Vesicular transporter protein

In addition to direct inhibition of the vesicular transport protein, storage of neurotransmitters can be reduced by dissipation of the proton electrochemical gradient. Bafilomycin (a specific inhibitor of the vacuolar H+-ATPase), as well as the proton ionophores carbonyl cyanide m-chlorophenylhydrazone (CCCP) and carbonylcyanide p-(trifluoromethoxy) phenylhy-drazone (FCCP) are used experimentally to reduce the vesicular storage of neurotransmitters. Weak bases including amphetamines and ammonium chloride are used to selectively reduce ApH. 

Liu Y, Edwards RH (1997) The role of vesicular transport proteins in synaptic transmission and neural degeneration. Annu Rev Neurosci 20 125-156... 

Rice, L. M., and Brunger, A. T. (1999). Crystal structure of the vesicular transport protein Seel 7 implications for SNAP function in SNARE complex disassembly. Mol. Cell 4, 85-95. 

The exocytotic release of neurotransmitters from synaptic vesicles underlies most information processing by the brain. Since classical neurotransmitters including monoamines, acetylcholine, GABA, and glutamate are synthesized in the cytoplasm, a mechanism is required for their accumulation in synaptic vesicles. Vesicular transporters are multitransmembrane domain proteins that mediate this process by coupling the movement of neurotransmitters to the proton electrochemical gradient across the vesicle membrane. 

Certain other non-KDEL-containing proteins destined for the membranes of the ER also pass to the Golgi and then return, by retrograde vesicular transport, to the ER to be inserted therein (see below). 

Based largely on a proposal by Rothman and colleagues, anterograde vesicular transport can be considered to occur in eight steps (Figure 46-7). The basic concept is that each transport vesicle bears a unique address marker consisting of one or more v-SNARE proteins, while each target membrane bears one or more complementary t-SNARE proteins with which the former interact specifically. 

Figure 46-7. Model of the steps in a round of anterograde vesicular transport. The cycle starts in the bottom left-hand side of the figure, where two molecules of ARF are represented as small ovals containing GDP. The steps in the cycle are described in the text. Most of the abbreviations used are explained in Table 46-7. The roles of Rab and Seel proteins (see text) in the overall process are not dealt with in this figure. (CGN, cis-Golgi network BFA, Brefeldin A.) (Adapted from Rothman JE Mechanisms of intracellular protein transport Nature 1994 372 55.) (Courtesy of E Degen.)...

Note that in equilibria (2) the subscripts per and cyt are omitted where substrate S is concerned. This is obvious when the binding is measured to a solubilized transport protein, but also in the case where the enzyme is embedded in the membrane of closed vesicular structures, internal and external substrate will have equal concentrations at equilibrium (see Eig. 5). Consequently, the binding is independent of the orientation of the enzyme in the membrane. 

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]. 

Protein prenylation (also called isoprenylation) attaches a 15-carbon, farnesyl diphosphate or a 20-carbon geranylgeranyl diphosphate to the cysteine residue near the C termini of the target proteins (Overmeyer et al., 1998 Rodrfguez-Concepcion et al., 1999a). This reaction is conserved both in animals and plants. The functions of the target proteins include signal transduction, nuclear architecture, and vesicular transport. 

Proteins can undergo different rounds of palmitoylation and depalmitoylation, either constitutively or as a response to signals." " Here the Ras proteins are the most commonly discussed examples. As described above, all Ras proteins are expressed with the CAAX-box and are subject to post-translational modifications. First, they get farnesylated and after proteolysis and methylation of the C-terminus, H-/N-Ras as well as K-Ras 4A get further palmitoylated at additional cysteines present in their C-terminus. Palmitoylation occurs in the Golgi apparatus and via vesicular transport the farnesylated and palmitoylated proteins are directed to the plasma membrane (PM). The palmitoyl thioester is hydrolyzed at multiple cellular sites and the protein is transported back to the Golgi via a nonvesicular pathway (Scheme 3)." ... 

Figure 1.6 Vesicular transport of proteins within the cell. Vesicles from the endoplasmic reticulum [A] carry protein to the Golgi complex, they are repackaged in the Golgi from which they leave to form primary lysosomes [B] or fuse with the plasma membrane this is to add proteins or to be secreted from the cell [C]. In the Golgi, new vesicles are formed to transport the proteins to the plasma membrane (e.g. transport proteins or proteins for export) or the lysosomes. This system transports, safely, dangerous hydrolytic enzyme to the lysosomes and it also protects membrane proteins, or proteins for export, from degradation in the cytosol.

Vesicular transport of bile acids has not been demonstrated under normal conditions, shown by using isolated rat hepatocyte couplets and fluorescently labelled bile acids. In these experiments confocal microscopy found no evidence of sequestering into clusters and colchicine disruption of microtubular function did not affect bile-acid transport. This makes it unlikely that vesicle transport plays a role and it is now believed that bile acids traverse the hepatocyte by diffusion through the cytosol while bound to soluble proteins. It is worth considering the caveat that fluorescently labelled bile acids, while very useful tools, do differ structurally from endogenous bile acids with increased hydro-phobicity leading to greater retention by cells. ... 

The illustration shows an interaction between a virus-infected body cell (bottom) and a CD8-carrying cytotoxic T lymphocyte (top). The infected cell breaks down viral proteins in its cytoplasm (1) and transports the peptide fragments into the endoplasmic reticulum with the help of a special transporter (TAP) (2). Newly synthesized class I MHC proteins on the endoplasmic reticulum are loaded with one of the peptides (3) and then transferred to the cell surface by vesicular transport (4). The viral peptides are bound on the surface of the a2 domain of the MHC protein in a depression formed by an insertion as a floor and two helices as walls (see smaller illustration). 

The norepinephrine transporter (NET) and the vesicular monoamine transporter (VMAT) are presynaptic components of the sympathetic neurons. NET is a Na+ /Cl -dependent transport protein and responsible for the neurotransmitter uptake from the synaptic cleft into the cytoplasm of the neurons. This transport process, called uptake-1, reduces the amount and, thus, the effect of NE released into the synaptic cleft. NE is stored in the cytoplasm of the neurons in specialized vesicles by the H+-dependent transport protein VMAT. Two isoforms VMAT1 and VMAT2, are known. VMAT is localized in the vesicle membranes, and the vesicular storage protects NE from metabolism by monoamine oxidase (MAO), which is localized on the surface membrane of the mitochondria. Vice versa, nerve depolarisation causes NE release from the vesicles into the synaptic cleft by Ca+-mediated exocytose (Fig. 12) [79,132-136],... 

In M phase, new phosphorylation of many proteins is observed that starts, in particular, from the CDC2-cyclin B complex. The phosphorylation mostly affects proteins involved in the reorganization of the cytoskeleton, the nuclear membrane and the formation of the spindle apparatus. As a consequence of phosphorylation events, inhibition of vesicular transport and general inhibition of transcription occur. 

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