Figure 5.

The internalized HRP-Boss chimera labels typical early and late endocytic structures in R7 cells. Internalized HRP-Boss was identified in electron micrographs by its HRP activity. Adjacent to the R8–R7 cell interface, HRP activity could be detected in vesicles with an internal vesicle (A), likely to represent patches of R8 cell surface pulled into R7 cells by the Boss–Sevenless interaction. Inside R7 cells, HRP-Boss was detected in all the typical endocytic structures: tubular (B) and tubular-vacuolar (C) large, mature MVBs (D and E), and multilammelar late endosomes or lysosomes (G). Structures of character intermediate to those of MVBs and multilammelar endosomes (F) were consistent with the previously described direct fusion between these compartments (Futter et al., 1996). Bar: F, 120 nm; A–E and G, 200 nm.