Critical tyrosine residues regulate the enzymatic and biological activity of Raf-1 kinase

  title={Critical tyrosine residues regulate the enzymatic and biological activity of Raf-1 kinase},
  author={John R. Fabian and Ira O. Daar and Deborah K. Morrison},
  journal={Molecular and Cellular Biology},
  pages={7170 - 7179}
The serine/threonine kinase activity of the Raf-1 proto-oncogene product is stimulated by the activation of many tyrosine kinases, including growth factor receptors and pp60v-src. Recent studies of growth factor signal transduction pathways demonstrate that Raf-1 functions downstream of activated tyrosine kinases and p21ras and upstream of mitogen-activated protein kinase. However, coexpression of both activated tyrosine kinases and p21ras is required for maximal activation of Raf-1 in the… 

Phosphorylation of Raf-1 serine 338-serine 339 is an essential regulatory event for Ras-dependent activation and biological signaling

Raf-1 residues 338 to 341 constitute a unique phosphoregulatory site in which the phosphorylation of serine and tyrosine residues contributes to the regulation of Raf by Ras, Src, and Ras-independent membrane localization.

Phosphorylation of Raf-1 by p21-activated Kinase 1 and Src Regulates Raf-1 Autoinhibition*

It is demonstrated that the autoinhibitory domain of Raf-1 is functional in mammalian cells and that its interaction with the Raf- 1 catalytic domain is regulated by phosphorylation of serine 338 and tyrosines 340 and 341.

Regulation of the Raf-1 kinase domain by phosphorylation and 14-3-3 association.

The results support a model of Raf regulation in which the activity of the Raf-1 catalytic domain is directly upregulated by phosphorylation, following relief of inhibition by the N-terminal regulatory domain upon Ras-GTP binding.

Raf Activation Is Regulated by Tyrosine 510 Phosphorylation in Drosophila

It is demonstrated that phosphorylation of tyrosine 510 is essential for activation of Drosophila Raf (Draf), which is an ortholog of mammalian B-Raf, and that Y510 regulates Draf activation by affecting the autoinhibitory interaction between the N- and C-terminal fragments of the protein.

Ras-induced activation of Raf-1 is dependent on tyrosine phosphorylation.

It is demonstrated that Raf-1 purified form Sf9 cells coinfected with baculovirus Ras but not Src could be inactivated by protein tyrosine phosphatase PTP-1B and that steady-state activated Ras is insufficient to activate B-Raf in vivo.

Mechanism of inhibition of Raf-1 by protein kinase A

Using purified proteins, it is shown that in addition to weakening the interaction of Raf-1 with Ras, PKA can inhibit Raf- 1 function directly via phosphorylation of the Raf-2 kinase domain.

Tyrosine Phosphorylation of the Proto-Oncoprotein Raf-1 Is Regulated by Raf-1 Itself and the Phosphatase Cdc25A

New findings indicate that Raf-1 tyrosine phosphorylation is regulated in vivo not only by itself but also by Cdc25A, which is found to dephosphorylate Raf- 1 on tyrosines that resulted in a significant decrease in Raf-2 kinase activity.

Serine 338 Phosphorylation Is Dispensable for Activation of c-Raf1*

In U937 cells stimulation of c-Raf1 activity by GTPγS did not correlate with p21-activated kinase activity and Ser-338 phosphorylation, suggesting an essential role for serine 338osphorylation in c- raf1 activation under the conditions used, but Ser- 338 phosphorylated appears dispensable for c- Raf1activation under the circumstances used.


The current model of Raf regulation is discussed and it is reported that phosphorylation of Thr598 and Ser601, which lie between kinase subdomains VII and VIII, is essential for B-Raf activation by Ras, and replacement of these two sites with acidic residues renders B- Raf constitutively active.

Regulation of the Raf kinase by phosphorylation.

The current model of Raf regulation is discussed and the recent findings that phosphorylation of Thr598 and Ser601, which lie between kinase subdomains VII and VIII, is essential for B-Raf activation by Ras are reported.



Raf-1 activates MAP kinase-kinase

Results indicate that c-Raf-1 is an immediate upstream activator of MAPK-K in vivo, the first physiological substrate of the c-raf-l protooncogene product to be identified.

Identification of the major phosphorylation sites of the Raf-1 kinase.

It is found that Ser43, Ser259, and Ser621 are the major sites of Raf-1 which are phosphorylated in mammalian cells and in Sf9 insect cells infected with a recombinant baculovirus encoding human Raf- 1.

The role of Raf-1 phosphorylation in signal transduction.

Ras controls coupling of growth factor receptors and protein kinase C in the membrane to Raf-1 and B-Raf protein serine kinases in the cytosol.

It is concluded that Raf kinase activation may be sufficient to account for this aspect of NGF function and that Ras mediates signal transfer from activated membrane receptors to Raf kinases in the cytosol.

Interleukin 2 regulates Raf-1 kinase activity through a tyrosine phosphorylation-dependent mechanism in a T-cell line.

Findings provide evidence that the IL-2-stimulated phosphorylation of Raf-1 on tyrosines plays an important role in upregulating the activity of this serine/threonine-specific kinase in CTLL-2 cells and, as such, provides a model system for studying the transfer of growth factor-initiated signals from protein tyrosine kinases to serine/.

A divergence in the MAP kinase regulatory network defined by MEK kinase and Raf

MEKK was expressed in all mouse tissues tested, and it phosphorylated and activated MEK, independent of Raf, a growth factor-regulated protein kinase that also phosphorylates MEK.

Signal transduction from membrane to cytoplasm: growth factors and membrane-bound oncogene products increase Raf-1 phosphorylation and associated protein kinase activity.

Findings suggest that proliferative signals generated at the membrane result in the phosphorylation of the Raf-1 protein and the activation of its serine/threonine-specific kinase activity, which may serve to transduce signals from the membrane to the cytoplasm and perhaps on to the nucleus.

Both p21ras and pp60v-src are required, but neither alone is sufficient, to activate the Raf-1 kinase.

It is shown that either pp60v-src or p21c-ras can independently activate the autokinase activity of Raf-1, but only to a limited extent, and surprisingly, both pp60 v-src and p21 c-ras are required to fully activate Raf- 1.