Estrogen receptor ␣ (ER␣) is a modular protein of the steroid/nuclear receptor family of transcriptional regulators that upon binding to the hormone undergoes structural changes, resulting in its nuclear translocation and docking to specific chromatin sites. In the nucleus, ER␣ assembles in multiprotein complexes that act as final effectors of estrogen signaling to the genome through chromatin remodeling and epigenetic modifications, leading to dynamic and coordinated regulation of hormoneresponsive genes. Identification of the molecular partners of ER␣ and understanding their combinatory interactions within functional complexes is a prerequisite to define the molecular basis of estrogen control of cell functions. To this end, affinity purification was applied to map and characterize the ER␣ interactome in hormone-responsive human breast cancer cell nuclei. MCF-7 cell clones expressing human ER␣ fused to a tandem affinity purification tag were generated and used to purify native nuclear ERcontaining complexes by IgG-Sepharose affinity chromatography and glycerol gradient centrifugation. Purified complexes were analyzed by two-dimensional DIGE and mass spectrometry, leading to the identification of a liganddependent multiprotein complex comprising -actin, myosins, and several proteins involved in actin filament organization and dynamics and/or known to participate in actin-mediated regulation of gene transcription, chromatin dynamics, and ribosome biogenesis. Time course analyses indicated that complexes containing ER␣ and actin are assembled in the nucleus early after receptor activation by ligands, and gene knockdown experiments showed that gelsolin and the nuclear isoform of myosin 1c are key determinants for assembly and/or stability of these complexes. Based on these results, we propose that the actin network plays a role in nuclear ER␣ actions in breast cancer cells, including coordinated regulation of target gene activity, spatial and functional reorganization of chromatin, and ribosome biogenesis. Molecular & Cellular Proteomics 9:1352-1367, 2010.Estrogens are potent tumor promoters for the mammary gland due to their growth-promoting actions in mammary epithelial cells (1). The mechanisms underlying stimulation of breast cell proliferation and control of the cell state by estrogens are still poorly defined despite the evident causal relationships between these hormonal actions and mammary gland carcinogenesis and cancer progression. Estrogen-responsive cells are endowed with specific estrogen receptors, ER␣ 1 and ER, members of the steroid/nuclear receptor superfamily of transcription factors that directly modulate the gene transcription rate (2). In addition, estrogens can trigger rapid and transient cellular responses through a mechanism(s) independent from this "genomic" pathway of steroid receptor action (3, 4). Such "extragenomic" effects include cell typespecific, rapid, and transient responses of signal transduction pathways; induction of intracellular calcium mobilization; and From the Departmen...