Raf oncogene

EXPERIMENTAL FIGURE 23-4 Transformation of mouse cells with DNA from a human cancer cell permits identification and molecular cloning of the raf oncogene. 

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. 

Rapp, U. R. (1991). Role of Raf-1 serine/threonine protein kinase in growth factor signal transduction. Oncogene 6 495-500. 

Ral has attracted much interest in recent years, not least because it was demonstrated to mediate part of Ras function as described above. In contrast to Rap, which rather inhibits Ras signaling, Ral is part of one of the essential Ras-activated pathways. Moreover, it has proved to be acting in parallel with the Raf pathway in cell transformation induced by oncogenic Ras [37, 77]. The case of Ral demonstrates the complexity - and the incomplete knowledge and understanding - of signal transduction. Ral can also be activated by Rap mediated by Rif [103] and, alternatively, by binding of a calcium/calmodulin complex to the Ral C-terminus which obviously does not affect the nucleotide state of Ral [111]. 

Oncogene-transformed mouse fibroblasts have a more decondensed chromatin structure than parental cell lines [59]. Phosphorylated Hl -3 levels are elevated in oncogene (ras, raf, fes, mos, myc) and aberrantly expressed MAPKK (MEK) transformed mouse fibroblasts, which have elevated activities of MAPK (ERKl and 2) [59] (Fig. 6). Further, RZ)-deficient human fibroblasts have increased levels of phosphorylated HI and a relaxed chromatin structure [60]. Cyclin E-Cdk2 was directly involved in increasing the levels of phosphorylated HI [60]. Elevated cyclin E-Cdk2 activity resulting from persistent activation of the Ras-MAPK pathway is also responsible for increased level of phosphorylated HI in oncogene-transformed mouse fibroblasts [61]. 

The effects of insulin on transcription are shown on the left of the illustration. Adaptor proteins Crb-2 and SOS ( son of sevenless ) bind to the phosphorylated IRS (insulin-receptor substrate) and activate the G protein Ras (named after its gene, the oncogene ras see p.398). Ras activates the protein kinase Raf (another oncogene product). Raf sets in motion a phosphorylation cascade that leads via the kinases MEK and ERK (also known as MARK, mitogen-activated protein kinase ) to the phosphorylation of transcription factors in the nucleus. 

Takahashi T, Ueno H, Shibuya, M. VEGF activates protein kinase C-dependent, but Ras-indepen-dent Raf-MEK-MAP kinase pathway for DNA synthesis in primary endothelial cells. Oncogene 1999 18(13) 2221-2230. 

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. 

Mutations may lead to loss of cellular control over the activity of a proto-oncogene. Frequently, this brings about constitutive activation of the signal protein. Thus, in the transforming v-raf gene, the N-terminal sequence section of Raf kinase is missing, on which both the autoinhibitory function and the phosphorylation sites of Raf kinase are locahzed (see 9.6). 

The activity of Ser/Thr-specific protein kinases is often controlled by autoinhibitory sequences (see 7.1.5). Loss or lack of function of autoregulatory sequences due to an oncogenic mutation can remove Ser/Hir kinase activity bound into mitogenic signaling pathways from normal control and thereby promote tumors. An example is the Raf kinase (see 9.6). Viral v-Raf oncoproteins are characterized by a deletion of the NH2-terminal regulatory sequences. 

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