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Ras-induced activation of Raf-1 is dependent on tyrosine phosphorylation
T Jelinek1, P Dent, T W Sturgill
1Department of Microbiology and Cancer Center, University of Virginia, Charlottesville 22908, USA.
Abstract:
Although Rafs play a central role in signal transduction, the mechanism(s) by which they become activated is poorly understood. Raf-1 activation is dependent on the protein's ability to bind Ras, but Ras binding is insufficient to activate Raf-1 tyrosine phosphorylation to this Ras-induced activation, in the absence of an over-expressed tyrosine kinase. We demonstrate that Raf-1 purified form Sf9 cells coinfected with baculovirus Ras but not Src could be inactivated by protein tyrosine phosphatase PTP-1B. 14-3-3 and Hsp90 proteins blocked both the tyrosine dephosphorylation and inactivation of Raf-1, suggesting that Raf-1 activity is phosphotyrosine dependent. In Ras-transformed NIH 3T3 cells, a minority of Raf-1 protein was membrane associated, but essentially all Raf-1 activity and Raf-1 phosphotyrosine fractionated with plasma membranes. Thus, the tyrosine-phosphorylated and active pool of Raf-1 constitute a membrane-localized subfraction which could also be inactivated with PTP-1B. By contrast, B-Raf has aspartic acid residues at positions homologous to those of the phosphorylated tyrosines (at 340 and 341) of Raf-1 and displays a high basal level of activity. B-Raf was not detectably tyrosine phosphorylated, membrane localized, or further activated upon Ras transformation, even though B-Raf has been shown to bind to Ras in vitro. We conclude that tyrosine phosphorylation is an essential component of the mechanism by which Ras activates Raf-1 kinase activity and that steady-state activated Ras is insufficient to activate B-Raf in vivo.
Insights
Ras activation of Raf-1 kinase activity requires tyrosine phosphorylation. This phosphorylation is essential for Raf-1 function and membrane localization, unlike B-Raf, which remains inactive.
Area of Science:
- Molecular Biology
- Cell Signaling
- Biochemistry
Background:
- Raf proteins are central to signal transduction pathways.
- The precise mechanisms of Raf activation, particularly Raf-1, remain incompletely understood.
- Ras binding alone is insufficient for full Raf-1 activation.
Purpose of the Study:
- To elucidate the role of tyrosine phosphorylation in Ras-induced Raf-1 activation.
- To investigate the differential activation mechanisms of Raf-1 and B-Raf by Ras.
- To determine the localization and regulation of active Raf-1 within cells.
Main Methods:
- Purification and manipulation of Raf-1 in Sf9 cells using baculovirus expression.
- Enzymatic assays using protein tyrosine phosphatase PTP-1B.
- Analysis of Raf-1 and B-Raf in Ras-transformed NIH 3T3 cells, including fractionation and tyrosine phosphorylation assessment.
Main Results:
- Raf-1 inactivation by PTP-1B was blocked by 14-3-3 and Hsp90, indicating phosphotyrosine-dependent activity.
- Active, tyrosine-phosphorylated Raf-1 was localized to plasma membranes in Ras-transformed cells.
- B-Raf, lacking tyrosine phosphorylation at key sites and not membrane-localized, showed high basal activity but was not further activated by Ras.
Conclusions:
- Tyrosine phosphorylation is a critical, indispensable step for Ras-mediated activation of Raf-1 kinase.
- The active pool of Raf-1 is a membrane-associated, tyrosine-phosphorylated fraction.
- Steady-state activated Ras is insufficient to activate B-Raf in vivo, highlighting distinct regulatory mechanisms.