EGF-induced tyrosine phosphorylation of Endofin is dependent on PI3K activity and proper localization to endosomes

In our previous study, Endofin was validated to be a novel tyrosine phosphorylation target downstream of EGFR. Here, we attempted to map the signaling events associated with Endofin following activation of EGFR with EGF. Tyrosine phosphorylation of endogenous Endofin peaked around 15 min and was modulated within 30 min of EGF treatment. Phosphatidylinositol 3-kinase (PI3K) activity and FYVE domain-mediated localization of Endofin to EEA1-marked endosomes were shown to be necessary for the tyrosine phosphorylation of Endofin. Tyrosine 515 was mapped to be a major phosphorylation site on Endofin but disruption of phosphorylation at Y515 neither affected Endofin's localization nor its co-localization with EGFR in the endosomes. Instead, abrogation of Y515 phosphorylation and mislocalization of Endofin were found to enhance the amplitude of the MAPK cascade, suggesting a possible role of Endofin in the modulation of MAPK pathway. Our study has identified a novel signaling cascade involving EGFR, PI3K, Endofin and MAPK in the EGFR signaling network.