EVIDENCE FOR RETROGRADE TRAFFIC BETWEEN TERMINAL LYSOSOMES AND THE PRELYSOSOMAL LATE ENDOSOME COMPARTMENT

We have investigated the interactions occuring between the prelysosoma l compartment, PLC/late endosome, and terminal lysosomes using an appr oach that allowed us to internalize and deliver specific tracers to th ese compartments, and look for evidence of their meeting. After intern alization of sucrose, the lysosomes (sucrosomes), but not the PLC/late endosomes, became significantly swollen and visible in the light micr oscope. If invertase is then added to the medium it reaches the lysoso mes where it cleaves sucrose into fructose and glucose. These sugars, unlike sucrose, can be transported into the cytosol, resulting in the disappearance of the sucrosomes. We previously showed that phagocytose d latex beads are delivered specifically to, and reside in, the PLC/la te endosome, a stage earlier than the lysosomes in the endocytic pathw ay (Rabinowitz et al. (1992) J. Cell Biol. 116, 95-112). In the presen t study, we demonstrate that invertase conjugated to the latex beads, and thus immobilized in late endosomes, has access to the sucrose pres ent in the more distal lysosomes. Experiments using nocodazole indicat e that this retrograde fusion event requires the presence of an intact microtubule network. The simplest interpretation of our results is th at the two compartments fuse, allowing for a retrograde transport of s ucrose from the lysosomes to the PLC/late endosomes.