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. The effects of this transcriptional coregulation, transactivation or silencing, may vary depending upon the biological context. Enhances the sumoylation of MTA1 and may participate in its paralog-selective sumoylation. Sumoylates CCAR2 which promotes its interaction with SIRT1. Diminishes the sumoylation of ZFHX3 by preventing the colocalization of ZFHX3 with SUMO1 in the nucleus (By similarity). Protein modification; protein sumoylation. Binds SUMO1 and UBE2I. Interacts with AR, BCL11A, GFI1, HMGA2, IRF1, MITF, NCOA2, as well as with STAT3, after treatment with IL6, CNTF or OSM and with STAT5, after PRL stimulation. Interacts with PLAG1 (By similarity). Interacts with ZFHX3. 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. Colocalizes with GFI1 in nuclear dots. Colocalizes with SUMO1 in nuclear granules. Widely expressed, with highest levels in lung, kidney and spleen. The LXXLL motif is a transcriptional coregulator signature. Sumoylated. Belongs to the PIAS family. Has been shown to interact with voltage-gated potassium channels, including the KCNB1 subunit, and to be critical for current enhancement. However, in view of its mostly nuclear subcellular location and its established function as a transcriptional coregulator, promoting the sumoylation of several transcription factors, the effect on potassium channels awaits further experimental confirmation.