text: Characterization of the nuclear and cytoplasmic components of the lymphoid-specific nuclear factor of activated T cells (NF-AT) complex. 
  The lymphoid-specific transcription complex, NF-AT, is involved in early gene activation in T cells and is assembled from a pre-existing, T cell restricted cytoplasmic factor and an inducible ubiquitous nuclear component within 30 min after activation through the antigen receptor. Recent studies have implicated the family of AP1 factors as components of the murine NF-AT complex. Evidence is provided here that the nuclear component of human NF-AT contains the phorbol ester-inducible transcription factor AP1 (Jun/Fos). We further characterize which AP1 family members can assume this role. Antisera to Fos inhibits NF-AT DNA binding as does an oligonucleotide containing a binding site for AP1. Constitutive expression in vivo of Fos, and to a lesser extent Fra-1, eliminates the requirement for phorbol 12-myristate 13-acetate (PMA) stimulation, leaving NF-AT-directed transcription responsive to calcium ionophore alone. Overexpression of cJun or JunD, but not JunB, also eliminates the requirement for PMA, indicating that many but not all Jun- and Fos-related proteins functionally activate NF-AT-dependent transcription in the presence of the cytoplasmic component. NF-AT DNA binding can be reconstituted in vitro using semi-purified AP1 proteins mixed with cytosol from T lymphocytes. Fos proteins are not needed for this reconstitution, and although JunB is not functional, it can participate in the NF-AT DNA binding complex. Finally, we have partially purified the cytoplasmic component of NF-AT and show by elution and renaturation from SDS-polyacrylamide gel electrophoresis gels that it has a molecular mass between 94 and 116 kDa and may have multiple differentially modified forms. 