Membrane vesicle transport

G proteins comprise several families of diverse cellular proteins that subserve an equally diverse array of cellular functions. These proteins derive their name from the fact that they bind the guanine nucleotides guanosine triphosphate (GTP) and guanosine diphosphate (GDP) and possess intrinsic GTPase activity. G proteins play a central role in signal transduction as well as in a myriad of cellular processes, including membrane vesicle transport,... 

Liver Uptake Blood Parenchymal cells Isolated, cultured cryopreserved hepatocytes, sinusoidal membrane vesicles, transporter expressions system... 

Excretion Parenchymal cells Bile Canalicular membrane vesicles, transporter expression system... 

R516 L. Rosendahl, A. Rudbeck, A. M. Scharff and P. Mouritzen, Assimilation of Fixed Nitrogen Studied by Membrane Vesicle Transport and in Vivo N-NMR Spectroscopy in Pea , Curr. Plant Set Biotechnol Agric., 2000, 38, 351... 

As a functional assay, the membrane vesicle transport assay has been used to investigate binding sites (Shapiro et al., 1997,1999), interspecies differences in transporter activity (Ishizuka et al., 1999 Ninomiya et al., 2005), polymorphisms in transporter activity (Center et al., 1998 Hirano et al., 2005), and substrate or inhibitor specificity for a given efflux pump (Keppler et al., 1998 Volk and Schneider, 2003). 

Once the proteins have passed the quality control system of the early secretory pathway, they are transported in vesicles via the individual compartments of the Golgi apparatus to the plasma membrane. Soluble proteins are transported in the vesicle lumen, membrane proteins are integrated in the vesicle membrane. The transport to the cell surface is the default pathway for secretory and membrane proteins. Proteins may also become part of one of the intracellular compartments along the secretory pathway, but only if they contain specific retention signals. 

The vesicular monoamine transporters (VMATs) were identified in a screen for genes that confer resistance to the parkinsonian neurotoxin MPP+ [2]. The resistance apparently results from sequestration of the toxin inside vesicles, away from its primary site of action in mitochondria. In addition to recognizing MPP+, the transporter s mediate the uptake of dopamine, ser otonin, epinephrine, and norepinephrine by neurons and endocrine cells. Structurally, the VMATs show no relationship to plasma membrane monoamine transporters. 

In eukaryotes there is also evidence that Met(O) is actively transported. It has been reported that Met(O) is transported into purified rabbit intestinal and renal brush border membrane vesicles by a Met-dependent mechanism and accumulates inside the vesicles against a concentration gradient102. In both types of vesicles the rate of transport is increased with increasing concentrations of Na+ in the incubation medium. The effect of the Na+ is to increase the affinity of Met(O) for the carrier. Similar to that found in the bacterial system, the presence of Met and other amino acids in the incubation medium decreased the transport of Met(O). These results suggest that Met(O) is not transported by a unique carrier. 

Even though dynein, kinesin, and myosin serve similar ATPase-dependent chemomechanical functions and have structural similarities, they do not appear to be related to each other in molecular terms. Their similarity lies in the overall shape of the molecule, which is composed of a pair of globular heads that bind microtubules and a fan-shaped tail piece (not present in myosin) that is suspected to carry the attachment site for membranous vesicles and other cytoplasmic components transported by MT. The cytoplasmic and axonemal dyneins are similar in structure (Hirokawa et al., 1989 Holzbaur and Vallee, 1994). Current studies on mutant phenotypes are likely to lead to a better understanding of the cellular roles of molecular motor proteins and their mechanisms of action (Endow and Titus, 1992). 

Many proteins synthesized on membrane-bound polyribosomes proceed to the Golgi apparatus and the plasma membrane in transport vesicles. 

Figure 20.1 Schematic diagram illustrating how antidepressants increase the concentration of extraneuronal neurotransmitter (noradrenaline and/or 5-HT). In the absence of drug (b), monoamine oxidase on the outer membrane of mitochondria metabolises cytoplasmic neurotransmitter and limits its concentration. Also, transmitter released by exocytosis is sequestered from the extracellular space by the membrane-bound transporters which limit the concentration of extraneuronal transmitter. In the presence of a MAO inhibitor (a), the concentration of cytoplasmic transmitter increases, causing a secondary increase in the vesicular pool of transmitter (illustrated by the increase in the size of the vesicle core). As a consequence, exocytotic release of transmitter is increased. Blocking the inhibitory presynaptic autoreceptors would also increase transmitter release, as shown by the absence of this receptor in the figure. In the presence of a neuronal reuptake inhibitor (c), the membrane-bound transporter is inactivated and the clearance of transmitter from the synapse is diminished...

Biochemical studies of plasma membrane Na /H exchangers have been directed at two major goals (1) identification of amino acids that are involved in the transport mechanism and (2) identification and characterization of the transport pro-tein(s). To date, most studies have been performed on the amiloride-resistant form of Na /H" exchanger that is present in apical or brush border membrane vesicles from mammalian kidney, probably because of the relative abundance of transport activity in this starting material. However, some studies have also been performed on the amiloride-sensitive isoform present in non-epithelial cells. 

Kaczorowski G, L Shaw, R Laura, C Walsh (1975) Active transport in Escherichia coli B membrane vesicles. Differential inactivating effects from the enzymatic oxidation of P-chloro-L-alanine and P-chloro-D-alanine. J Biol Chem 250 8921-8930. 

Garcia Ruiz, C., Fernandez Checa, J. and Kaplowitz, N. (1992). Bidirectional mechanism of plasma membrane transport of reduced glutathione in intact rat hepatocytes and membrane vesicles. J. Biol. Chem. 267, 2256-2264. 

Heijn, M., Oude Elferink, R. and Jansen, P. (1992). ATP-dependent multispecific organic anion transport system in rat erythrocyte membrane vesicles. Am. J. Physiol. 262, 104-110. 

P. Askerlund and C. Larsson, Transmembrane electron transport in plasma membrane vesicles loaded with an NADH-generating system or ascorbate. Plant Phy-.i-iol. 96 1178 (1991). 

Although several allelochemicals (primarily phenolic acids and flavonoids) have been shown to inhibit mineral absorption, only the phenolic acids have been studied at the physiological and biochemical levels to attempt to determine if mineral transport across cellular membranes can be affected directly rather than indirectly. Similar and even more definitive experiments need to be conducted with other allelochemicals that are suspected of inhibiting mineral absorption. Membrane vesicles isolated from plant cells are now being used to elucidate the mechanism of mineral transport across the plasma membrane and tonoplast (67, 68). Such vesicle systems actively transport mineral ions and thus can serve as simplified systems to directly test the ability of allelochemicals to inhibit mineral absorption by plant cells. 

Solioz, M. and Odermatt, A., Copper and silver transport by CopB-ATPase in membrane vesicles of Enterococcus hirae, J Biol Chem, 270 (16), 9217-9221, 1995. 

Solute uptake can also be evaluated in isolated cell suspensions, cell mono-layers, and enterocyte membrane vesicles. In these preparations, uptake is normalized by enzyme activity and/or protein concentration. While the isolation of cells in suspension preparations is an experimentally easy procedure, disruption of cell monolayers causes dedifferentiation and mucosal-to-serosal polarity is lost. While cell monolayers from culture have become a popular drug absorption screening tool, differences in drug metabolism and carrier-mediated absorption [70], export, and paracellular transport may be cell-type- and condition-depen-dent. 

Mucosal brush border membrane vesicles and basolateral membrane vesicles can be isolated to study solute uptake across specific enterocyte boundaries. These more isolated vesicle systems allow for investigation of solute transport across a particular membrane barrier and permit separation of membrane trans-... 

N Piyapolrungroj, C Li, RL Pisoni, D Fleisher. Cimetidine transport in brush-border membrane vesicles from rat small intestine. J Pharmacol Exp Ther 289 346-353, 1999. 

Figure 1 General pathways through which molecules can actively or passively cross a monolayer of cells. (A) Endocytosis of solutes and fusion of the membrane vesicle with the opposite plasma membrane in an active process called transcytosis. (B) Similar to A, but the solute associates with the membrane via specific (e.g., receptor) or nonspecific (e.g., charge) interactions. (C) Passive diffusion between the cells through the paracellular space. (C, C") Passive diffusion (C ) through the cell membranes and cytoplasm or (C") via partitioning into and lateral diffusion within the cell membrane. (D) Active or carrier-mediated transport of an otherwise poorly membrane permeable solute into and/or out of a cellular barrier.

In addition to the aforementioned effects on paracellular drug transport, Ca2+ also plays an important role in the transcytosis of macromolecules. The entry of the plant lectins abrin, modeccin, and viscumin into Vero cells was inhibited in a Ca2+-free medium a well as in a Ca2+-containing medium containing verapamil and Co2+, both inhibitors of Ca2+ [217], Ca2+ is therefore required at a stage after the binding of the above lectins, perhaps in the fusion and exocytosis of membrane vesicles. 

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