A molecular model of the membrane of a synaptic vesicle. Within nerve cells, a synaptic vesicle consists of a cellular membrane surrounding a soluble compartment filled with neurotransmitter molecules. Vesicles of this type are assembled in the vicinity of a nerve cell’s nucleus and then transported to the tip of the axon. There the vesicle awaits the arrival of a nerve impulse that will induce it to fuse with the overlying plasma membrane, releasing its contents into the narrow cleft that separates the nerve cell from a neighboring cell. The three dimensional model of this membrane was constructed using known structures of the various proteins along with information on their relative numbers obtained from the analysis of purified synaptic vesicles. The image on the front cover shows a synaptic vesicle that has been cut in half; the lipid bilayer that forms the core of the vesicle membrane is shown in green. The image on the back cover shows the surface structure of an intact vesicle. Most of the proteins present in this membrane are required for the interaction of the vesicle with the plasma membrane. The large blue protein at the lower right of the vesicle contains a ring of subunits that rotates within the lipid bilayer as the protein pumps hydrogen ions into the vesicle. The elevated concentration of hydrogen ions within the vesicle is subsequently used as an energy source for the uptake of neurotransmitter molecules from the surrounding cytosol. These images provide the most comprehensive model of any cellular membrane yet to be studied and they reveal how much this membrane is dominated by protein—both within the bilayer itself and on both membrane surfaces.
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