Fig. 2. Ligand diffusion and internalization leads to STAT nuclear localization in S2 cells. (A-C) Cells expressing either Upd-GFP or Dome were mixed and cultured together for 0.5-4 hours. The 0 hour time-point corresponds to cells transfected with Dome only. Upd-GFP diffuses in the medium and binds to Dome-expressing cells in a time-dependent manner. Cells that do not express Dome are outlined by broken lines. Upd-GFP is first found at the membrane of Dome-expressing cells, and (B) becomes internalized in Dome-containing vesicles (arrows) after 6 hours. (C) Binding of Upd-GFP leads to the nuclear accumulation of Stat in a Dome- and time-dependent manner. Dome-expressing cells in C are stained with a -galactosidase marker (purple). Bars, 10 µm.