Synaptic vesicles contain a characteristic pattern 
of intrinsic membrane proteins, of which proteins 
with apparent molecular weights of 65, 38 (syn- 
aptophysin), and 18 kDa (synaptobrevin) are major 
constituents and are being intensely studied in Dr. 
Siidhof's laboratory. They have been purified, 
cloned, and expressed. Specific antibodies have 
been raised against each of them, and their mem- 
brane topology and possible functions are being in- 
vestigated. All three proteins are comparatively 
abundant in synaptic vesicles but completely ab- 
sent from other neuronal organelles. They are pres- 
ent in all synaptic vesicles independent of their 
neurotransmitter type and do not represent sub- 
units of the proton pump or neurotransmitter 
transporters. Presumably these proteins have a 
function that is specific to synaptic vesicles but in- 
dependent of neurotransmitter content, a function 
that may be related to the cell biological life cycle 
of the vesicles. 
Synaptophysin is an evolutionarily well-con- 
served synaptic vesicle protein that contains four 
transmembrane regions and a tyrosine-rich re- 
peated sequence at its carboxyl terminus that is 
probably phosphorylated. Specific antipeptide anti- 
bodies and limited proteolysis of synaptic vesicles 
were used to establish its transmembrane topology. 
Synaptophysin is known to form higher molecular 
weight complexes, and it has been hypothesized 
that it might form a transient pore in the mem- 
brane during exocytosis. The self- association of syn- 
aptophysin in the synaptic vesicle membrane is cur- 
rently being studied to determine if a membranous 
pore is formed that is lined by transmembrane re- 
gions or if the self-association is mediated by the ex- 
tramembranous parts of the protein to form a kind 
of membrane skeleton. 
Synaptic vesicle membrane proteins are ex- 
pressed not only in neurons but also in endocrine 
cells. Analysis of the expression of different synaptic 
vesicle proteins in endocrine cells revealed a ratio 
between the synaptic vesicle proteins that is differ- 
ent from that observed in native synaptic vesicles. 
WhUe the expression of p65, synaptobrevin, and 
the four synapsins can be demonstrated in endo- 
crine cells but appears to be low, synaptophysin is 
expressed at levels comparable with those observed 
in brain. Upon analysis by immunocytochemistry or 
subcellular subfractionation, >95% of the synaptic 
vesicle membrane protein in endocrine cells was lo- 
calized to a small vesicular compartment distinct 
from the large secretory granules. The secretory 
granules in endocrine cells contained low-to-unde- 
tectable amounts. Surprisingly, when synaptophy- 
sin was expressed by transfection in fibroblastic 
CHO cells, it was also targeted into a small vesicu- 
lar compartment indistinguishable from that ob- 
served in endocrine cells. In collaboration with Drs. 
Pietro DeCamilli and Reinhard Jahn, biochemical 
and immunocytochemical techniques were used to 
identify the synaptophysin-containing compart- 
ment in endocrine and fibroblastic cells. This com- 
partment corresponds to the vesicular pathway of 
recycling receptors, such as the transferring recep- 
tor. The receptor-mediated endocytosis pathway 
shares with synaptic vesicles the property of recy- 
cling independently of the Golgi complex. The tar- 
geting of synaptic vesicle proteins into this pathway 
in endocrine cells suggests that the pathway of syn- 
aptic vesicles may represent a specialization of that 
of recycling receptors. 
Synaptobrevin is an 18 kDa membrane protein of 
synaptic vesicles that has a single transmembrane 
region at its carboxyl terminus. A homologue of this 
protein was cloned from Drosophila melanogaster 
in Dr. Siidhof's laboratory. Of the characterized 
synaptic vesicle proteins, synaptobrevin is the most 
conserved. It contains an evolutionarily poorly con- 
served amino terminus that is rich in proline or as- 
paragine, followed by a highly conserved 70-amino 
acid sequence (78% identity between the Dro- 
sophila and bovine sequences). The structure of 
the protein suggests that the conserved region is lo- 
cated at the interface between the synaptic vesicle 
lipid bilayer and the cytoplasm, while the carboxyl- 
terminal halves of the transmembrane region and 
the small intravesicular sequences are not as well 
conserved. Although the function of synaptobrevin 
is unknown, its structure on the cytoplasmic face of 
the synaptic vesicle suggests that it serves to bind 
an unidentified cellular factor. Efforts are under 
way in Dr. Siidhof's laboratory to use recombinant 
proteins to identify possible ligands for syn- 
aptobrevin. 
The work in Dr. Siidhof's laboratory has resulted 
in the characterization of several major compo- 
nents of synaptic vesicles and determination of the 
relation of the synaptic vesicle pathway to the gen- 
eral membrane traffic in eukaryotic cells. Future 
work will focus on understanding the interactions 
of synaptic vesicle proteins with each other and 
with other cellular proteins. 
Dr. Siidhof is also Associate Professor of Molecu- 
lar Genetics at the University of Texas Southwest- 
ern Medical Center at Dallas. 
Continued 
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