FRS2α attenuates FGF receptor signaling by Grb2- mediated recruitment of the ubiquitin ligase Cbl

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Abstract
Attenuation of growth factor signaling is essential for the regulation of developmental processes and tissue homeostasis in most organisms. The product of Cbl protooncogene is one such regulator, which functions as an ubiquitin ligase that ubiquitinates and promotes the degradation of a variety of cell signaling proteins. Here, we demonstrate that Grb2 bound to tyrosine-phosphorylated FRS2α forms a ternary complex with Cbl by means of its Src homology 3 domains resulting in the ubiquitination of fibroblast growth factor (FGF) receptor and FRS2α in response to FGF stimulation. These observations highlight the importance of FRS2α in the assembly of both positive (i.e., Sos, phosphatidylinositol 3-kinase) and negative (i.e., Cbl) signaling proteins to mediate a balanced FGF signal transduction. However, the partial inhibition of FGF receptor down-regulation in FRS2α−/− cells indicates that the attenuation of signaling by FGF receptor is regulated by redundant or multiple mechanisms.