Rab GTPase signatures define different SC subcellular compartments. (A–D) SCs from 3-d-old males expressing different Rab-YFP constructs and stained with LysoTracker red. SCs have 5–10 small acidic Rab7-YFP–positive iLEs (A, arrowheads; most of these are usually located more apically than this confocal section; see Fig. 3) and 2.4 ± 0.9 (n = 11) Rab7-YFP–positive large vacuoles, 85% of which are acidic mMVBLs (A, asterisks; similar numbers [2.9 ± 0.5, n = 9] of large acidic compartments are seen in CD63-GFP–expressing SCs at this stage). Other Rab-YFP lines reveal Rab11-YFP–positive, nonacidic SVs (B, +; and D), many Rab5-YFP–positive small nonacidic compartments (C, arrowheads), and more rarely (4/16 SCs), a Rab5-YFP–positive small acidic compartment (C, arrows). In some fixed tub>Rab11-YFP SCs stained for Rab7, Rab7 colocalizes with Rab11-YFP in parts of the limiting membrane (D, arrow) of a single SV (D, #). DAPI stains nuclei (blue). Approximate outline of SC is marked in all images. Bars, 5 µm.