Membrane vesicle shuttle

Transport vesicle shuttles lipids and proteins between ER, Golgi, and plasma membrane... 

In the membrane flow model of Fig. 1-7, membranes move through the cell from ER—> Golgi apparatus— secretory vacuoles— plasma membrane. In the membrane shuttle proposal, the vesicles shuttle between ER and Golgi apparatus, while secretory vacuoles shuttle back and forth between the Golgi apparatus and the plasma membrane. 

The presently known mammalian AQP0-AQP12 have been localized in tissues involved in fluid transport as well as in nonfluid-transporting tissues (Table 1). Most AQPs are constitutively present in the plasma membrane, whereas some water channels can be triggered to shuttle between intracellular vesicles and the plasma membrane [2]. 

In contrast, receptor-mediated endocytosis is very specific. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer consisting of a protein called clathrin. When ligands bind to the receptor sites, they are carried into the cell by the inward budding of a coated pit to form a coated vesicle. Clathrin-coated vesicles become uncoated and fuse to form an endosome. Ligand and receptor dissociate within the endosome, and the receptor shuttles back to the cell surface. The endosome fuses with the lysosome on which ligand degradation occurs. 

FIGURE 10.25 Utilization of transferrin and its receptor for delivering iron into the cell. The iron/transferrin complex circulates throughout the bloodstream. Eventually, the iron/transferrin complex binds to the transferrin receptor, a membrane-bound protein (Step 1). Part of the plasma membrane pinches off, creating an endocytotic vesicle, which resides in the cytoplasm (Step 2). The interior of the vesicles becomes acidified (Step 3), and the iron atoms leave the vesicle (Step 4). MobUferrin is a cytosolic protein that is thought to bind the released iron atoms, and to shuttle them to newly synthesized iron metalloproteins (Conrad et al, 1996). Finally, the transferrin receptor is inserted back into the plasma membrane (Steps 5 and 6). 

Fig. 19 a Schematic (not drawn to scale) of polymerized electroactive vesicles tethered to a gold electrode via mercaptoethyl tethers. Electrons flow reversibly from the electrode to the entrapped electron acceptor (Fe(CN)g-) shuttled by an electron mediator (BQ) which also transports protons to maintain electrical neutrality, b Chemical structures of polymerizable DCg q PC (major component) and PDP-DCs PE (minor component) lipids and electroactive components incorporated within the vesicle core (Fe(CN)g-) and membrane (BQ). Reprinted with permission from [109]. Copyright 2004, American Chemical Society... 

Finally, a protein has been identified that is unique from the previously discussed ABC proteins. This protein, called lung resistance protein or major vault protein, has a molecular weight of 110 kD and is found not in the cell membrane but rather the nuclear membrane or the cytoplasm. The role of this protein is to prevent the drug from accessing the nucleus by trapping the drug in vesicles called vaults. The vaults then shuttle... 

DHA is very abundant in excitable membranes in the retina and brain, particularly in PL of the rod outer segment of retina and of synaptic vesicles, and is important in vision. However, the mechanism by which DHA functions in retina is not well understood. Chen et al. (Y. Chen, 1993) suggest that DHA in retina might be involved in shuttling 11-c/j-retinal to photoreceptors, whereas Niu et al. (S. Niu, 2004) propose that DHA in PL increases the efficiency of G-protein-mediated signal transduction of rhodopsin. In humans, supplementation of infant formula with DHA accelerates the development of visual functions in pre-term infants. The novel protective lipid mediator docosanoids, namely, Protectin D1 (C. Serhan, 2002) and 17S-hydroperoxy-DHA (V. Marcheselli, 2003), have been suggested to mediate the beneficial effects of DHA. 

The localization of enzymes of the oxylipin pathway has yet to be unequivocally elucidated. Nonetheless, LOX has been localized in plastids, vacuoles and the cytoplasm, e.g. [1], and in lipid bodies [36,37]. Since enzymes of the jasmonic acid biosynthetic pathway are thought to be localized mainly in plastids, mechanisms must exist to shuttle fatty acids released from the plasma membrane to the plastids. Furthermore, since p-oxidation of fatty acids normally occurs in peroxisomes, transport vesicles that carry the cargo between organelles may exist in plant cells. It is tempting to speculate that there could be fusion or mixing of compartments that contain either enzymes, fatty acids, and/or intermediate products, thus resulting in oxylipin biosynthesis. Intensive research is needed to address these questions as well as the cell-specific and the subcellular localization of oxylipin synthesis and the mechanisms that regulate this process. 

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