KLF8 transcriptional activity and of its cell cycle progression into G(1) phase (PubMed:16617055). Interacts with MTA1 (PubMed:21965678). Interacts with PRDM1/Blimp-1 (PubMed:28842558). Interacts with TRIM32 upon treatment with UVB and TNF-alpha (By similarity). Colocalizes with SUMO1 and TCF7L2/TCF4 and LEF1 in a subset of PML (promyelocytic leukemia) nuclear bodies. Highly expressed in testis and, at lower levels, in spleen, prostate, ovary, colon and peripheral blood leukocytes. The LXXLL motif is a coregulator signature that is essential for transcriptional corepression. Sumoylated. Lys-35 is the main site of sumoylation. Sumoylation is required for TCF4 sumoylation and transcriptional activation. Represses LEF1 transcriptional activity. SUMO1 is the preferred conjugate. Belongs to the PIAS family.