the Raf-1 protein, was readily phosphorylated by 
the KI deletion mutant of the receptor, both in 
intact cells and in vitro. This is consistent with the 
abiUty of this mutant to stimulate PI hydrolysis in 
intact cells. 
C. 3' -Phosphatidylinositol kinase (PI kinase). 
This enzyme, which phosphorylates phosphatidyl- 
inositol at the 3 position of the inositol ring, was 
the first protein shown to be associated directly 
with the ligand-activated PDGF receptor in intact 
cells. This year. Dr. Kaplan showed that purified 
and immobilized PDGF receptor bound to PI kinase 
in 3T3 cells in vitro. 
Together these experiments show that ligand-ac- 
tivated PDGF receptor physically associates with 
several transducing molecules and phosphorylates 
the molecules on tyrosine residues. Preliminary ex- 
periments suggest that the receptor domains that 
are involved in these protein associations are dis- 
tinct for each signaling molecule. Physical associa- 
tion of the receptor with the signal-transducing 
molecules may be involved in localizing the activi- 
ties of these enzymes. The experiments on the Raf 
kinase provide the first direct evidence of a biologi- 
cal activity that is directly altered by a receptor tyro- 
sine kinase. 
II. Autophosphorylation Sites of the PDGF Receptor. 
Dr. Escobedo and Dr. Wendy Fantl showed that 
PUBLICATIONS 
substitution of phenylalanine for tyrosine at posi- 
tion 825 of the mouse PDGF P-receptor caused a 
critical alteration in the substrate specificity of the 
receptor's tyrosine kinase and a concomitant defi- 
ciency in the ability of the receptor to mediate 
PDGF-stimulated DNA synthesis. The roles of this 
and other PDGF receptor autophosphorylation sites 
in ligand-mediated association of the receptor with 
signaling molecules is currently under investigation. 
III. Fibroblast Growth Factor Receptor. 
The fibroblast growth factor (FGF) receptor was 
purified from 50,000 chicken embryos by Dr. Pau- 
line Lee in Dr. Williams's laboratory. Amino acid se- 
quences of tryptic digest fragments of the receptor 
were obtained by Dr. Victor Fried (St. Jude 
Children's Research Hospital). Dr. Lee and Dr. Dan- 
iel Johnson cloned the cDNA for the chicken FGF 
receptor and for the human analogues of the recep- 
tor. The chicken FGF receptor, expressed in 
Xenopus oocytes by microinjection of mRNA, medi- 
ated FGF-stimulated changes in calcium flux. Ongo- 
ing work on the role of this receptor and the devel- 
opmental biology in angiogenesis will be supported 
in the future by the National Institutes of Health; 
HHMI will support work on the structure of this re- 
ceptor. 
Dr. Williams is also Professor of Medicine at the 
University of California at San Francisco. 
Articles 
Coughlin, S.R., Escobedo, J.A., and WiUiams, L.T. 1989. Role of phosphatidylinositol kinase in PDGF receptor 
signal transduction. Science 243:1191-1194. 
Escobedo, J.A. , Barr, RJ., and Williams, L.T. 1988. Role of tyrosine kinase and membrane-spanning domains in 
signal transduction by the platelet-derived growth factor receptor. Mol Cell Biol 8:5126-515'^^. 
Escobedo, J.A., and Williams, L.T. 1988. A PDGF receptor domain essential for mitogenesis but not for many 
other responses to PDGF. Nature 335:85-87. 
Harsh, G.R., ly Kavanaugh, WM., Starksen, N.F., and WiUiams, L.T. 1989. Cyclic AMP blocks expression of the 
c-sis gene in tumor cells. Oncogene Res 4:65-73. 
Keating, M.T, Escobedo, J.A., and Williams, L.T. 1988. Ligand activation causes a phosphorylation-dependent 
change in platelet-derived growth factor receptor conformation. J Biol Chem 263:12805-12808. 
Keating, M.T, Harryman, C.C., and WiUiams, L.T. 1989- Platelet-derived growth factor receptor inducibility is 
acquired immediately after translation and does not require glycosylation. J Biol Chem 264:9129-9132. 
Lee, PL., Johnson, D.E., Cousens, L.S., Fried, VA., and WUliams, L.T. 1989. Purification and complementary 
DNA cloning of a receptor for basic fibroblast growth factor. Science 245:57-60. 
Orchansky PL., Escobedo, J. A., and WUliams, L.T . 1988. Phosphatidylinositol linkage of a truncated form of 
the platelet-derived growth factor receptor. J Biol Chem 263:15159-15165. 
Continued 
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