Spatial segregation of degradation- and recycling-trafficking pathways in COS-1 cells.

After endocytosis, most membrane proteins and lipids return to the plasma membrane (recycling pathway), but some membrane components are delivered to lysosomes (degradation pathway). These two pathways diverge in early endosomes. The recycling pathway involves recycling endosomes and the degradation pathway incorporates late endosomes and lysosomes. In many cell lines, these organelles often are located in the perinuclear region where they visually intermix. The present study, by tracking specific ligands (epidermal growth factor and transferrin) and expression of Rab proteins (Rab5, Rab7, and Rab11), demonstrated that, in COS-1 cells, the two pathways were spatially segregated. Recycling endosomes were mostly confined within the ring-shaped structure of the Golgi complex ("the Golgi ring"), whereas late endosomes and lysosomes were excluded from inside the Golgi ring. Thus, the unique organization of endocytic organelles in COS-1 cells can be utilized to visualize endocytic trafficking pathways in detail.