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. 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 (By similarity). 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 (By similarity). Protein modification; protein sumoylation. Interacts with NCOA2 and AR. Interacts with NR2C1; the interaction promotes its sumoylation (By similarity). Interacts with DDX21, CSRP2, AXIN1, JUN, UBE2I, SUMO1, 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. Interacts with CHUK/IKKA; this interaction induces PIAS1 phosphorylation. Interacts with PTK2/FAK1; the interaction promotes its sumoylation. Interacts with DDX5. Interacts with PML (By similarity). Interacts with MTA1. Interacts with SUMO1P1/SUMO5 (PubMed:27211601). Interacts with PRDM1/Blimp-1 (PubMed:28842558). (Microbial infection) Interacts with ebolavirus VP35; this interaction mediates the sumoylation of IRF7 and contributes to the viral inhibition of IFN-type I production. Interaction with CSRP2 may induce a partial redistribution along the cytoskeleton. Expressed in numerous tissues with highest level in testis. 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. A paper showing that PRMT1-mediated arginine methylation of PIAS1 regulates STAT1 signaling has been retracted, because some of the data was found to be deliberately falsified.