Phosphorylation and Protein-Protein Interactions 
in Signal Transduction 
Tony Pawson, Ph.D. — International Research Scholar 
Dr. Pawson is Senior Scientist at the Samuel Lunenfeld Research Institute of Mount Sinai Hospital, 
Toronto, where he holds the Apotex Chair of Molecular Oncology; Professor of Molecular and Medical 
Genetics at the University of Toronto, and a Terry Fox Cancer Research Scientist of the National Cancer 
Institute of Canada. He received his Ph.D. degree from the University of London while working at the 
Imperial Cancer Research Fund. Prior to moving to Toronto, Dr. Pawson did postdoctoral work at the 
University of California, Berkeley, and was a faculty member of the University of British Columbia. 
MANY of the polypeptide hormones that con- 
trol development in the embryo and cell 
growth, differentiation, and metabolism in the 
adult bind to the extracellular region of receptors 
that span the plasma membrane. These receptors, 
in their cytoplasmic region, contain a protein ki- 
nase domain that phosphorylates tyrosine resi- 
dues. Normally these catalytic receptors are only 
active when engaged by the appropriate growth 
factor. However, structural alterations, resulting 
from cancer-causing mutations in the relevant 
genes, can render them active even in the absence 
of a growth factor. Such aberrantly active recep- 
tors can trigger unrestricted cancerous cell 
Activated growth factor receptors can elicit 
many changes in stimulated cells, including 
changes in gene expression, cellular architec- 
ture, cell-cell communications, and cellular me- 
tabolism. An important question involves the 
identities of the intracellular targets of receptor 
tyrosine kinases — targets that control signal 
transduction pathways within the cell. 
SH2 -containing Proteins Bind Activated 
Growth Factor Receptors 
The first thing growth factor receptors do upon 
activation, apparently, is phosphorylate them- 
selves on tyrosine. Receptor autophosphoryla- 
tion serves as a switch to elicit the tight binding 
of intracellular signaling proteins to the receptor. 
These receptor-binding proteins include Ras 
GTPase-activating protein (GAP), phospholipase 
C-7, phosphatidylinositol (PI) 3'-kinase, and Src- 
family cytoplasmic tyrosine kinases. They have 
the attributes expected of signaling proteins, in 
the sense that they regulate specific intracellular 
signal transduction pathways and in some cases 
are known to be phosphorylated, and activated, 
by receptor tyrosine kinases. How are these func- 
tionally diverse signaling proteins all able to rec- 
ognize autophosphorylated receptors? They each 
contain one or two copies of a protein domain, 
the Src homology 2 (SH2) domain, which recog- 
nizes specific autophosphorylated sites. Thus ty- 
rosine phosphorylation of the receptor itself 
creates a tight binding site for SH2 domains and 
promotes a strong interaction between the acti- 
vated receptor and its target SH2-containing 
proteins. 
There is evidently some specificity to these 
SH2-receptor interactions. Different SH2 do- 
mains bind tenaciously to distinct phosphory- 
lated sites on a receptor, suggesting that the 
amino acid sequence surrounding the phospho- 
tyrosine residue is important in determining tar- 
get specificity. 
A Network of SH2 -mediated Interaction 
An increasing number of SH2-containing pro- 
teins that can bind to activated growth factor re- 
ceptors have been identified. These are all likely 
to regulate some aspect of intracellular signaling, 
though in several cases the relevant biochemical 
pathways have not been defined. 
The association of SH2 domains with auto- 
phosphorylated growTih factor receptors is one in- 
dication of their general ability to bind 
tyrosine-phosphorylated sites. As an example, 
considerable data suggest that the SH2 domain of 
the c-Src cytoplasmic tyrosine kinase interacts 
with an inhibitory tyrosine phosphorylated site in 
its own carboxyl-terminal tail, thereby repressing 
c-Src tyrosine kinase activity. However, once 
c-Src is enzymatically (and oncogenically) acti- 
vated by removal of the carboxyl-terminal phos- 
phorylation site, the SH2 domain has a positive 
transforming effect, probably by virtue of its abil- 
ity to bind substrates of the kinase domain and 
participate in the formation of signaling com- 
plexes. Thus the c-Src SH2 domain has multiple 
functions. GAP is another example of a signaling 
protein whose SH2 domains can interact with 
several different phosphotyrosine-containing 
proteins. 
We are pursuing the structure and function of 
SH2 domains, using a variety of biochemical, cel- 
lular, and genetic approaches. 
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