Identification of Y589 and Y599 in the juxtamembrane domain of Flt3 as ligand-induced autophosphorylation sites involved in binding of Src family kinases and the protein tyrosine phosphatase SHP2

Early signal relay steps upon ligand binding to the receptor tyrosine kinase Flt3 (ie, sites of Flt3 autophosphorylation and subsequent docking partners) are mainly unresolved. By immunoprecipitation of specific tryptic peptides contained in the juxtamembrane region of human Flt3 and subsequent radiosequencing, we identified the tyrosine residues 572, 589, 591, and 599 as in vivo autophosphorylation sites. Focusing on Y589 and Y599, we examined FIt3 ligand (FL)-mediated responses in wild-type-Flt3- (WT-Flt3-), Y589F-Flt3-, and Y599F-FIt3-expressing 32D cells. Compared with WT-Flt3-32D cells upon ligand stimulation, 32D-Y589F-Flt3 showed enhanced Erk activation and proliferation/survival, whereas 32D-Y599F-Flt3 cells hereby displayed substantially diminished responses. Both pY589 and pY599 were identified as association sites for signal relay molecules including Src family kinases and SHP2. Consistently, 32D-Y589F-Flt3 and 32D-Y599F-Flt3 showed decreased FL-triggered activation of Src family kinases. Interference with the Src-dependent negative regulation of Flt3 signaling may account for the enhanced mitogenic response of Y589F-Flt3. Y599 was additionally found to interact with the protein tyrosine phosphatase SHP2 in a phosphorylation-dependent manner. As Y599F-Flt3-32D was unable to associate with and to phosphorylate SHP2 and since silencing of SHP2 in WT-Flt3-expressing cells mimicked the Y599F-Flt3 phenotype, we hypothesize that recruitment of SHP2 to pY599 contributes to FL-mediated Erk activation and proliferation.