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. Plays a crucial role as a transcriptional coregulation in various cellular pathways, including the STAT pathway and the steroid hormone signaling pathway. Repressor of STAT3 signaling via inhibiting STAT3 DNA-binding and suppressing cell growth. Repressor of MITF transcriptional activity. Enhances the sumoylation of MTA1 and may participate in its paralog-selective sumoylation. Sumoylates CCAR2 which promotes its interaction with SIRT1 (By similarity). Diminishes the sumoylation of ZFHX3 by preventing the colocalization of ZFHX3 with SUMO1 in the nucleus (By similarity). Protein modification; protein sumoylation. Monomer. Interacts with PLAG1 and ZFHX3. Interacts with STAT5A; the interaction occurs on stimulation by PRL (By similarity). Binds SUMO1 and UBE2I. Interacts with AR, BCL11A, HMGA2, IRF1 and NCOA2. Interacts with MITF; the interaction inhibits the transcriptional activity of MITF. Interacts with STAT3; the interaction occurs on stimulation by IL6, CNTF or OSM and inhibits the DNA binding activity of STAT3. Interacts with GFI1; the interaction relieves the inhibitory effect of PIAS3 on STAT3-mediated transcriptional activity. Interacts with MTA1. Interacts with CCAR2 (via N-terminus) (By similarity). Interacts with TRIM8 (By similarity). Interacts with PRDM1 (By similarity). Colocalizes with MITF in the nucleus (PubMed:11709556). Colocalizes with GFI1 in nuclear dots (PubMed:11060035). Colocalizes with SUMO1 in nuclear granules (PubMed:12077349). Expressed in kidney, heart, spleen, brain and cerebellum; weak expression, if any, in liver and lung. Expressed as early as 7.6 dpc. Expression remains high through 15.5 dpc. The LXXLL motif is a transcriptional coregulator signature. Sumoylated. Belongs to the PIAS family.