Figure 4.

Sec20 is essential for proper lysosomal biogenesis but not for autophagosome-lysosome fusion. (A,B) Starved fat cells expressing Lamp1-GFP and 3xmCherry-Atg8a. Colocalizing dots (arrows) are evident both in control (A) or Sec20 RNAi (B) cells, suggesting autophagosome-lysosome fusion proceeds normally in both cases. (A). (C,D) Lamp1-GFP-expressing fat cells were immunostained against CathL to detect lysosomes. In control cells (C), Lamp1-GFP clearly colocalizes with CathL (arrows), while their overlap is decreased upon Sec20 knockdown (D) suggesting a failure in lysosome biogenesis. (E,F) Quantification of the area (E) and number (F) of Lamp1-GFP dots showing a significant decrease in size but increase in number of autolysosomes. Medians are shown as horizontal black lines within the boxes. Bars show the upper and lower quartiles, and the whiskers plot the smallest and largest observations. (G,H) The red color of the compound eye of a control fly (G) becomes darker upon eye-specific expression of Sec20 RNAi (H) suggesting disturbed biogenesis of lysosome-related eye pigment granules. Combined GMR- and ey-Gal4-expressing flies in wild type background were used as controls (G) *** p < 0.001, scale bars: 10 μm.