Sumoylation of a small isoform of NFATc1 is promoted by PIAS proteins and inhibits transactivation activity.

The NFAT family of transcription factors plays an important role in immune system development and function. NFATc1 and NFATc2 are highly expressed in peripheral T cells, and several isoforms are produced via the use of different promoters and polyadenylation sites. The specific isoforms with relatively long C-termini, NFATc1/C and NFATc2/A, have been shown to be modified by SUMO within their specific C-terminal regions, which regulates NFAT protein localization and transactivation activity. Here, we demonstrate that an isoform NFATc1/A, which has a short C-terminus and does not contain the sumoylation sites found in the long isoforms, is also modified by SUMO. NFATc1/A sumoylation increased with low level expression of SUMO E3 ligases, specifically PIAS1, PIAS3, and PIASy, in co-transfected cells. PIAS3 interacted with NFATc1/A and an active site mutant failed to promote NFATc1/A sumoylation, indicating a role for PIAS3 as a SUMO E3 ligase. A lysine residue at 351 within the central regulatory domain was identified as the major SUMO attachment site in both co-transfection and in vitro assays. Sumoylation of NFATc1/A did not affect nuclear translocation upon ionomycin and phorbol 12-myristate 13-acetate treatment. However, although sumoylation of NFATc1/A slightly increased protein stability, it inhibited transactivation activity for reporter genes driven by promoters containing NFAT sites. Our results indicate that the transactivation activity of NFATc1/A is negatively regulated by PIAS protein-mediated sumoylation, and that SUMO is a general regulator of NFAT family members with either long or short C-termini.