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RAF is an acronym for Rapidly Accelerated Fibrosarcoma. [12] RAF kinases participate in the RAS-RAF-MEK-ERK signal transduction cascade, also referred to as the mitogen-activated protein kinase (MAPK) cascade. [11] Activation of RAF kinases requires interaction with RAS-GTPases. The three RAF kinase family members are: A-RAF; B-RAF; c-Raf
Activated Ras then activates the protein kinase activity of a RAF kinase. [4] The RAF kinase phosphorylates and activates a MAPK/ERK Kinase (MEK1 or MEK2). The MEK phosphorylates and activates a mitogen-activated protein kinase (MAPK). RAF and MAPK/ERK are both serine/threonine-specific protein kinases. MEK is a serine/tyrosine/threonine kinase.
The MAP2 kinase of this pathway is called Pbs2 (related to mammalian MKK3/4/6/7), the dedicated MAP3 kinases involved in activation are Ssk2 and SSk22. The system in S. cerevisiae is activated by a sophisticated osmosensing module consisting of the Sho1 and Sln1 proteins, but it is yet unclear how other stimuli can elicit activation of Hog1.
Another protein kinase, Raf-1, was shown to phosphorylate a "MAP kinase-kinase", thus qualifying as a "MAP kinase kinase kinase". [4] The MAP kinase-kinase, which activates ERK, was named "MAPK/ERK kinase" . [5] Receptor-linked tyrosine kinases, Ras, Raf, MEK, and MAPK could be fitted into a signaling cascade linking an extracellular signal to ...
RAF proto-oncogene serine/threonine-protein kinase, also known as proto-oncogene c-RAF or simply c-Raf or even Raf-1, is an enzyme [4] that in humans is encoded by the RAF1 gene. [ 5 ] [ 6 ] The c-Raf protein is part of the ERK1/2 pathway as a MAP kinase (MAP3K) that functions downstream of the Ras subfamily of membrane associated GTPases. [ 7 ]
HRas is a small G protein in the Ras subfamily of the Ras superfamily of small GTPases. Once bound to Guanosine triphosphate , H-Ras will activate a Raf kinase like c-Raf , the next step in the MAPK/ERK pathway .
GAP eventually returns Ras to its inactive state. Activation of Ras has the potential to initiate three signaling pathways downstream of Ras: Ras→Raf→MAP kinase pathway, PI3 kinase pathway, and Ral pathway. Each pathway leads to the activation of transcription factors which enter the nucleus to alter gene expression. [9]
However, Raf activation does not exclusively depend on Ras. Protein kinase C, which is activated by phorbol esters, can fulfill the same function as Ras. [22] MEK kinase (MEKK) can also activate MEKs, which then activate MAPKs, making Raf unnecessary at times. [23] Various signal transduction pathways, therefore, funnel through MEKs and MAPKs ...
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