Molecular Mechanisms of Neurotransmitter Release 
Thomas C. Sudhof, M.D. — Associate Investigator 
Dr. Sudhof is also Associate Professor of Molecular Genetics at the University of Texas Southwestern Med- 
ical Center at Dallas. He received his medical degree from the Georgia Augusta University of Gottingen, 
FRG. He obtained postdoctoral training first with Victor Whittaker at the Max-Planck-Institut fUr biophys- 
ikalische Chemie in Gottingen and then with Michael Brown and Joseph Goldstein in Dallas. 
MANY cells send out signals by secreting mes- 
senger molecules. These molecules are rec- 
ognized by specific receptors on target cells. The 
most striking example of this communication pro- 
cess is found in neurotransmission — the relay of 
information from one neuron to another in the 
nervous system. Neurotransmission occurs at syn- 
apses, highly specialized contact sites between 
two nerve cells. At a synapse, a presynaptic neu- 
ron secretes a signal that is recognized by the ad- 
jacent postsynaptic cell. A similar communica- 
tion process is also responsible for the regulation 
of body metabolism by hormones, except that in 
this case several messenger-secreting endocrine 
cells are clustered together and send their signals 
to widely distributed cells in the body. 
My laboratory explores one aspect of this im- 
portant communication process — the mecha- 
nism by which signal molecules like neurotrans- 
mitters or hormones are released from cells. 
These molecules are synthesized in the secreting 
cells and packaged into intracellular membrane- 
bound vesicles. Upon stimulation, the cell's sig- 
nal molecules are released by exocytosis, which 
consists of the fusion of the vesicle membranes 
with the plasma membrane and the concurrent 
secretion of the vesicle contents. Our goal is to 
characterize the components and mechanisms of 
the cell that are involved in the assembly, exocy- 
tosis, and recycling of synaptic vesicles. 
The first goal in exploring synaptic vesicle 
functions consists of the characterization of their 
protein constituents. The cellular processes lead- 
ing to the accumulation of neurotransmitters in 
synaptic vesicles and their release by exocytosis 
must act on the synaptic vesicle membrane. 
Therefore we are focusing on the proteins of syn- 
aptic vesicles, with the aim of understanding 
their structure and function in the context of 
membrane traffic in neurons. 
Since synaptic vesicles have rather specialized 
functions, they are comparatively simple organ- 
elles. Preliminary estimates suggest that synaptic 
vesicles have fewer than 50 major protein compo- 
nents. Although the composition of these organ- 
elles has not yet been determined in molecular 
detail, they may be amenable to such an analysis. 
In addition to providing insight into synaptic 
functions, the study of the molecular structure of 
these vesicles will provide insight into the struc- 
tures of organelles in general. 
The membrane proteins of synaptic vesicles 
fall into two basic functional categories: trans- 
port proteins instrumental in the active uptake of 
neurotransmitters by synaptic vesicles, and struc- 
tural proteins that mediate the vesicles' interac- 
tions with other cellular membranes and the cyto- 
skeleton during exocytosis and endocytosis of the 
vesicles. The number and abundance of proteins 
in the second class is much greater than in the 
first class. This correlates with the greater com- 
plexity of the transport, fusion, and fission reac- 
tions of synaptic vesicles as compared with their 
neurotransmitter uptake processes. 
In the past several years, more than 10 proteins 
that are abundant in the synaptic vesicle mem- 
brane have been characterized in my laboratory. 
The transport functions of synaptic vesicles de- 
pend on a proton pump in their membrane; the 
activity of this pump establishes an energy gra- 
dient across the vesicle membrane. This energy 
gradient is utilized for the active accumulation of 
neurotransmitters. Two subunits of the synaptic 
vesicle proton pump have been cloned in my lab- 
oratory, and we are currently studying its assem- 
bly and regulation. Some synaptic vesicles also 
contain an electron transport protein in their 
membrane to convey reducing equivalents for in- 
travesicular enzymes. This protein, cytochrome 
b56l, has also been purified and cloned in this 
laboratory. We are now in the process of trying to 
identify and characterize the actual neurotrans- 
mitter transport proteins in the synaptic vesicle 
membrane. 
The second class of proteins are those presum- 
ably involved in the exocytosis and re-endocyto- 
sis of synaptic vesicles. These proteins should be 
present on all vesicles independent of their neu- 
rotransmitter content. The most abundant among 
these are the three integral membrane proteins — 
synaptotagmin {M^ 65,000), synaptophysin {M^ 
38,000), and synaptobrevin {M, 18,000) — and 
the synapsins, a group of four related peripheral 
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