The 22 members of the fibroblast growth factor (FGF) family of growth factors mediate their cellular responses by binding to and activating the different isoforms encoded by the four receptor tyrosine kinases (RTKs) designated FGFR1, FGFR2, FGFR3 and FGFR4. These receptors are key regulators of several developmental processes in which cell fate and differentiation to various tissue lineages are determined. Unlike other growth factors, FGFs act in concert with heparin or heparan sulfate proteoglycan (HSPG) to activate FGFRs and to induce the pleiotropic responses that lead to the variety of cellular responses induced by this large family of growth factors. An alternative, FGF-independent, source of FGFR activation originates from the interaction with cell adhesion molecules, typically in the context of interactions on neural cell membranes and is crucial for neuronal survival and development.
Upon ligand binding, receptor dimers are formed and their intrinsic tyrosine kinase is activated causing phosphorylation of multiple tyrosine residues on the receptors. These then serve as docking sites for the recruitment of SH2 (src homology-2) or PTB (phosphotyrosine binding) domains of adaptors, docking proteins or signaling enzymes. Signaling complexes are assembled and recruited to the active receptors resulting in a cascade of phosphorylation events.
This leads to stimulation of intracellular signaling pathways that control cell proliferation, cell differentiation, cell migration, cell survival and cell shape, depending on the cell type or stage of maturation.
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Fukumoto T, Kubota Y, Kitanaka A, Yamaoka G, Ohara-Waki F, Imataki O, Ohnishi H, Ishida T, Tanaka T.; ''Gab1 transduces PI3K-mediated erythropoietin signals to the Erk pathway and regulates erythropoietin-dependent proliferation and survival of erythroid cells.''; PubMedEurope PMCScholia
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Cantwell-Dorris ER, O'Leary JJ, Sheils OM.; ''BRAFV600E: implications for carcinogenesis and molecular therapy.''; PubMedEurope PMCScholia
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Ong SH, Lim YP, Low BC, Guy GR.; ''SHP2 associates directly with tyrosine phosphorylated p90 (SNT) protein in FGF-stimulated cells.''; PubMedEurope PMCScholia
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Cseh B, Doma E, Baccarini M.; ''"RAF" neighborhood: protein-protein interaction in the Raf/Mek/Erk pathway.''; PubMedEurope PMCScholia
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Davies H, Bignell GR, Cox C, Stephens P, Edkins S, Clegg S, Teague J, Woffendin H, Garnett MJ, Bottomley W, Davis N, Dicks E, Ewing R, Floyd Y, Gray K, Hall S, Hawes R, Hughes J, Kosmidou V, Menzies A, Mould C, Parker A, Stevens C, Watt S, Hooper S, Wilson R, Jayatilake H, Gusterson BA, Cooper C, Shipley J, Hargrave D, Pritchard-Jones K, Maitland N, Chenevix-Trench G, Riggins GJ, Bigner DD, Palmieri G, Cossu A, Flanagan A, Nicholson A, Ho JW, Leung SY, Yuen ST, Weber BL, Seigler HF, Darrow TL, Paterson H, Marais R, Marshall CJ, Wooster R, Stratton MR, Futreal PA.; ''Mutations of the BRAF gene in human cancer.''; PubMedEurope PMCScholia
Brown MD, Sacks DB.; ''Protein scaffolds in MAP kinase signalling.''; PubMedEurope PMCScholia
Xu H, Lee KW, Goldfarb M.; ''Novel recognition motif on fibroblast growth factor receptor mediates direct association and activation of SNT adapter proteins.''; PubMedEurope PMCScholia
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Ruhe JE, Streit S, Hart S, Wong CH, Specht K, Knyazev P, Knyazeva T, Tay LS, Loo HL, Foo P, Wong W, Pok S, Lim SJ, Ong H, Luo M, Ho HK, Peng K, Lee TC, Bezler M, Mann C, Gaertner S, Hoefler H, Iacobelli S, Peter S, Tay A, Brenner S, Venkatesh B, Ullrich A.; ''Genetic alterations in the tyrosine kinase transcriptome of human cancer cell lines.''; PubMedEurope PMCScholia
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Fong CW, Leong HF, Wong ES, Lim J, Yusoff P, Guy GR.; ''Tyrosine phosphorylation of Sprouty2 enhances its interaction with c-Cbl and is crucial for its function.''; PubMedEurope PMCScholia
PD173074 is potent pan-FGFR reversible inhibitor that interacts with residues in the ATP-binding pocket and inhibits tyrosine kinase activity and autophosphorylation (Mohammadi, 1998; Ezzat, 2005). PD173074 is not suitable for therapeutic use due to issues with toxicity.
PD173074 is potent pan-FGFR reversible inhibitor that interacts with residues in the ATP-binding pocket and inhibits tyrosine kinase activity and autophosphorylation (Mohammadi, 1998; Ezzat, 2005). PD173074 is not suitable for therapeutic use due to issues with toxicity.
Signaling by AKT is one of the key outcomes of receptor tyrosine kinase (RTK) activation. AKT is activated by the cellular second messenger PIP3, a phospholipid that is generated by PI3K. In ustimulated cells, PI3K class IA enzymes reside in the cytosol as inactive heterodimers composed of p85 regulatory subunit and p110 catalytic subunit. In this complex, p85 stabilizes p110 while inhibiting its catalytic activity. Upon binding of extracellular ligands to RTKs, receptors dimerize and undergo autophosphorylation. The regulatory subunit of PI3K, p85, is recruited to phosphorylated cytosolic RTK domains either directly or indirectly, through adaptor proteins, leading to a conformational change in the PI3K IA heterodimer that relieves inhibition of the p110 catalytic subunit. Activated PI3K IA phosphorylates PIP2, converting it to PIP3; this reaction is negatively regulated by PTEN phosphatase. PIP3 recruits AKT to the plasma membrane, allowing TORC2 to phosphorylate a conserved serine residue of AKT. Phosphorylation of this serine induces a conformation change in AKT, exposing a conserved threonine residue that is then phosphorylated by PDPK1 (PDK1). Phosphorylation of both the threonine and the serine residue is required to fully activate AKT. The active AKT then dissociates from PIP3 and phosphorylates a number of cytosolic and nuclear proteins that play important roles in cell survival and metabolism. For a recent review of AKT signaling, please refer to Manning and Cantley, 2007.
The RAS-RAF-MEK-ERK pathway regulates processes such as proliferation, differentiation, survival, senescence and cell motility in response to growth factors, hormones and cytokines, among others. Binding of these stimuli to receptors in the plasma membrane promotes the GEF-mediated activation of RAS at the plasma membrane and initiates the three-tiered kinase cascade of the conventional MAPK cascades. GTP-bound RAS recruits RAF (the MAPK kinase kinase), and promotes its dimerization and activation (reviewed in Cseh et al, 2014; Roskoski, 2010; McKay and Morrison, 2007; Wellbrock et al, 2004). Activated RAF phosphorylates the MAPK kinase proteins MEK1 and MEK2 (also known as MAP2K1 and MAP2K2), which in turn phophorylate the proline-directed kinases ERK1 and 2 (also known as MAPK3 and MAPK1) (reviewed in Roskoski, 2012a, b; Kryiakis and Avruch, 2012). Activated ERK proteins may undergo dimerization and have identified targets in both the nucleus and the cytosol; consistent with this, a proportion of activated ERK protein relocalizes to the nucleus in response to stimuli (reviewed in Roskoski 2012b; Turjanski et al, 2007; Plotnikov et al, 2010; Cargnello et al, 2011). Although initially seen as a linear cascade originating at the plasma membrane and culminating in the nucleus, the RAS/RAF MAPK cascade is now also known to be activated from various intracellular location. Temporal and spatial specificity of the cascade is achieved in part through the interaction of pathway components with numerous scaffolding proteins (reviewed in McKay and Morrison, 2007; Brown and Sacks, 2009). The importance of the RAS/RAF MAPK cascade is highlighted by the fact that components of this pathway are mutated with high frequency in a large number of human cancers. Activating mutations in RAS are found in approximately one third of human cancers, while ~8% of tumors express an activated form of BRAF (Roberts and Der, 2007; Davies et al, 2002; Cantwell-Dorris et al, 2011).
The phospho-tyrosine positions for FRS2-beta were inferred by similarity to the analogous positions in FRS2-alpha. Five out of six tyrosine positions in alpha are present in beta.
SPRY2 translocates to the plasma membrane upon activation of cells with FGF, and translocation is required for the inhibition of growth factor-stimulated cell migration, proliferation and differentiation. Translocation may be mediated by interactions with PIP2 in the membrane, palmitoylation of the C-terminal region of SPRY2 and/or interactions with caveolin-1.
Sprouty 2 protein is phosphorylated on tyrosine residue 55. The ability of SRC kinase to catalyze this reaction has been demonstrated with purified proteins in vitro (Li et al. 2004) and in cultured cells with studies of the effects of SRC-family pharmacological inhibitors and of dominant-negative mutant SRC proteins (Mason et al. 2004). SRC kinase also phosphorylates numerous tyrosine residues in the C terminal region of SPRY2 including Y227, in response to FGF but not EGF stimulation.
Some evidence suggests that SPRY2 may exert its negative effect by binding to GRB2 and competing with the GRB2:SOS1 interaction that is required for MAPK activation. SPRY2 phosphorylation at Y55 is stimulated in response to both FGF and EGF, and is required for SPRY2 to act as a negative regulator of FGF signaling. Y55 is not thought to be a GRB2 binding site, however. Instead, phosphorylation at Y55 is thought to cause a conformational change in SPRY2 that reveals a cryptic PXXPXPR GRB2-docking site in the C-terminal of SPRY2. SPRY2 has also been shown to be phosphorylated at multiple tyrosine residues in its C-terminal in response to FGF, but not EGF, stimulation. This phosphorylation, in particular at residue 227, is thought to augment the ability of SPRY2 to inhibit FGF signaling through the MAPK cascade, although the mechanism remains to be elucidated.
The N terminal TKB domain of CBL binds to the phospho-tyrosine 55 of SPRY2, targeting SPRY2 for degradation by the 26S proteasome. Y55 is also a binding site for PP2A, which dephosphorylates numerous serine and threonine residues on SPRY2, allowing a conformational change that may promote a SPRY2:GRB2 interaction and limit the extent of MAPK activation following FGF stimulation.
In unstimulated cells, SPRY2 has been shown to be phosphorylated on multiple serine and threonine residues. In these cells, SPRY2 exists in a complex with the regulatory and catalytic subunits (A and C, respectively) of the serine/threonine phosphatase PP2A. After stimulation with FGF, the catalytic activity of PP2A increases and the phosphatase dephophorylates SPRY at serine 112 and serine 115. This is thought to promote changes in tertiary structure that promote GRB2 binding and phosphorylation of Y55 and Y227.
Some evidence suggests that SPRY2 can exert its negative role on FGF signaling at the level of RAF activation. Hypophosphorylated SPRY2 binds to inactive B-RAF, preventing it from activating ERK signaling. MAPK activation results in phosphorylation of SPRY2 on six serine residues (S7, S42, S111, S120, S140 and S167), and inhibits B-RAF binding. Phosphorylation at S111 and S120 directly affects B-RAF binding while the remaining four sites appear to contribute indirectly. Oncogenic forms of B-RAF such as B-RAF V600E, which adopt active kinase conformations, do not associate with SPRY2, regardless of its phosphorylation status. This suggests that two mechanisms affect the SPRY2:B-RAF interaction: SPRY2 phosphorylation and B-RAF conformation.
After being bound by BetaKlotho and FGF19, FGFR4 undergoes autophosphorylation on tyrosine residues in the intracellular portion of the receptor. The phosphorylation sites on FGFR4 have not been accurately determined in vitro or in vivo but are predicted based on sequence comparison with the other FGF receptors.
PPTN11 (also known as SHP2) may exert its positive effects on MAPK activation in response to FGF stimulation by catalyzing the dephosphorylation of tyrosine resides on SPRY2. This dephosphorylation promotes dissociation of the GRB2/SPRY2 complex and as a consequence stimulates GRB2 association with the activated receptor, leading to sustained MAPK signaling.
The Src homology 2 (SH2) domain of the phosphatidylinositol 3-kinase (PIK3) regulatory subunit (PIK3R1, i.e. PI3Kp85) binds to GAB1 in a phosphorylation-independent manner. GAB1 serves as a docking protein which recruits a number of downstream signalling proteins. PIK3R1 can bind to either GAB1 or phosphorylated GAB1.
In this reaction, FGF receptor in the plasma membrane binds an associating extracellular ligand, a requisite step for subsequent activation. The resulting complex consists of dimerized receptor, two ligand molecules, and heparan sulfate.
The intrinsic protein tyrosine kinase activity of activated FGF receptor 4 catalyzes multiple phosphorylation events, creating a number of binding sites for membrane bound docking proteins to gather intracellular signaling mediators. Based on sequence alignment, FGFR4 contains 5 of the 8 cytoplasmic tyrosine residues identified in FGFR1.
The Y367C mutation in FGFR4 was identified in a breast cancer cell line in a cDNA screen of kinase mutants (Ruhe, 2007). This residue is paralogous to the FGFR2 Y375C and FGFR3 Y373C mutations that have been shown to result in increased receptor activation (Prezylepa, 1996; Rousseau, 1996; d'Avis, 1998). Biochemical characterization of the FGFR4 Y367C mutation revealed that it undergoes spontaneous dimerization independent of ligand stimulation, presumably mediated by the aberrant cysteine residues in the extracellular region of the receptor (Roidl, 2009), thus affecting FGFR4 activity without directly altering its kinase activity.
Expression of the FGFR4 Y367C mutant in MDA-MB453 breast cancer cell line results in constitutive tyrosine phosphorylation of the receptor and serum-independent activation of downstream signaling as monitored by Erk phosphorylation. Ectopic expression of FGFR4 Y367C in HEK cells also leads to Erk activation and enhanced cellular proliferation. Akt and phospho-AKT levels were not affected by overexpression of the FGFR4 Y367C mutant, however (Roidl, 2010)
FGFR4 N535K and FGFR4 V550E have been shown to undergo autophosphorylation when transfected into a murine rhabdomysarcoma (RMS) cell line and to promote transformation in NIH 3T3 cells. Receptor activation leads to increased signaling through STAT3, but decreased levels of phospho-AKT and phospho-Erk1/2 compared to vector control. Cells transfected with the N535K and V550E mutants also showed upregulation of cell cycle and DNA replication gene pathways and significantly higher growth rates (Taylor, 2009).
FGFR4 is highly expressed in rhabdomyosarcoma (RMS) tissue, and high levels of expression are correlated with lower survival. Sequencing of exons from 94 RMS tumors identified 14 missense variants, 6 of which were localized in the tyrosine kinase domain, and four of which were in two amino acids (N535K/D and V550E/L). Mutations at amino acid 535 are predicted to eliminate an inhibitory H-bond that restricts receptor autophosphorylation, and mutations at amino acid 550 are believed to alter the ATP-binding site. Functional studies on N535K and V550 show that they undergo autophosphorylation when transfected into a murine RMS lines and transformed NIH 3T3 cells, leading to a metastatic phenotype (Taylor, 2009).
Treatment of cells expressing FGFR4 N535K or V550E with the in vitro FGFR inhibitor PD170734 reduces the levels of receptor autophosphorylation and increases the activation of caspase-3 and the number of cells undergoing apoptosis (Taylor, 2009).
PLC gamma is phosphorylated by activated FGFR, resulting in PLC gamma activation, stimulation of phosphatidyl inositol hydrolysis and generation of two second messengers, diacylglycerol and inositol (1,4,5) P3. Tyrosine phosphorylation of PLCgamma by FGFR4 is weaker than that seen by other isoforms of FGFR.
Recruitment of PLC-gamma by FGF receptors has been best studied in FGFR1c signaling, where it has been shown that autophosphorylation of Tyr766 in the C-terminal tail of FGFR1c creates a specific binding site for the SH2 domain of PLC-gamma. A mutant FGFR1c in which Y766 is replaced by phenylalanine is unable to activate PI hydrolysis and Ca2+ release in response to FGF stimulation. Membrane recruitment of PLC-gamma is also aided by binding of the Pleckstrin homology (PH) domain of this enzyme to PtIns(3,4,5) P3 molecules that are generated in response to PI-3 kinase stimulation. By sequence comparison, Y766 is conserved in all FGFR isoforms, and PLC-gamma signaling is observed, to a greater or lesser extent, downstream of all FGFR receptors upon stimulation with FGFs.
Dissociation from the activated receptor quickly follows phosphorylation of PLC-gamma. Phosphorylated PLC-gamma catalyzes the hydrolysis of phosphatidylinositol(4, 5)bisphosphate to generate two second messengers, diacylglycerol and inositol (1,4,5) triphosphate.
FRS2 (also known as FRS alpha is activated through tyrosine phosphorylation catalyzed by the protein kinase domain of the activated FGFR. FRS2 contains four binding sites for the adaptor protein GRB2 at residues Y196, Y306, Y349 and Y392, and two binding sites for the protein tyrosine phosphatase PPTN11/SHP2 at residues Y436 and Y471. Different FGFR isoforms may generate different phosphorylation patterns on FRS2 leading to alternate downstream signaling.
FRS2 (also known as FRS2alpha) is broadly expressed in adult and fetal tissues. Membrane-bound FRS2 interacts with FGFR as a first step in the phosphorylation of this docking protein. The juxtamembrane binding site for FRS2 does not contain tyrosine, so binding may be independent of receptor activation and/or constitutive. Activation of the FGFR receptor is required for FRS2 phosphorylation and subsequent recruitment of downstream effectors.
Phosphorylated SHC1 links FGFR to Grb2 (Klint et al. 1995) leading to the formation of a signaling complex including Shc, Grb2 and Sos. Transformation of NIH 3T3 cells with v-Src produced a strong constitutive association of FGFR1 with Shc, Grb2 and Sos (Curto et al. 1998) suggesting Src involvement. Recruitment of Grb2-Sos links FGFR to the Ras pathway.
The p46 and p53 isoforms of SHC1 have been shown to be phosphorylated upon FGF stimulation. Three consensus RTK phosphoryation sites are present in SHC1, although phosphorylation of these specific tyrosine residues has not been explicitly demonstrated in response to FGF stimulation. In contrast, the p66 isoform of SHC1 does not appear to undergo FGF-dependent phosphorylation.
FRS2 has 8 canonical MAPK phosphorylation sites which are phosphorylated by activated ERK1/2 after FGF stimulation. Phosphorylation of these 8 threonine residues counteracts the activating effect of tyrosine phosphorylation of FRS2, although the exact mechanism for this negative regulation is not known. Expression of a version of FRS2 in which the 8 threonine residues are mutated to valine results in enhanced tyrosine phosphorylation of FRS2, enhanced GRB2-SOS1 recruitment and a more sustained MAPK response. The 8 threonine residues are not conserved in FRS3; as a result, signaling through FRS3 complexes do not appear to be subject to this downregulation.
FRS3 (also known as FRS2beta) is predominantly expressed in the developing and adult neuroepithelium. As is the case for FRS2 (also known as FRS2alpha), binding of FRS3 to FGFR may be constitutive and/or independent of receptor activation. Elements of the downstream signaling mediated by the two FRS family members appear to be at least partially conserved, as FRS3 is phosphorylated upon FGF stimulation, binds PPTN11/SHP2 and GRB2 and results in ERK activation. Moreover, expression of FRS3 in FRS2-/- MEFs restores ERK activation.
FRS3 (also known as FRS2 beta) is activated through tyrosine phosphorylation catalyzed by the protein kinase domain of the activated FGFR. By sequence comparison, FRS3 has the 2 PPTN11/SHP2-binding sites and has three of the four GRB2-binding sites.
Tyrosine phosphorylation of PPTN11/SHP2 by FGFR kinase is required for activation of the phosphatase activity of PPTN11 and for downstream signaling. Tyrosine phosphorylated PPTN11 plays a major role in the activation of RAS-MAP kinase pathway, although the precise role is not yet clear.
Tyrosine phosphorylated FRS2 recruits GRB2:SOS1 complex by means of the SH3 domain of GRB2, leading to RAS-MAP kinase activation. The FRS2:GRB2-mediated pathway plays a minor role in the activation of RAS-MAP kinase pathway compared to that mediated by FRS2:PPTN11.
Grb2 bound to tyrosine phosphorylated FRS2 forms a ternary complex with Cbl through the binding of the SH3 domains of Grb2 to a proline rich region in Cbl. Grb2-mediated recruitment of Cbl results in ubiquitination of FGFR and FRS2. Cbl is a multidomain protein that posses an intrinsic ubiquitin ligase activity and also functions as a platform for recruitment of a variety of signaling proteins. Multiple mechanisms appear to be required for downregulation of FGFR, as internalization of the receptor is reduced but not abolished if recruitment of CBL to FRS2 is compromised by mutation of GRB2-binding sites.
Once recruited to the activated receptor, PI3K phosphorylates PIP2 to PIP3, leading to activation of AKT signaling. PI3K signaling has been demonstrated in ZMYM2-, FOP- and BCR-FGFR1 fusions (Chen, 2004; Demiroglu, 2001; Guasch, 2001), as well as downstream of a number of other FGFR mutants (see for instance, Byron, 2008; Kunii, 2008; Agazie, 2003; Takeda, 2007).
The ubiquitin ligase CBL exists in a complex with GRB2 and is recruited to tyrosine-phosphorylated FRS2 after FGF stimulation. In addition to promoting the ubiquitination, endocytosis, and degradation of the activated receptor complex, recruitment of the p-CBL:GRB2 complex seems to attenuate FGFR signaling by competing with GRB2:SOS1 for binding to the direct GRB2-binding sites on p-FRS2.
RAS nucleotide is stimulated downstream of activated FGFR4 in a p-PTPN11-dependent manner. The phosphatase activity of PTPN11 is required for activation of the RAS-MAP kinase pathway, although the mechanism for RAS pathway activation is not yet clear (Hadari et al, 1998; reviewed in Mohi et al, 2007; Gotoh et al, 2008).
In humans, the phosphorylated MNK1 kinase phosphorylates the adaptor protein Sprouty2 on Ser112 and Ser121, and also at some other serine and threonine residues. MNK1 appears not to form a complex with Sprouty2. Some of these (including the two main sites mentioned above) conform to the serine-containing consensus sites for phosphorylation by MNK1 kinase (K/R-X-X-S, R-X-S). It appears that serine phosphorylation is required to protect Sprouty2 from degradation.
In the absence of serine phosphorylation, phosphorylation of Tyr55 and subsequent binding to E3 ubiquitin ligase, CBL, is enhanced. Serine phosphorylation of Sprouty2 appears to stabilise the protein by interfering with its potential phosphorylation of Tyr55 (Sprouty2 appears to be a poor substrate for c-Src kinase) in response to growth factor stimulation.
In humans, the phosphorylated adaptor protein Sprouty2 is ubiquitinated by the E3 ubiquitin ligase CBL, marking it for degradation by the 26S proteasome.
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bound to
FGF19:BetaKlothoenhanced kinase
mutantskinase mutant
dimerswith enhanced kinase
activity:PD173074(A:C):S112/S115
p-SPRY2The importance of the RAS/RAF MAPK cascade is highlighted by the fact that components of this pathway are mutated with high frequency in a large number of human cancers. Activating mutations in RAS are found in approximately one third of human cancers, while ~8% of tumors express an activated form of BRAF (Roberts and Der, 2007; Davies et al, 2002; Cantwell-Dorris et al, 2011).
Annotated Interactions
bound to
FGF19:BetaKlothoenhanced kinase
mutantskinase mutant
dimerskinase mutant
dimerskinase mutant
dimerskinase mutant
dimerswith enhanced kinase
activity:PD173074(A:C):S112/S115
p-SPRY2(A:C):S112/S115
p-SPRY2SPRY2 has also been shown to be phosphorylated at multiple tyrosine residues in its C-terminal in response to FGF, but not EGF, stimulation. This phosphorylation, in particular at residue 227, is thought to augment the ability of SPRY2 to inhibit FGF signaling through the MAPK cascade, although the mechanism remains to be elucidated.
In the absence of serine phosphorylation, phosphorylation of Tyr55 and subsequent binding to E3 ubiquitin ligase, CBL, is enhanced. Serine phosphorylation of Sprouty2 appears to stabilise the protein by interfering with its potential phosphorylation of Tyr55 (Sprouty2 appears to be a poor substrate for c-Src kinase) in response to growth factor stimulation.