COMPARATIVE-ANALYSIS OF NFAT (NUCLEAR FACTOR OF ACTIVATED T-CELLS) COMPLEX IN HUMAN T-LYMPHOCYTE AND B-LYMPHOCYTE

Nuclear factor of activated T cells (NFAT) is a transcriptional activa tor that binds to sequences in the interleukin-2 (IL-2) promoter and i s thought to be largely responsible for the T cell-specific inducibili ty of IL-2 expression. Electrophoretic mobility shift assays (EMSA) sh owed that specific NFAT binding activity could also be induced in huma n B cells. The B cell NFAT complex, however, was not functional, since it failed to activate transcription from an NFAT-driven chloramphenic ol acetyltransferase (CAT) construct. Competition with an AP-1 motif o r with anti-Jun and anti-Fos antibodies abolished binding to the NFAT motif in both T and B cells, indicating that Jun and Fos are critical for NFAT complex formation in both cell types. Purified recombinant Ju n and Fos proteins failed to bind directly to the NFAT motif. However, when combined with unstimulated B or T cell extracts, full-length, bu t not truncated, Jun/Fos heterodimers were able to form an NFAT comple x, indicating the presence of a constitutively expressed nuclear facto r(s) in B and T cells necessary for the formation of the NFAT complex in both cell types. An NFAT oligonucleotide carrying mutations in the 5' purine-rich part of the NFAT sequence failed to form a complex and to compete with the wild type motif for NFAT complex formation in both T and B cells. We therefore propose a model whereby a core NFAT compl ex consisting of Jun, Fos, and a constitutive nuclear factor is formed in both T and B cells, but an additional factor and/or post-translati onal modification of a factor, missing in B cells, might be required f or transactivation by NFAT.