Mechanism of Vesicle Fusion

The fusion of vesicle membrane with the plasma membrane is not a unique feature of synaptic transmission. Many other cellular processes require the fusion of intracellular vesicles with the plasma membrane. For instance, plasma membrane proteins are synthesized intracellularly within the Golgi apparatus and are then conveyed to their target sites by transport vesicles, which must then fuse with the plasma membrane to deliver their cargo. Also, secretion of substances to the extracellular space frequently occurs via exo-cytosis. The molecular mechanism of synaptic vesicle exocytosis shares common features with other forms ofexocytosis. However, the requirement for rapid triggering of exocytosis in response to Ca2+ influx sets synaptic vesicle exocytosis apart from other forms of exocytosis. The delay time between a presynaptic action potential and the first appearance of the postsynaptic response is <0.5 msec. Therefore, there is little time for complex, multistage processes to prepare vesicles for membrane fusion. For this reason, vesicles must be placed very near the membrane at the active zone (Figure 8-10), ready for fusion when Ca2+ enters during an action potential.

Three membrane proteins that play a central role in synaptic vesicle fusion are synaptobrevin, which is associated with the vesicle membrane, and two plasma membrane proteins, syntaxin and SNAP-25. These proteins bind to each other to form the core complex, which brings the vesicle in close proximity to the plasma membrane, as shown in Figure 8-11. Formation of the core complex is required for neurotransmitter release. It is not yet clear, however, whether the core complex is directly involved in fusion or plays a vital role in preparing vesicles for fusion, a process called priming. Energy to prime vesicles for fusion is provided by hydrolysis of ATP, which is carried out by an ATPase called NSF that interacts with proteins of the core complex.

In other forms of exocytosis, fusion follows immediately after priming. Primed synaptic vesicles, however, must be prevented from fusing until influx of Ca2+ triggers the process. Therefore, the molecular machinery of fusion

Figure 8-11 Proteins of the synaptic vesicle and the plasma membrane participate in synaptic vesicle exocytosis at the active zone in the presynaptic terminal.

requires a brake, which is removed when Ca2+ enters during an action potential. This role is carried out by synaptotagmin, a protein associated with the synaptic vesicle (Figure 8-11). Synaptotagmin includes two binding sites for Ca2+ and also interacts with the proteins of the core complex. This interaction prevents fusion from proceeding until calcium ions bind to synaptotagmin. If the gene for synaptotagmin is knocked out by genetic manipulation, rapid coupling between calcium influx and neurotransmitter release is lost.

The final component of the complex of proteins that regulate calcium-dependent fusion of synaptic vesicles is the calcium channel itself. Voltage-dependent Ca2+ channels of the synaptic terminal directly bind to syntaxin, which is part of the core complex. Thus, the source of the calcium ions that trigger neurotransmitter release is held in close proximity to the calcium sensor molecule (synaptotagmin) and the rest of the fusion machinery.

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