摘要：

Rab5 and rab7 proteins belong to a superfamily of small molecular weight GTPases known to be associated with early and late endosomes, respectively. The rab5 protein plays an important regulatory role in early endocytosis, yet the function of rab7 protein was previously uncharacterized. This question was addressed by comparing the kinetics of vesicular stomatitis virus (VSV) G protein internalization in baby hamster kidney cells overexpressing wild-type or dominant negative mutant forms of the rab7 protein (rab7N125I and rab7T22N). Overexpression of wild-type rab7 protein allowed normal transport to late endosomes (mannose 6-phosphate receptor positive), while the rab7N125I mutant caused the VSV G protein to accumulate specifically in early (transferrin receptor positive) endosomes. Horseradish peroxidase and paramyxo-T HE late endosome represents the convergence point of numerous pathways. It is the delivery site of endocytosed material from early endosomes and of newly synthesized lysosomal proteins exiting the TGN. Endocytosed material (both fluid phase and receptor bound) is internalized in coated vesicles formed-at the plasma membrane, which then fuse with and deliver their contents to the early endosomes. Here, receptors and ligands are uncoupled by the acidic pH and molecules may either recycle to the plasma membrane or be transported to the perinuclear late endosome (Goldstein et al., 1985; Yamashiro et al., 1984). In addition, there is evidence to suggest that it is the meeting point of both the autophagic route, targeting organellar and cytosolic components for destruction, and the phagocytic pathway, operative in specialized cells for the uptake of large particles (Beron et al., 1995; Desjardins et al., 1994b; Olkkonen et al., 1993; Rabinowitz et al., 1992). Molecules exiting the late endosome can either recycle back to the TGN, with the most notable example being the mannose 6-phosphate receptor, or they can be routed to lysosomes (Griffiths et al., 1990;

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