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Mol. Cell. Biol., Mar 1997, 1702-1713, Vol 17, No. 3
DD Schlaepfer, MA Broome and T Hunter
The focal adhesion kinase (FAK), a protein-tyrosine kinase (PTK),
associates with integrin receptors and is activated by cell binding to
extracellular matrix proteins, such as fibronectin (FN). FAK
autophosphorylation at Tyr-397 promotes Src homology 2 (SH2) domain binding
of Src family PTKs, and c-Src phosphorylation of FAK at Tyr-925 creates an
SH2 binding site for the Grb2 SH2-SH3 adaptor protein. FN- stimulated Grb2
binding to FAK may facilitate intracellular signaling to targets such as
ERK2-mitogen-activated protein kinase. We examined FN-stimulated signaling
to ERK2 and found that ERK2 activation was reduced 10-fold in Src-
fibroblasts, compared to that of Src- fibroblasts stably reexpressing
wild-type c-Src. FN-stimulated FAK phosphotyrosine (P.Tyr) and Grb2 binding
to FAK were reduced, whereas the tyrosine phosphorylation of another
signaling protein, p130cas, was not detected in the Src- cells. Stable
expression of residues 1 to 298 of Src (Src 1-298, which encompass the SH3
and SH2 domains of c-Src) in the Src- cells blocked Grb2 binding to FAK;
but surprisingly, Src 1-298 expression also resulted in elevated p130cas
P.Tyr levels and a two- to threefold increase in FN-stimulated ERK2
activity compared to levels in Src- cells. Src 1-298 bound to both FAK and
p130cas and promoted FAK association with p130cas in vivo. FAK was observed
to phosphorylate p130cas in vitro and could thus phosphorylate p130cas upon
FN stimulation of the Src 1-298-expressing cells. FAK-induced
phosphorylation of p130cas in the Src 1-298 cells promoted the SH2
domain-dependent binding of the Nck adaptor protein to p130cas, which may
facilitate signaling to ERK2. These results show that there are additional
FN-stimulated pathways to ERK2 that do not involve Grb2 binding to FAK.
Copyright © 1997, American Society for Microbiology
Fibronectin-stimulated signaling from a focal adhesion kinase-c-Src complex: involvement of the Grb2, p130cas, and Nck adaptor proteins
Molecular Biology and Virology Laboratory, Salk Institute for Biological Studies, La Jolla, California 92037, USA.
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