Concerted dephosphorylation of the transcription factor NFAT1 induces a conformational switch that regulates transcriptional activity

NFAT transcription factors are highly phosphorylated proteins that are regu lated by the calcium-dependent phosphatase calcineurin. We show by mass spe ctrometry that NFAT1 is phosphorylated on fourteen conserved phosphoserine residues in its regulatory domain, thirteen of which are dephosphorylated u pon stimulation. Dephosphorylation of all thirteen residues is required to mask a nuclear export signal (NES), cause full exposure of a nuclear locali zation signal (NLS), and promote transcriptional activity. An inducible pho sphorylation site in the transactivation domain contributes to transcriptio nal activity. Our data suggest that dephosphorylation promotes NFAT1 activa tion by increasing the probability of an active conformation, in a manner a nalogous to that by which depolarization increases the open probability of voltage-gated ion channels. This conformational switch paradigm may explain modification-induced functional changes in other heavily phosphorylated pr oteins.