Characterization of a new isoform of the NFAT (nuclear factor of activated T cells) gene family member NFATc

The cyclosporin A (CsA)/FK506-sensitive nuclear factor of activated T cells (NFAT) plays a key role in the inducible expression of cytokine genes in T cells. Although NFAT has been recently shown to be inducible in several non-T immune cells, the NFAT gene family members characterized to date have been isolated only from T cells. To further characterize NFAT function in human B cells and to demonstrate cytokine gene specificity of NFAT proteins, we report here the isolation and characterization of a cDNA clone from the Raji B cell line. The cDNA clone encodes a new isoform, NFATc-β, of the NFAT gene family member NFATc (designated here NFATc.α). The amino acid sequence of NFATc.β differs from that of NFATc.α in the first NH₂-terminal 29 residues and contains an additional region of 142 residues at the COOH terminus. Northern analysis using a probe encompassing a common region of both isoforms showed two mRNA species of 2.7 and 4.5 kilobase pairs, while an NFATc.β-specific probe detected only the 4.5-kilobase pair mRNA which was preferentially expressed in the spleen. Transient expression of NFATc.β was capable of activating an interleukin-2 NFAT-driven reporter gene in stimulated Jurkat cells in a CsA-sensitive manner. However, NFATc.β neither bound to the κ3 element (an NFAT-binding site) in the tumor necrosis factor- α promoter nor activated the tumor necrosis factor-α promoter in cotransfection assays. These data suggest that different members or isoforms of NFAT gene family may regulate inducible expression of different cytokine genes.