Formin-mediated nuclear actin assembly at androgen receptors promotes transcriptional droplet formation

Abstract Steroid hormone receptors are ligand-binding transcription factors essential for mammalian physiology. The androgen receptor (AR) binds androgens mediating gene expression for sexual, somatic, and behavioral functions, and is involved in various conditions including androgen-insensitivity-syndrome (AIS) or prostate cancer. Here we identified functional AIS-patient mutations in the formin and actin nucleator DAAM2. DAAM2 localized to the nucleus and colocalized with the AR to form actin-dependent transcriptional droplets in response to dihydrotestosterone. DAAM2-AR-droplets ranged from 0.02 to 0.06 µm 3 in size and associated with active RNA polymerase II. DAAM2 polymerizes actin directly at the AR to promote droplet fusion in a highly dynamic manner and actin polymerization is required for prostate-specific antigen expression in cancer cells. Our data uncover signal-regulated nuclear actin assembly at a steroid hormone receptor necessary for transcription.