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. Mediates sumoylation of CEBPA, PARK7, HERC2, MYB, TCF4 and RNF168. 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. Involved in gene silencing. In Wnt signaling, represses LEF1 and enhances TCF4 transcriptional activities through promoting their sumoylations. Enhances the sumoylation of MTA1 and may participate in its paralog-selective sumoylation (By similarity). Binds to AT-rich DNA sequences, known as matrix or scaffold attachment regions (MARs/SARs) (PubMed:11731474). Catalyzes conjugation of SUMO2 to KAT5 in response to DNA damage, facilitating repair of DNA double-strand breaks (DSBs) via homologous recombination (HR) (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 sumoylation. Interacts with AR, AXIN1, GATA2, TP53 and STAT1 (IFNG-induced) (By similarity). Interacts with LEF1 (PubMed:11731474). Interacts with TICAM1 (By similarity). 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 (By similarity). Interacts with MTA1 (By similarity). Interacts with PRDM1/Blimp-1 (By similarity). Interacts with TRIM32 upon treatment with UVB and TNF-alpha (PubMed:16816390). (Microbial infection) Interacts ewith Moloney murine leukemia virus Capsid protein p30. Colocalizes with SUMO1 and TCF7L2/TCF4 and LEF1 in a subset of PML (promyelocytic leukemia) nuclear bodies. Accumulates in the cytoplasm upon treatment with UVB and TNF-alpha. Widely expressed, with highest levels in testis. Also expressed in vascular endothelial cells, in primary keratinocytes and in the CNS, including cortex, olfactory bulb, spinal cord, thalamus and trigeminal ganglion. Low expression, if any, in liver and lung. At 8.5 dpc, expressed primarily in the anterior part of the neural tube. At 10.5 dpc, expressed in the neuroepithelium of the forebrain and hindbrain. At 11.5 dpc, detected in the neural tube, eye, limb buds and brachial arches. At 12.5 dpc, expressed in the hindlimbs and forelimbs, as well as in the forebrain. At 12.5 and 13.5 dpc, detected in single cells in the marginal zone of the developing cortex, as well as in other developing tissues and organs. At 13.5 dpc, expressed in the developing limb buds, in single cells in the mesenchyme surrounding future digit structures. At 15.5 dpc, detected in the inner root sheath of vibrissa hair follicle. Expression in the inner root sheath of the hair follicle continues later in life as it can also be detected in the back skin of newborn at postnatal day 3. At 16.5 dpc, expressed in the epithelium of olfactory and in the retina. 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. Ubiquitinated by TRIM32 upon treatment with UVB and TNF-alpha. Belongs to the PIAS family.