Raf kinase Activation

In addition to Raf kinase activation, Ras protein also mediates stimulation of other protein kinases, known as MEK kinases. These are signal proteins in the MARK pathway (see Chapter 10) and transmit signals at the level of gene expression. 

The MEK proteins are themselves substrates for another type of protein kinase further upstream, the MEK kinases (MEKKs, also known as MAPKK kinases, MAPKKK). The MEK kinases include the various Raf kinases activated by Ras protein, Mos kinase and the protein kinases MEKKl, MEKK2 and MEKKS. 

Cell-permeable ATP-competitive inhibitor of Raf kinase activity. Also a potent inhibitor of a variety of other protein kinases. 

Raf phosphoserine-259 then is dephosphorylated (by an unknown phosphatase) and other serine or threonine residues on Raf become phosphorylated by yet other kinases. These reactions Incrementally Increase the Raf kinase activity by mechanisms that are not fully understood. 

Proposed phosphorylation-signaling cascade from EGF receptor to Fos and Jun through a sequential series of kinase-activation steps. The role of Ras in stimulating Raf kinase activity is thought to involve Ras-mediated membrane localization of Raf, which would explain the requirement for Ras famesylation. 

This lipid binding, coupled with loss of interaction with 14-3-3 proteins, resulted in decreased Raf kinase activity. Moreover, it was postulated that ceramide generation might regulate Raf translocation to microdomains where Raf interactions with KSR and Ras may be facilitated (Hekman et al, 2002). 

Sorafenib (Nexavar) TKI Raf, VEGFR2, c-Kit, PDGFR, Flt-3, FGFR1 Inhibition of kinase activity - ATP-competitive RCC... 

In chronic myelogenous leukemia (CML) as well as in a subset of acute lymphoblastic leukemia (ALL) Bcr-Abl, a fusion protein of c-Abl and the breakpoint cluster region (bcr), is expressed in the cytosol of leukemic cells. This fusion protein forms homo-oligomeric complexes that display elevated kinase activity and is the causative molecular abnormality in CML and certain ALL. The transforming effect of Bcr-Abl is mediated by numerous downstream signaling pathways, including protein kinase C (PKC), Ras-Raf-ERK MAPK, JAK-STAT (see below), and PI3-kinase pathways. 

App, H., Hazan, R., Zilberstein, A., Ullrich, A., Schlessinger, J., and Rapp, U. (1991). Epidermanl growth factor (EGF) stimulates association and kinase activity of raf-1 with the EGF receptor. Mol. Cell. Biol. 11 913-919. 

Morrison, D. K., Kaplan, D. R., Rapp, U., and Roberts, T. M. (1988). Signal transduction from membrane to cytoplasm growth factors and membrane-bound oncogene products increase Raf-1 phosphorylation and associated protein kinase activity. Proc. Natl. Acad. Sci. USA 85 8855-8859. 

All steps of the Ras pathway from ligand binding to receptor tyrosine kinases, down to activation of effectors like Raf kinase, occur at the plasma membrane. However, most biophysical studies on protein/protein interactions involved in this scenario have been carried out with bacterially synthesized proteins lacking... 

The membrane-associated kinase KSR plays an important role in ceramide-mediated regulation of BAD, a pro-apoptotic protein belonging to the Bcl-2-like protein family (Basu et al, 1998). Ceramide indirectly activates BAD via a pathway involving KSR, Ras, c-Raf-1, and MEK-1. Stimulation of this pathway results in Akt inactivation. Since the kinase activity of Akt maintains Bad in the inactive form, inhibition of Akt in turn releases BAD and, finally, permits BAD-triggered cell death. 

Fig. 7.10. Functions and regulation of protein kinase C. Receptor-controlled signal pathways lead to formation of the intracellular messenger substances and diacylglycerol (DAG), that, like phorbol ester (TPA), activate protein kinase C (PKC). Translocation to the cell membrane is linked with activation of protein kinase C receptors for protein kinase C, the RACK proteins, are also involved. Substrates of protein kinase C are the MARCKS proteins and other proteins associated with the cytoskeleton. Other substrates are the Raf kinase (see Chapter 10) and the receptor for vitamin D3 (VDR, see Chapter 4).

Further examples of substrates of protein kinase C are the epidermal growth factor receptor (see Chapter 8), a Na7H exchanger protein, and Raf kinase (Chapter 9). Activation of protein kinase C may, as the examples show, act on other central signal transduction pathways of the cell it may have a regulating activity on transcription processes and it is involved in the regulation of transport processes. Many substrates of protein kinase C are membrane proteins and it is evident that membrane association of protein kinase C is of great importance for the phosphorylation of these proteins. 

What is the mechanism of activation of effector molecules such as the Raf kinase ... 

Ser and Thr residues that serve as regulatory phosphorylation sites. Mutations are also described for these regions leading to oncogenic activation of Raf kinase. The protein kinase activity is found in the CR3 domain. 

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