Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

TRANSPORT FROM THE TRANS GOLGI NETWORK TO THE CELL EXTERIOR: EXOCYTOSIS
747
plasma
membrane
of nerve
terminal
synaptic
vesicles
fused
synaptic
vesicle
active zone
200 nm
lipid bilayer
H +
v-SNARE (synaptobrevin)
ATP
H + /glutamate
transporter
glutamate
H +
ADP
V-ATPase
Figure 13–69 Scale models of a brain presynaptic
terminal and a synaptic vesicle. The illustrations show
sections through a pre-synaptic terminal (A; enlarged in
B) and a synaptic vesicle (C) in which proteins and lipids
are drawn to scale based on their known stoichiometry
and either known or approximated structures. The relative
localization of protein molecules in different regions of the
presynaptic terminal was inferred from super-resolution
imaging and electron microscopy. The model in (A)
contains 300,000 proteins of 60 different kinds that vary in
abundance from 150 copies to 20,000 copies. In the model
in (C), only 70% of the membrane proteins present in the
membrane are shown; a complete model would therefore
show a membrane that is even more crowded than this
picture suggests (Movie 13.7). Each synaptic vesicle
membrane contains 7000 phospholipid molecules and
5700 cholesterol molecules. Each also contains close to
50 different integral membrane protein molecules, which
vary widely in their relative abundance and together
contribute about 600 transmembrane α helices. The
transmembrane v-SNARE synaptobrevin is the most
abundant protein in the vesicle (~70 copies per vesicle).
By contrast, the V-ATPase, which uses ATP hydrolysis to
pump H + into the vesicle lumen, is present in 1–2 copies
per vesicle. The H + gradient provides the energy for
neurotransmitter import by an H + /neurotransmitter antiport,
which loads each vesicle with 1800 neurotransmitter
molecules, such as glutamate, one of which is shown
to scale. (A and B, from B.G. Wilhelm et al., Science
344:1023–1028, 2014. With permission from AAAS;
C, adapted from S. Takamori et al., Cell 127:831–846,
2006. With permission from Elsevier.)