Fusion of vesicles

Cellular secretion from macromolecules mediated by fusion of vesicles with the plasma membrane... 

A second mechanism that impinges on the localization of transporters is through the association with proteins, the most prominent example being syntaxin. Syntaxin is a t-SNARE protein necessary for the fusion of vesicles with the plasma membrane (see the chapter on exocytosis). On the cell surface syntaxin consistently stabilizes the localization of GABA, noradrenaline, glycine, and 5HT transporters the PKCa isoform can sever the interaction with syntaxin suggesting a general mechanism for transporter internalization. 

These considerations are important also in view of the processes of division and/or fusion of vesicles. In particular, when a vesicle divides up, and the total surface area remains constant, the total volume must decrease. This means that water must be eliminated in the process, so as to keep the volume to surface ratio constant. Conversely, when two vesicles fuse with each other, with a constant surface area (no fresh surfactant being added), the total volume must increase to keep the volume/surface constant and water must come in. This important, characteristic feature of vesicles is represented in Figure 9.27. 

Figure 10.17 Fusion of vesicles as a way to foster reactivity and to increase the molecular complexity of the water-pool content S, I and P are enzymatic substrate and reaction products. This is also a method to circumvent the problem of substrate permeability in liposomes. It can be seen as a model of synthetic symbiogenesis.

The fusion of vesicles with opposite charges appears to be an efficient method to increase the molecular complexity. Suppose all enzymes of the Krebs cycle were distributed into five or six different charged vesicle families (some positively, some negatively charged) - then mixed. Would the Krehs cycle be reconstituted ... 

Release Liberation of the neurotransmitter from vesicle electrical firing of a nerve leads to fusion of vesicles with the cell wall, allowing the neurotransmitter to be released into the synaptic junction... 

Fig. 15. A schematic representation for the polymerization-induced phase separation of DOPE and poly 16 leading to fusion of vesicles due to bilayer contact of the enriched PE domains. See text for description.

Schematic illustration of a generalized cholinergic junction (not to scale). Choline is transported into the presynaptic nerve terminal by a sodium-dependent choline transporter (CHT). This transporter can be inhibited by hemicholinium drugs. In the cytoplasm, acetylcholine is synthesized from choline and acetyl -A (AcCoA) by the enzyme choline acetyltransferase (ChAT). Acetylcholine is then transported into the storage vesicle by a second carrier, the vesicle-associated transporter (VAT), which can be inhibited by vesamicol. Peptides (P), adenosine triphosphate (ATP), and proteoglycan are also stored in the vesicle. Release of transmitter occurs when voltage-sensitive calcium channels in the terminal membrane are opened, allowing an influx of calcium. The resulting increase in intracellular calcium causes fusion of vesicles with the surface membrane and exocytotic expulsion of acetylcholine and cotransmitters into the junctional cleft (see text). This step can he blocked by botulinum toxin. Acetylcholine s action is terminated by metabolism by the enzyme acetylcholinesterase. Receptors on the presynaptic nerve ending modulate transmitter release. SNAPs, synaptosome-associated proteins VAMPs, vesicle-associated membrane proteins.

The preparation of giant liposomes79) made it possible to apply electrical field-induced fusion to the fusion of vesicles made from natural and polymerizable lipids. Since in dielectrophoresis the net force pulling vesicles towards higher field intensities is proportional to the volume of the particle, only large vesicles can be aligned in parallel to the field lines between the electrodes. 

Figure 16.3 Neurotransmitter release, (a) Presynaptic nerve terminal containing vesicles and other organelles, (b) Neurotransmitter-containing vesicles are made of lipid bilayers. Associated proteins participate in the release process, (c) The vesicle associates with the presynaptic membrane via protein complexes that mediate release, (d) Release of neurotransmitter into the synapse is by protein-mediated fusion of vesicle and presynaptic membranes.

Schematic diagram of a generalized noradrenergic junction (not to scale). Tyrosine is transported into the noradrenergic ending or varicosity by a sodium-dependent carrier (A). Tyrosine is converted to dopamine (see Figure 6-5 for details), which is transported into the vesicle by a carrier (B) that can be blocked by reserpine. The same carrier transports norepinephrine (NE) and several other amines into these granules. Dopamine is converted to NE in the vesicle by dopamine-B-hydroxylase. Release of transmitter occurs when an action potential opens voltage-sensitive calcium channels and increases intracellular calcium. Fusion of vesicles with the surface membrane results in expulsion of norepinephrine, cotransmitters, and dopamine-13-hydroxylase.

A key determinant of the final urine concentration is antidiuretic hormone (ADH also called vasopressin). In the absence of ADH, the collecting tubule (and duct) is impermeable to water, and dilute urine is produced. However, membrane water permeability of principal cells can be increased by ADH-induced fusion of vesicles containing preformed water channels with the apical membranes (Figure 15-6). ADH secretion is regulated by serum osmolality and by volume status. 

E. Kalb, S. Frey, and L. K. Tamm, Formation of supported planar bilayers by fusion of vesicles to supported phospholipid monolayers, Biochim. Biophys. Acta 1103, 307-316 (1992). 

In principle, the activation of a presynaptic ionotropic receptor may either increase or decrease the amount of transmitter being released. However, the effect may depend on the situation prior to the activation of the receptor. In this context one has to mention that transmitter release occurs either due to spontaneous fusion of vesicles with the plasma membrane or as a consequence of an event that triggers increases in the intracellular Ca2+ concentration at the active zone where vesicle exocytosis takes place. The Ca2+ concentrations required to promote exocytosis towards maximal rates lie between approximately 10 and 100 pM (Augustine 2001 Schneggenburger and Neher 2005), depending on the synapse being investigated. These two types of transmitter release are usually named spontaneous and stimulated (or stimulation-evoked) release, respectively. 

In a second series of experiments, Hanczyc et al. [52] found that the myristoleic acid vesicles could be induced to grow by addition of fatty acid to the medium, presumably by incorporating fatty acid molecules into the membrane, rather than by fusion of vesicles. If the resulting suspension of large vesicles was then filtered through a polycarbonate filter having pores 0.2 pm in diameter, the larger vesicles underwent a kind of shear-induced division to produce smaller vesicles. This process could be repeated several times (Fig. 5). 

In eukaryotes, soluble N-ethylmaleimide-sensitive factor (NSF) adaptor proteins (SNAPs) receptors (SNAREs) are known to be required for docking and fusion of intracellular transport vesicles with acceptor/target membranes. The fusion of vesicles in the secretory pathway involves target-SNAREs (t-SNAREs) on the target membrane and vesicle-SNAREs (v-SNAREs) on vesicle membranes that recognize each other and assemble into trans-SNARE complexes (Sollner et al., 1993). 

Influx of calcium causes fusion of vesicle with cell membrane... 

Ptdlns-3,4,5-P3, which is implicated as a second messenger in insulin action (see below), has been reported to activate membrane fusion of vesicles containing glucose transporters, but the molecular details have not yet been worked out. But it is to be expected... 

Insulin stimulates the rate of transport into various cells, such as those of adipose tissue and muscle. How does a rise in this hormone in thepia ima provoke an increase in flow of glucose to the cell s interior Insulin stimulates the fusion of vesicles with the plasma membrane. These vesicles contain membrane-bound proteins that can transport glucose thus, the fusion event results in the insertion of glucose transporters into the plasma membrane, with the consequent increase in glucose transport inlc) the cell. I his scenario is illustrated in Figure 4-12, in which the signal transmitted into the ccD by insulin is represented by a spark. 

The cubic membrane in the apothecial cells is of special interest in many ways. It is not static, but reversible, and its appearance seems to be correlated with the development of the apothecium. It is believed to form de novo from an "amorphous lipoprotein matrix" [61]. This would indicate that true phase condensation could occur in some instances of the formation of cubic membranes. However, the interpretation presented [61] is, in view of our analysis, slightly inaccurate. In particular, it is incorrect to suppose that the development of the "lattice body" takes place through the formation and subsequent fusion of vesicles. The membranous "vesicles" are properly interpreted as sections normal to any lattice plane of the gyroid in which two axes are close to zero. It should be emphasised that similar images cm also be produced by projections of the D- and the P-PCS s. 

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