Abstract

NFAT proteins constitute a family of transcription factors involved in mediating signal transduction. Using a panel of specific antisera in immunoprecipitation assays, we found that NFATp (135 kDa) is constitutively expressed in normal human T cells, while synthesis of NFATc (predominant form of 86 kDa) is induced by ionomycin treatment. NFAT4/x was very weakly expressed in unstimulated cells, and its level did not increase upon treatment with activating agents. NFAT3 protein was not observed under any conditions. Higher-molecular-weight species of NFATc (of 110 and 140 kDa) were also detected. In addition, translation of NFATc mRNA apparently initiates at two different AUG codons, giving rise to proteins that differ in size by 36 amino acids. Additional size heterogeneity of both NFATc and NFATp results from phosphorylation. In contrast to ionomycin treatment, exposure of cells to phorbol myristate acetate (PMA) plus anti-CD28 did not induce NFATc, indicating that under these conditions, interleukin-2 synthesis by these cells is apparently independent of NFATc. In DNA binding assays, both PMA plus anti-CD28 and PMA plus ionomycin resulted in nuclear NFAT. Surprisingly, the PMA-ionomycin-induced synthesis of NFATc that was detected by immunoprecipitation was not mirrored in the DNA binding assays: nearly all of the activity was due to NFATp. This is the first study of expression of all family members at the protein level in normal human T cells.