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Fibroblast Growth Factor Receptor 1 (FGFR1) Tyrosine Phosphorylation Regulates Binding of FGFR Substrate 2 (FRS2) But Not FRS2β to the Receptor

Center for Cancer and Stem Cell Biology, Institute of Biosciences and Technology, Texas A&M Health Science Center, Houston, Texas 77030-3303

Address all correspondence and requests for reprints to: F. Wang, Center for Cancer and Stem Cell Biology, Institute of Biosciences and Technology, Texas A&M Health Science Center, 2121 West Holcombe Boulevard, Houston, Texas 77030-3303. E-mail: fwang{at}ibt.tamhsc.edu.

Binding of the fibroblast growth factor (FGF) to the FGF receptor (FGFR) tyrosine kinase leads to receptor tyrosine autophosphorylation as well as phosphorylation of multiple downstream signaling molecules that are recruited to the receptor either by direct binding or through adaptor proteins. The FGFR substrate 2 (FRS2) family consists of two members, FRS2 and FRS2β, and has been shown to recruit multiple signaling molecules, including Grb2 and Shp2, to FGFR1. To better understand how FRS2 interacted with FGFR1, in vivo binding assays with coexpressed FGFR1 and FRS2 recombinant proteins in mammalian cells were carried out. The results showed that the interaction of full-length FRS2, but not FRS2β, with FGFR1 was enhanced by activation of the receptor kinase. The truncated FRS2 mutant that was comprised only of the phosphotyrosine-binding domain (PTB) bound FGFR1 constitutively, suggesting that the C-terminal sequence downstream the PTB domain inhibited the PTB-FGFR1 binding. Inactivation of the FGFR1 kinase and substitutions of tyrosine phosphorylation sites of FGFR1, but not FRS2, reduced binding of FGFR1 with FRS2. The results suggest that although the tyrosine autophosphorylation sites of FGFR1 did not constitute the binding sites for FRS2, phosphorylation of these residues was essential for optimal interaction with FRS2. In addition, it was demonstrated that the Grb2-binding sites of FRS2 are essential for mediating signals of FGFR1 to activate the FiRE enhancer of the mouse syndecan 1 gene. The results, for the first time, demonstrate the specific signals mediated by the Grb2-binding sites and further our understanding of FGF signal transmission at the adaptor level.

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