Intracellular sorting endocytosis

Fig. 3.3 Intracellular sorting in epithelial cells after clathrin-mediated endocytosis. AEE apical early endosome ARE apical recycling endosome CE common endosome ...

Moisenovich, M., Tonevitsky, A., Agapov, I., Niwa, H., Schewe, H. and Bereiter-Hahn, J. (2002) Differences in endocytosis and intracellular sorting of ricin and viscumin in 3T3 cells. Eur J Cell Biol, 81, 529-538. 

Webster, P. and Fish, W. R. (1989) Endocytosis by African trypanosomes. II. Occurrence in different life-cycle stages and intracellular sorting. Eur. J. Cell Biol. 49 303-310. 

Lipids are transported between membranes. As indicated above, lipids are often biosynthesized in one intracellular membrane and must be transported to other intracellular compartments for membrane biogenesis. Because lipids are insoluble in water, special mechanisms must exist for the inter- and intracellular transport of membrane lipids. Vesicular trafficking, cytoplasmic transfer-exchange proteins and direct transfer across membrane contacts can transport lipids from one membrane to another. The best understood of such mechanisms is vesicular transport, wherein the lipid molecules are sorted into membrane vesicles that bud out from the donor membrane and travel to and then fuse with the recipient membrane. The well characterized transport of plasma cholesterol into cells via receptor-mediated endocytosis is a useful model of this type of lipid transport. [9, 20]. A brain specific transporter for cholesterol has been identified (see Chapter 5). It is believed that transport of cholesterol from the endoplasmic reticulum to other membranes and of glycolipids from the Golgi bodies to the plasma membrane is mediated by similar mechanisms. The transport of phosphoglycerides is less clearly understood. Recent evidence suggests that net phospholipid movement between subcellular membranes may occur via specialized zones of apposition, as characterized for transfer of PtdSer between mitochondria and the endoplasmic reticulum [21]. 

Figure 11.1 The intracellular trafficking pathway of plasmid DNA complexed by poly cationic lipid (lipoplex). Critical steps are indicated by numbers (1) endocytosis, sorting and recycling via vesicular compartments comprising the early (EE) and sorting endosomes (2) entrapment and degradation in the late-endosomes (LE) and lysosomes (3) destabilization of the endo-lysosomal membrane and release into the cytosol, (the precise location of this step is not known) (4) diffusion toward the nuclear pore complex (NPC) and degradation in the cytoplasm, and (5) nuclear translocation across the NPC.

Figure 9.1. Receptor-mediated endocytosis. Exogenous ligands (yellow ovals) bind specifically to their cell surface receptors. The plasma membrane invaginates around the ligand-receptor complexes to form an intracellular vesicle (endosome). As the lumen of the maturing endosome acidifies, the receptor often releases the ligand. Eventually, the fates of both the ligand and the receptor are determined during a sorting process within late endosomal compartments.

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