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Identification of the mitogen-activated protein kinase phosphorylation sites on human Sos1 that regulate interaction
S Corbalan-Garcia1, S S Yang, K R Degenhardt
1Department of Molecular Genetics and Microbiology, State University of New York at Stony Brook 11794-8621, USA.
Abstract:
The Son of sevenless proteins (Sos) are guanine nucleotide exchange factors involved in the activation of Ras by cytoplasmic and receptor tyrosine kinases. Growth factor stimulation rapidly induces the phosphorylation of Sos on multiple serine and threonine sites. Previous studies have demonstrated that growth factor-induced Sos phosphorylation occurs at the C-terminal region of the protein and is mediated, in part, by mitogen-activated protein (MAP) kinase. In this report, we describe the identification of five MAP kinase sites (S-1137, S-1167, S-1178, S-1193, and S-1197) on hSos1. We demonstrate that four of these sites, S-1132, S-1167, S-1178, and S-1193, become phosphorylated following growth factor stimulation. The MAP kinase phosphorylation sites are clustered within a region encompassing three proline-rich SH3-binding sites in the C-terminal domain of hSos1. Replacing the MAP kinase phosphorylation sites with alanine residues results in an increase in the binding affinity of Grb2 to hSos1. Interestingly, hSos2 contains only one MAP kinase phosphorylation site and, as demonstrated previously, has an increased affinity toward Grb2 compared with hSos1. These results suggest a role for MAP kinase in the regulation of Grb2-Sos interactions. Since the binding of Grb2 is important for Sos function, the phosphorylation-dependent modulation of Grb2-Sos association may provide a means of controlling Ras activation.
Insights
Mitogen-activated protein (MAP) kinase phosphorylates Son of sevenless (Sos) proteins, impacting Ras activation. This phosphorylation enhances Grb2 binding to Sos, suggesting a regulatory mechanism for controlling Ras signaling pathways.
Area of Science:
- Cellular signaling pathways
- Protein phosphorylation
- Signal transduction
Background:
- Son of sevenless (Sos) proteins are guanine nucleotide exchange factors crucial for Ras activation.
- Growth factor stimulation induces Sos phosphorylation, primarily mediated by MAP kinase.
- Previous studies localized growth factor-induced Sos phosphorylation to the C-terminal region.
Purpose of the Study:
- Identify specific MAP kinase phosphorylation sites on human Sos1 (hSos1).
- Investigate the functional consequences of these phosphorylation events on Grb2 binding.
- Compare phosphorylation patterns and Grb2 affinity between hSos1 and hSos2.
Main Methods:
- Site-directed mutagenesis to replace identified MAP kinase sites with alanine residues.
- Analysis of protein phosphorylation status following growth factor stimulation.
- Assessment of Grb2 binding affinity to wild-type and mutant hSos1 proteins.
Main Results:
- Five MAP kinase phosphorylation sites (S-1137, S-1167, S-1178, S-1193, S-1197) were identified on hSos1.
- Four of these sites (S-1132, S-1167, S-1178, S-1193) are phosphorylated upon growth factor stimulation.
- Mutating these sites increased Grb2 binding affinity to hSos1, suggesting a regulatory role for phosphorylation.
- hSos2, with fewer MAP kinase sites, exhibits higher Grb2 affinity compared to hSos1.
Conclusions:
- MAP kinase-dependent phosphorylation of hSos1 regulates its interaction with Grb2.
- Phosphorylation-dependent modulation of Grb2-Sos association offers a mechanism to control Ras activation.
- Differential phosphorylation and Grb2 binding between hSos1 and hSos2 may contribute to distinct signaling outcomes.