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Sos1 rapidly associates with Grb2 and is hypophosphorylated when complexed with the EGF receptor after EGF
1Trescowthick Research Centre, Peter MacCallum Cancer Institute, Victoria, Australia.
Abstract:
The Son of sevenless (Sos) protein, a guanine nucleotide exchange factor for ras proteins, appears to play a central role in signalling between protein tyrosine kinase receptors and ras. The C-terminal region of Sos binds an adaptor protein, Grb2, which in turn binds to activated receptors including the EGF receptor (EGFR). Although the Sos protein is rapidly phosphorylated following cytokine stimulation, there is no evidence that this alters the enzymatic activity of Sos for ras proteins. Therefore, we investigated whether the ability of Sos1 to form complexes with Grb2 and with the EGF receptor (EGFR) changes following EGF stimulation, as a possible mechanism for regulating Sos activity. In contrast to earlier findings, we find that both the association and dissociation of Sos1 with Grb2 is responsive to EGF. Whilst the association of Sos1 and Grb2 following EGF stimulation is not cell type specific, we find that it is dependent on cell density and that the response to EGF differs to that induced by NGF. We find that following EGF stimulation, the Sos1 protein associated with the EGFR is markedly less phosphorylated than the majority of the Sos1 within the cell and there was reduced binding of Grb2 with phosphorylated Sos1 protein in a direct binding assay. A time course analysis showed that Sos1 dissociates from the EGFR more rapidly than does Grb2 following EGF stimulation. Collectively our findings are consistent with the notion that the phosphorylation of Sos1 affects its ability to complex with the EGFR and Grb2.
Insights
The Son of sevenless (Sos) protein
Area of Science:
- Cellular signaling pathways
- Molecular biology
- Signal transduction
Background:
- Son of sevenless (Sos) is a guanine nucleotide exchange factor for ras proteins.
- Sos links protein tyrosine kinase receptors to ras signaling.
- Grb2 adaptor protein links Sos to activated receptors like the EGF receptor (EGFR).
Purpose of the Study:
- Investigate how EGF stimulation affects Sos1 complex formation with Grb2 and EGFR.
- Determine if Sos1 phosphorylation regulates its activity in response to EGF.
Main Methods:
- Investigated Sos1 complex formation with Grb2 and EGFR following EGF stimulation.
- Analyzed Sos1 phosphorylation status in EGF-stimulated cells.
- Performed direct binding assays to assess Grb2 binding to phosphorylated Sos1.
- Conducted time course analysis of Sos1 and Grb2 dissociation from EGFR.
Main Results:
- Sos1 association and dissociation with Grb2 are responsive to EGF stimulation.
- EGF-induced Sos1-Grb2 association is cell density-dependent and differs from NGF response.
- Sos1 associated with EGFR is less phosphorylated than bulk Sos1.
- Reduced Grb2 binding to phosphorylated Sos1 was observed.
- Sos1 dissociates from EGFR faster than Grb2 post-EGF stimulation.
Conclusions:
- Sos1 phosphorylation influences its ability to form complexes with EGFR and Grb2.
- EGF signaling dynamically regulates Sos1 interactions with Grb2 and EGFR.
- Phosphorylation state of Sos1 is a key factor in modulating its signaling complex formation.