Normal and Oncogenic Forms of 
thestc Gene Product 
Joan S. Brugge, Ph.D. — Investigator 
Dr. Brugge is also Professor of Microbiology at the University of Pennsylvania School of Medicine. She re- 
ceived her B.A. degree in biology from Northwestern University and her Ph.D. degree in virology from 
Baylor College of Medicine. Her postdoctoral research was done with Raymond Erikson at the University 
of Colorado School of Medicine, Denver. Before moving to the University of Pennsylvania, Dr. Brugge 
was a member of the Department of Microbiology at the State University of New York at Stony Brook. 
IN 1 9 1 1 , Peyton Rous at the Rockefeller Univer- 
sity first demonstrated that tumors could be in- 
duced by a transmissible agent isolated from a 
chicken sarcoma. This agent, later shown to be a 
retrovirus, was designated Rous sarcoma virus 
(RSV). Since then, RSV has served as a model sys- 
tem to examine tumorigenesis. One of the four 
genes that RSV carries is both necessary and suffi- 
cient to cause tumor formation in chickens. This 
oncogene, v-src, is derived from a normal cellu- 
lar gene (c-src) that was captured by RSV during 
infection of chicken cells. The c-src gene is pres- 
ent in normal chromosomal DNA from humans 
and from lower eukaryotic species, such as fruit 
flies and sponges. The high degree of conserva- 
tion of this gene throughout evolution suggests 
that it plays a crucial role in normal cell pro- 
cesses. The Y-src gene from RSV contains many 
mutations that distinguish it from c-src. The src 
gene codes for a 60-kDa protein that is located on 
the cytoplasmic side of the plasma membrane. 
The src protein, an enzyme referred to as a pro- 
tein kinase, catalyzes the transfer of a phosphate 
group from ATP to tyrosine residues on other cel- 
lular proteins. The addition of phosphate to pro- 
teins is commonly employed by cells to regulate 
the activity of cellular proteins. It is believed that 
the mutated RSV src protein induces tumor for- 
mation by the inappropriate phosphorylation of 
cellular proteins that are involved in regulating 
cellular growth. Our major objectives are 1) to 
determine the function of the src protein in nor- 
mal cells, 2) to understand how this protein is 
regulated in normal cells, 3) to determine how 
mutations in the structure of the src protein acti- 
vate the ability of this protein to convert a normal 
cell to a tumor cell, and 4) to elucidate the mech- 
anism whereby these mutated forms induce tu- 
mor formation. 
We have found that two types of normal cells 
express high levels of the src protein: 1) neurons 
from embryonic and adult neural tissues and 2) 
peripheral blood platelets. To investigate the 
function of the src protein in neurons, we have 
microinjected antibodies directed against the src 
protein into PC 12 cells, a neural tumor cell line 
that differentiates into neuron-like cells after 
treatment with nerve growth factor (NGF) . These 
studies were performed in collaboration with 
Simon Halegoua (State University of New York at 
Stony Brook) . 
Introduction of purified monoclonal antibod- 
ies to the src protein inhibited the production of 
neurite extensions, the characteristic morpholog- 
ical marker of neuron-like differentiation. Using 
this antibody, we have begun to examine how src 
communicates with other regulatory proteins be- 
lieved to participate in NGF-induced neurite out- 
growth. We have provided evidence that src col- 
laborates with another proto-oncogene, ras, to 
provide essential functions that act on a single 
pathway involved in NGF-induced neurite out- 
growth. We have also found that the oncogenic 
forms of several tyrosine kinases, in addition to 
v-src (as shown by Alema and co-workers), are 
able to induce neurite outgrowth in PCI 2 cells. 
These studies strongly support the possibility that 
tyrosine phosphorylation is important in the 
regulation of critical events in neural cell 
development. 
We have characterized a uniquely modified 
form of the src protein that is specifically pro- 
duced in central nervous system neurons. This 
variant form of the src protein {src*) , which con- 
tains six additional amino acids that are not found 
in the src protein expressed in any other cell 
type, is generated by an alternative processing of 
the src-specific messenger RNA in neurons. The 
src* protein displays an elevated level of kinase 
activity. This unique modification suggests that 
src plays a specific role in these specialized cells. 
Using antibodies that exclusively recognize this 
unique form of the src protein, we have localized 
src* to specific subclasses of neurons in different 
regions of the brain. Further studies are designed 
to determine the precise intracellular localiza- 
tion of this form of src in developing and mature 
neurons to provide additional clues to the func- 
tion of this protein. 
Platelets are small anucleate cells in peripheral 
blood that contain many intracellular vesicles 
whose components are released upon activation 
63 
