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Receptor-tyrosine-kinase- and G beta gamma-mediated MAP kinase activation by a common signalling pathway
T van Biesen1, B E Hawes, D K Luttrell
1Howard Hughes Medical Institute, Department of Medicine (Cardiology), Duke University Medical Center, Durham, North Carolina 27710, USA.
Abstract:
Mitogen-activated protein (MAP) kinases mediate the phosphorylation and activation of nuclear transcription factors that regulate cell growth. MAP kinase activation may result from stimulation of either tyrosine-kinase (RTK) receptors, which possess intrinsic tyrosine kinase activity, or G-protein-coupled receptors (GPCR). RTK-mediated mitogenic signalling involves a series of SH2- and SH3-dependent protein-protein interactions between tyrosine-phosphorylated receptor, Shc, Grb2 and Sos, resulting in Ras-dependent MAP kinase activation. The beta gamma subunits of heterotrimeric G proteins (G beta gamma) also mediate Ras-dependent MAP kinase activation by an as-yet unknown mechanism. Here we demonstrate that activation of MAP kinase by Gi-coupled receptors is preceded by the G beta gamma-mediated tyrosine phosphorylation of Shc, leading to an increased functional association between Shc, Grb2 and Sos. Moreover, disruption of the Shc-Grb2-Sos complex blocks G beta gamma-mediated MAP kinase activation, indicating that G beta gamma does not mediate MAP kinase activation by a direct interaction with Sos. These results indicate that G beta gamma-mediated MAP kinase activation is initiated by a tyrosine phosphorylation event and proceeds by a pathway common to both GPCRs and RTKs.
Insights
Mitogen-activated protein (MAP) kinase activation by G-protein-coupled receptors (GPCRs) involves tyrosine phosphorylation of Shc. This initiates a signaling cascade through Shc, Grb2, and Sos, common to both GPCRs and tyrosine-kinase receptors (RTKs).
Area of Science:
- Cellular signaling pathways
- Molecular biology
- Signal transduction
Background:
- Mitogen-activated protein (MAP) kinases regulate cell growth through transcription factor phosphorylation.
- MAP kinase activation can be triggered by tyrosine-kinase receptors (RTKs) or G-protein-coupled receptors (GPCRs).
- RTK-mediated signaling involves Shc, Grb2, and Sos for Ras-dependent MAP kinase activation.
Purpose of the Study:
- To elucidate the mechanism by which G-protein beta gamma subunits (G beta gamma) activate MAP kinases.
- To determine if G beta gamma-mediated MAP kinase activation utilizes a pathway distinct from RTK signaling.
- To investigate the role of Shc phosphorylation in GPCR-mediated MAP kinase activation.
Main Methods:
- Investigated MAP kinase activation in response to Gi-coupled receptor stimulation.
- Analyzed tyrosine phosphorylation of Shc protein.
- Examined the functional association between Shc, Grb2, and Sos.
- Disrupted the Shc-Grb2-Sos complex to assess its role in signaling.
Main Results:
- G beta gamma-mediated MAP kinase activation by Gi-coupled receptors is preceded by Shc tyrosine phosphorylation.
- This phosphorylation enhances the association between Shc, Grb2, and Sos.
- Disruption of the Shc-Grb2-Sos complex abrogates G beta gamma-mediated MAP kinase activation.
- G beta gamma does not directly interact with Sos to activate MAP kinase.
Conclusions:
- G beta gamma-mediated MAP kinase activation is initiated by tyrosine phosphorylation of Shc.
- The signaling pathway for G beta gamma-mediated MAP kinase activation converges with that of RTKs.
- This study reveals a common mechanism for MAP kinase activation downstream of both GPCRs and RTKs.