Mitogen-activated protein kinase-mediated growth factor signals are known to augment the ligand-induced transactivation function of nuclear estrogen receptor ␣ (ER␣) through phosphorylation of Ser-118 within the ER␣ N-terminal transactivation (activation function-1) domain. We identified the spliceosome component splicing factor (SF)3a p120 as a coactivator specific for human ER␣ (hER␣) activation function-1 that physically associated with ER␣ dependent on the phosphorylation state of Ser-118. SF3a p120 potentiated hER␣-mediated RNA splicing, and notably, the potentiation of RNA splicing by SF3a p120 depended on hER Ser-118 phosphorylation. Thus, our findings suggest a mechanism by which growth factor signaling can regulate gene expression through the modulation of RNA splicing efficiency via phosphorylation of sequencespecific activators, after association between such activators and the spliceosome.nuclear receptor ͉ estrogen ͉ coactivator ͉ mitogen-activated protein kinase ͉ RNA splicing M ost of the actions of estrogen are thought to be mediated via the transcriptional control of target genes by nuclear estrogen receptors (ER) ␣ and , members of the steroid hormone receptor gene superfamily that act as ligand-inducible transcription factors. ERs bind as dimers to specific estrogen response elements in the promoters of some target genes (1). However, most ER target promoters appear to recruit ERs without specific DNA binding, presumably through associations with sequence-specific factors bound to the promoters (2). ERs contain two transactivation functions (AFs), AF-1 in the Nterminal A͞B domain and AF-2 in the C-terminal ligand-binding E͞F domain. Ligand-induced transactivation by ERs requires multiple distinct classes of coactivator complexes, as well as a number of coregulators. The best-characterized complex contains p160͞SRC family proteins (3, 4) and CBP͞p300 histone acetyltransferases (5, 6), along with the RNA coactivator steroid receptor RNA activator (7) and presumably other known and unknown coactivators (8, 9). Another histone acetyltransferasescontaining complex, the TBP-free TAF II -containing (TFTC)-like complex (10), can also coactivate ER transactivation, as can the nonhistone acetyltransferases DRIP (VDR interacting protein)͞TRAP (thyroid hormone receptor-associated protein)͞ SMCC (SRB͞MED cofactor complexes)͞ARC (activatorrecruited cofactor) complex (11-13).ER-mediated estrogen signaling is known to involve cross-talk with other signaling pathways (14). For instance, growth factors potentiate estrogen-induced cellular proliferation in female reproductive tissues (15). Phosphorylation of the Ser-118 residue in the human ER␣ (hER␣) A͞B domain by mitogen-activated protein kinase (MAPK) activated by growth factors (16, 17) and cyclin-dependent kinase-7 (18) results in the potentiation of AF-1 function (16). However, the molecular basis of ER␣ AF-1 potentiation by MAPK-mediated phosphorylation remains unclear. In a previous study, we identified the DEAD-box RNA helicase subfamily member p68͞p72...