Transport vesicle sorting mechanisms

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]. 

Le Borgne, R., and Hoflack, B. (1998a). Mechanisms of protein sorting and coat assembly insights from the clathrin-coated vesicle pathway. Curr. Opin. Cell Biol. 10, 499-503. Le Borgne, R., and Hoflack, B. (1998b). Protein transport from the secretory to the endocytic pathway in mammalian cells. Biochim. Biophys. Acta 1404, 195-209. 

Renal proximal tubular epithelium (e.g., convoluted tubules) reclaim protein, including immunoglobulin, from the urinary filtrate via sorting/recy-cling mechanisms in intracellular vesicles. The apical (lumen-facing) surface of these cells features a specialized brush border which expresses FcRn closely associated with (S2-microglobulin [110,116], Studies with cultured human renal proximal tubular epithelial cells have demonstrated bidirectional IgG transport and salvage by fully functional FcRn [110] as discussed further in the next section. 

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