The signalling cascade including Raf, mitogen-activated protein kinase (MAPK) kinase and extracellular-signal-regulated kinase (ERK) is important in many facets of cellular regulation1-3. Raf is activated through both Ras-dependent and Ras-independent mechanisms4-6, but the regulatory mechanisms of Raf activation remain unclear7-9. Two families of membrane-bound molecules, Sprouty and Sprouty-related EVH1-domain-containing protein (Spred) have been identified10-13 and characterized as negative regulators of growth-factor-induced ERK activation14-25. But the molecular functions of mammalian Sproutys have not been clarified. Here we show that mammalian Sprouty4 suppresses vascular epithelial growth factor (VEGF)-induced, Ras-independent activation of Raf1 but does not affect epidermal growth factor (EGF)-induced, Ras-dependent activation of Raf1. Sprouty4 binds to Raf1 through its carboxy-terminal cysteine-rich domain, and this binding is necessary for the inhibitory activity of Sprouty4. In addition, Sprouty4 mutants of the amino-terminal region containing the conserved tyrosine residue, which is necessary for suppressing fibroblast growth factor signalling19,25, still inhibit the VEGF-induced ERK pathway. Our results show that receptor tyrosine kinases use distinct pathways for Raf and ERK activation and that Sprouty4 differentially regulates these pathways.
All Science Journal Classification (ASJC) codes
- Cell Biology