(A) Overview of an egg chamber and schematic representation of the follicular epithelium (FE) polarity domains and germline cells (GCs). Longitudinal (Lg) section through a wildtype egg chamber expressing Coll IV-GFP (green), stained for F-Actin (red) and DNA (blue). (B–E) Lg-sections through follicle cell (FC) layers expressing Coll IV-GFP (green, B, C) or Pcan-GFP (green, D, E), stained for F-Actin (red) and DNA (blue). (B) Control FE. (C–E) FE expressing two different RNAi constructs against strat (CG7787) with traffic jam-Gal4 (tj-Gal4). In strat knockdown FCs, Coll IV-GFP (C′) and Pcan-GFP (D′, E′) accumulate apically. (F) Strat protein schematic and amino acid sequence alignment between Drosophila Strat and human Mss4 (hMss4/RabIF). Strat protein contains an Mss4 domain (red). (G–H) Stratum is the Drosophila homolog of human Mss4. (G) Lg-sections through FC layers expressing Pcan-GFP IV, stained for F-Actin (red) and DNA (blue). FE expressing an RNAi construct against strat (CG7787) and hMss4 with traffic jam-Gal4 (tj-Gal4). (H) The expression of hMss4 in strat-knocked down FCs leads to a decrease in the number of egg chambers exhibiting apical mislocalization of BM proteins, indicating a partial rescue of the phenotype observed in strat RNAi FCs. n≥100; ****, p<0.0001. (I–J) Lg-section through egg chambers containing strat mutant (I, stratMF13; J, stratMC2) FC clones expressing Pcan-GFP (green) and stained for F-Actin (red) and DNA (blue). strat mutant FCs are marked by the absence of intracellular GFP (green, dashed lines indicate clonal boundaries; asterisks (*) specify homozygous mutant FCs). In strat mutant cells, the polarized distribution of BM is disrupted, as revealed by the strong accumulation of Pcan-GFP at the apical side of FCs (arrowheads, I′ and J′). Bars, 10 μm.