![enroute 4 compartment enroute 4 compartment](https://i.ytimg.com/vi/2g7xrw3eUwc/hqdefault.jpg)
The remaining TGN38 and CI-MPR are delivered to the trans-Golgi. From the ERC, essentially all of the transferrin receptors recycle to the cell surface, and ∼85% of the internalized TGN38 and CI-MPR also return to the cell surface. Furin is retained in the sorting endosome as it begins to mature into a late endosome, and furin reaches the Golgi via the late endosomes. Most of the membrane proteins, including CI-MPR, rapidly exit this compartment and are either returned directly to the plasma membrane (PM) or are transported to the ERC.
![enroute 4 compartment enroute 4 compartment](https://i.ytimg.com/vi/RNkOckeDhqU/maxresdefault.jpg)
All of the membrane proteins concentrate into clathrin-coated pits, and the initial delivery site is sorting endosomes. The model compares the postendocytic itineraries of the CI-MPR (MR) and its enzyme ligands (E) with the transferrin receptor (TR), transferrin (Tf) with or without iron (Fe), the LDL receptor (LR), LDL, furin (F), and TGN38 (T). Schematic model of CI-MPR trafficking in CHO cells. These data demonstrate that CI-MPR takes a complex route that involves multiple sorting steps in both early and late endosomes. In CHO cells, only a small fraction of the receptor is ever detected in endosomes bearing substrates destined for lysosomes (kinetically defined late endosomes). Importantly, the receptor does not start to colocalize with late endosomal markers until after it has passed through the endocytic recycling compartment. Over the course of an hour, the endocytosed receptors achieve their steady-state distribution. A large fraction of the receptors return to the plasma membrane, but some are delivered to the trans-Golgi network and/or late endosomes. After endocytosis, the chimeric receptor first enters sorting endosomes, and it then accumulates in the endocytic recycling compartment. Detailed confocal microscopy analysis revealed that internalized chimeric CI-MPR overlaps almost completely with the endogenous CI-MPR but only partially with individual markers for the trans-Golgi network or other endosomal compartments. In this report, we describe the endocytic trafficking and steady-state localization of a chimeric form of the CI-MPR containing the ecto-domain of the bovine CI-MPR and the murine transmembrane and cytoplasmic domains expressed in a CHO cell line. Although the distribution of the cation-independent mannose 6-phosphate receptor (CI-MPR) has been well studied, its intracellular itinerary and trafficking kinetics remain uncertain.