Mechanism of Action of Polypeptide Growth Factors 
It has recently been postulated that the inositol 
phospholipids may function in the regulation of 
cell shape via interactions with proteins that con- 
trol the polymerization of actin. Using EGF and 
A43 1 cells, we have shown that there is no corre- 
lation between EGF-induced changes in cell 
shape and changes in the levels of the classical 
phosphoinositides, phosphatidylinositol 4-phos- 
phate and phosphatidylinositol 4,5-bisphos- 
phate. Further experiments have also failed to 
document the involvement of novel inositol phos- 
pholipids, phosphorylated on the 3 position of 
the inositol ring, in the regulation of cell shape 
by EGF. These data suggest that EGF may control 
the actin cytoskeleton simply by a mechanism 
that involves phosphorylation of cytoskeletal 
proteins rather than indirectly by causing alter- 
ations in inositol phospholipid levels. 
Desensitization of the EGF Receptor 
When A43 1 cells are treated with large doses of 
EGF, washed, and subsequently rechallenged 
with EGF, they fail to respond to the grovvT:h fac- 
tor. This phenomenon is known as desensitiza- 
tion. Our studies have shown that when the EGF 
receptor becomes desensitized, it is no longer in- 
ternalized into the cells, and EGF no longer stimu- 
lates phosphatidylinositol metabolism. This EGF- 
induced desensitization is specific for the EGF 
receptor, because the responsiveness of other re- 
ceptors is not decreased after EGF treatment. 
The EGF receptor itself is a monomeric pro- 
tein, a single chain. Upon binding of EGF to its 
receptor, two of the receptor monomers come 
together to form an EGF receptor dimer. This 
dimer is the form that is active in signal transduc- 
tion. We have shown that desensitized EGF re- 
ceptors do not transduce a signal because they 
cannot undergo this EGF-induced dimerization. 
Our data suggest that the desensitization of the 
EGF receptor results from its phosphorylation by 
a protein kinase. We have identified a protein ki- 
nase in A43 1 cell cytosol that is activated by EGF 
and appears to be involved in receptor desensiti- 
zation. The kinase catalyzes the phosphorylation 
of the EGF receptor in vitro. Consistent with 
what has been observed in whole cells, phos- 
phorylation of EGF receptor monomers by this 
kinase leads in vitro to an inhibition of the ability 
of the phosphorylated monomers to dimerize. 
The kinase phosphorylates the EGF receptor on 
a serine residue in the second half of the receptor 
molecule. Using techniques of molecular biol- 
ogy, we have altered this site and are in the pro- 
cess of characterizing the properties of cells ex- 
pressing this mutated form of the EGF receptor. 
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