AIRE (autoimmune regulator) promotes the establishment of self-tolerance by regulating gene expression in the thymus. Mutations in AIRE lead to an autoimmune disease, APECED. Here we have identified PIAS proteins as novel AIRE interaction partners. Although PIAS proteins function as E3 SUMO ligases, AIRE is not sumoylated. We expressed AIRE, wt PIAS1, and PIAS1 mutants with deleted SP-RING domain or SUMO interaction motif (SIM) in different cell lines and demonstrate that AIRE and PIAS1 localize to adjacent nuclear bodies (NBs). The expression of AIRE enhances the formation of PIAS1 NBs. The ability of PIAS1 to localize into NBs and interconnect with AIRE is neither dependent on the SP-RING domain nor the SIM. Further, we show that PIAS1 is able to attract AIRE into SUMO1-containing complexes and that the process is dependent on the SIM of PIAS1. PIASI and AIRE concurrently activate the human insulin promoter, a known target gene of AIRE, and the SP-RING is required for this activation. Moreover, AIRE represses and PIAS1 activates the CSTB promoter, used as a model for a housekeeping promoter, and both the SP-RING and SIM are needed for its activation by PIAS1. Collectively, our data suggest that AIRE and PIAS1 interact functionally to regulate the activities of the target genes of AIRE. (C) 2007 Elsevier Ltd. All rights reserved.