Regulation of Cellular Processes by Protein- Tyrosine Phosphorylation 
studies raise the possibility that integrin recep- 
tors in other cell types play similar roles in cou- 
pling with cytoplasmic tyrosine kinases. 
We have also found that another platelet mem- 
brane receptor, GPIV, or CD36, is tightly cou- 
pled with three other protein-tyrosine kinases. 
These kinases, Fyn, Lyn, and Yes, are members of 
the Src family of protein-tyrosine kinases, which 
have been shown to be linked with transmem- 
brane receptors in other cell types. The ligand 
that binds to GPIV/CD36 has not been identified; 
however, the evidence that this receptor is asso- 
ciated with protein-tyrosine kinases strongly im- 
plicates it is an intracellular signal transducer. 
The Fc receptors for the complement-binding 
domain of antibody molecules are also found on 
platelets. Activation of these receptors causes 
platelet aggregation and secretion, and the in- 
duction of tyrosine phosphorylation of the same 
proteins that are phosphorylated in thrombin- 
treated platelets. However, unlike other platelet 
receptors, the FC-7RII receptor is itself phos- 
phorylated on tyrosine. The site(s) of tyrosine 
phosphorylation of this receptor lies within a 
peptide motif that is shared with several other 
lymphocyte receptors in T cells, B cells, natural 
killer cells, mast cells, and basophils. This motif 
appears to be involved in coupling these recep- 
tors with protein-tyrosine kinases. Tyrosine phos- 
phorylation of the conserved tyrosine residues in 
this motif may be important for these coupling 
interactions. 
Tyrosine Phosphorylation in Neuronal 
PCI 2 Cells 
We are interested in defining the role of tyro- 
sine phosphorylation in mediating neuronal dif- 
ferentiation. PCI 2 cells, derived from a rat 
pheochromocytoma, provide a useful model sys- 
tem, since these cells differentiate into cells re- 
sembling sympathetic neurons after treatment 
with nerve growth factor (NGF) or fibroblast 
growth factor (FGF) . Both of these receptors are 
transmembrane proteins whose cytoplasmic do- 
mains contain protein-tyrosine kinase activity. 
Growth factor binding to these cells causes a 
burst of tyrosine phosphorylation of multiple 
cellular proteins. 
Recently, in collaboration with Simon Ha- 
legoua, we have found that tyrosine phosphoryla- 
tion of two serine/threonine protein kinases, 
p42MAPK p44MAPK jg dependent on the small 
GTP-binding protein Ras. Expression of a mutant 
of Ras that interferes with the activity of the en- 
dogenous Ras protein blocks tyrosine phosphory- 
lation of these proteins, and expression of a con- 
stitutively activated form of Ras leads to their 
tyrosine phosphorylation. These mutant forms of 
Ras have no effect on the activity of the NGF re- 
ceptor itself or on the phosphorylation of pro- 
teins that are direct substrates of this kinase. 
The p42 and p44 MAP kinases are referred to as 
"switch kinases." They are activated by phos- 
phorylation on tyrosine but phosphorylate other 
proteins on serine and threonine. MAP kinases are 
activated by a variety of mitogens and growth fac- 
tors, and are believed to serve as critical compo- 
nents of intracellular signaling pathways by inte- 
grating signals from a diverse array of receptors. 
Our studies suggest that tyrosine phosphoryla- 
tion of MAP kinase is dependent on pathways reg- 
ulated by the Ras GTP-binding protein, and that 
this event can thus be distinguished from phos- 
phorylation events that are directly mediated by 
the NGF receptor itself. Since Ras activity is es- 
sential for NGF- and FGF-induced neuronal dif- 
ferentiation of PC 12 cells, these results raise the 
possibility that MAP kinases are also essential 
components of this process. 
Dr. Brugge is now Scientific Director at Ariad 
Pharmaceuticals, Cambridge, Massachusetts. 
56 
