STRUCTURE AND FUNCTION OF THE PRESYNAPTIC NERVE TERMINAL 
Thomas C. Sudhof, M.D., Investigator 
Synapses are abundant neural structures (there 
are ~10' times more synapses in the mammalian 
brain than there are base pairs in the human ge- 
nome) that constitute the principal point of infor- 
mation transfer between neurons. At the synapse, a 
presynaptic nerve terminal contacts a postsynaptic 
cell, probably via cell-cell contact proteins. Infor- 
mation is transferred between pre- and postsynaptic 
cells by the release of neurotransmitters and chemi- 
cal signals from the presynaptic terminal, and the 
recognition of the neurotransmitters by the postsyn- 
aptic cell. In the presynaptic nerve terminal, neuro- 
transmitters are stored at high concentrations in syn- 
aptic vesicles, specialized secretory organelles of 
the presynaptic nerve terminal, and released on syn- 
aptic vesicle exocytosis at the active zone of the pre- 
synaptic plasma membrane. 
Over the past six years. Dr. Sudhof and his col- 
leagues have studied the nerve terminal by two ap- 
proaches. First, the structures and functions of the 
proteins of synaptic vesicles, the central organelle 
in neurotransmitter release, were investigated. 
These studies have led to the molecular description 
of a number of synaptic vesicle proteins, accounting 
for a significant part of the total synaptic vesicle 
proteins, and to the formulation of the functional 
importance of some of these proteins. The work on 
synaptic vesicles has also allowed generalizations 
that may be applicable to other organelles. Second, 
Dr. Siidhofs laboratory has initiated studies on the 
presynaptic plasma membrane and its interactions 
with synaptic vesicles during synaptic vesicle exo- 
cytosis/neurotransmitter release. These studies 
have led to the description of a family of synaptic 
cell surface proteins named neurexins that are re- 
lated to the a-latrotoxin receptor, and to the discov- 
ery that the a-latrotoxin receptor as well as other 
neurexins directly interacts with synaptotagmin, a 
synaptic vesicle membrane protein. 
With respect to synaptic vesicles, recent work in 
Dr. Siidhofs laboratory has focused on two aspects: 
the structure and function of synaptotagmin, a major 
synaptic vesicle protein with a putative function in 
the Ca^^-dependent exocytosis of synaptic vesicles, 
and the relation between the synaptic vesicle path- 
way and other cellular membrane trafficking events. 
These studies were carried out in collaboration with 
Dr. Reinhardjahn (HHMI, Yale University). 
Synaptotagmin is a 65,000-Af^ protein that is pres- 
ent on synaptic vesicles in several differentially dis- 
tributed isoforms (referred to as synaptotagmins I- 
IV). Native synaptotagmin forms a high-molecular- 
weight complex, probably a homotetramer, on the 
synaptic vesicle membrane, to which it is attached 
by a single transmembrane region. The primary 
structure of synaptotagmin contains an internal re- 
peat that is homologous to the C2 domain of protein 
kinase C and is highly conserved from humans to 
fruit flies. Purified synaptotagmin I was shown to 
bind Ca^^ in a ternary complex together with nega- 
tively charged phospholipids. Ca'^^ binding by syn- 
aptotagmin I was concentration dependent, with 
half-maximal binding observed at ~20 free 
Ca^^, a concentration thought to be reached physio- 
logically in the stimulated nerve terminal. These 
properties suggest that synaptotagmin may be the 
Ca^^ trigger on the synaptic vesicle surface that 
functions in Ca^^-dependent exocytosis. In addi- 
tion, evidence was obtained for a direct interaction 
of synaptotagmin I with neurexins and the a-Iatro- 
toxin receptor, which are thought to be synaptic 
plasma membrane proteins. These findings raised 
the possibility that synaptotagmin may also be in- 
volved in the docking of synaptic vesicles in the 
nerve terminal in addition to being a Ca^^ sensor. 
The pathway of synaptic vesicles in the nerve ter- 
minal, although unique in terms of its speed and 
regulation, contains many similarities to ubiquitous 
cellular membrane trafficking pathways, in particu- 
lar the receptor-mediated endocytosis pathway. In 
experiments performed several years ago in collabo- 
ration with Dr. Pietro De Camilli (HHMI, Yale Uni- 
versity), Dr. Siidhofs laboratory had shown that 
transfection of synaptophysin into fibroblasts results 
in the targeting of synaptophysin to a vesicular com- 
partment that was identified as part of the receptor- 
mediated endocytosis pathway. Recent findings 
have shown that in developing neurons, synaptic 
vesicle proteins are present in a constitutively recy- 
cling pathway that also contains transferrin recep- 
tors. Only with neuronal maturation can a segrega- 
tion of the pathways be seen. Furthermore, in 
endocrine cells both pathways seem to coexist. Fi- 
nally, a new form of one of the synaptic vesicle pro- 
teins was recently discovered that, despite extensive 
homologies, was absent from synaptic vesicles but 
present in the receptor-mediated endocytosis path- 
way. Together these findings suggest that the recep- 
tor-mediated endocytosis pathway and the synaptic 
vesicle pathway are evolutionarily and mechanisti- 
cally related. 
Apart from these two central themes regarding 
444 
