MFAP1 and PRPF38A segregate out of nuclear speckles and into droplet-like bodies adjacent to nuclear speckles after RNA pol II transcription inhibition. (A) Schematic depicting MFAP1/PRPF38A (green) before and after treatment with the RNA pol II transcription inhibitor DRB. (B and C) Maximum-intensity projections of optical sections of U2OS cells expressing GFP-MFAP1 (B) or GFP-PRPF38A (C, green; GFP-PR38A) treated with either DMSO (top panels) or DRB (bottom panels) for 2 h. Nuclear speckles (red, SON immunostaining) and DNA (blue, DAPI staining) are shown. Insets are magnifications of boxed areas. (D) SON immunostaining (magenta) of U2OS cells coexpressing GFP-PRPF38A (GFP-PR38A) and tagRFP-MFAP1 (RFP-MFAP1) treated with either DMSO or DRB. Boxed areas show magnified views (right panels). (E and F) Recovery of GFP-MFAP1 (E, green; Video 3) or GFP-PRPF38A (F, green; Video 4) back into nuclear speckles after release from DRB inhibition. Live cell imaging of U2OS cells coexpressing mCherry-SON after 2-h DRB treatment followed by rinsing with fresh growth media (t = 0). Times (1–45 min) are shown in the top left corner. Boxed areas are magnified (top panels) in grayscale. Scale bars: main panel, 10 µm; inset, 2 µm.