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, the p53 pathway and the steroid hormone signaling pathway. In vitro, binds A/T-rich DNA (By similarity). The effects of this transcriptional coregulation, transactivation or silencing, may vary depending upon the biological context. Sumoylates PML (at'Lys-65' and 'Lys-160') and PML-RAR and promotes their ubiquitin-mediated degradation. PIAS1-mediated sumoylation of PML promotes its interaction with CSNK2A1/CK2 which in turn promotes PML phosphorylation and degradation. Enhances the sumoylation of MTA1 and may participate in its paralog-selective sumoylation. Plays a dynamic role in adipogenesis by promoting the SUMOylation and degradation of CEBPB (PubMed:24061474). 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 NR2C1; the interaction promotes its sumoylation. Interacts with DDX21, CSRP2, AXIN1, JUN, SATB2, PLAG1, TP53 and STAT1 (dimer), following IFNA1-stimulation. Interacts with SP3 (preferentially when SUMO-modified). 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 CHUK/IKKA; this interaction induces PIAS1 phosphorylation. Interacts with PTK2/FAK1; the interaction promotes its sumoylation (By similarity). Interacts with SUMO1, UBE2I, NCOA2 and AR. Interacts with NR2C1; the interaction promotes its sumoylation. Interacts with DDX5. Interacts with MTA1 (By similarity). Interacts with PML (isoform PML-12). Interacts with PRDM1 (By similarity). Interaction with CSRP2 may induce a partial redistribution along the cytoskeleton. 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 (PubMed:10854042). In 3T3-L1 cells, expression is transiently induced during late adipocyte differentiation (PubMed:24061474). The LXXLL motif is a transcriptional coregulator signature. The SP-RING-type domain is required for promoting EKLF sumoylation. Sumoylated. Belongs to the PIAS family.