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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.
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
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.
The molecular events linking cell surface receptors to activation of ERKs are complex. It was found that Ras GTP-binding proteins are involved in the activation of ERKs. [3] Another protein kinase, Raf-1, was shown to phosphorylate a "MAP kinase-kinase", thus qualifying as a "MAP kinase kinase kinase". [4]
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]
Perhaps the best characterized MAP3K are the members of the oncogenic RAF family (RAF1, BRAF, ARAF), which are effectors of mitogenic ras signaling and which activate the ERK1/2 (MAPK3/MAPK1) pathway, through activation of MEK1(MAP2K1) and MEK2(MAP2K2). The JNKs are regulated by the MEKK 1/4, MLK 2/3, and ASK 1 MAPKKKs.
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]
Assembly and localization of the RAS-ERK pathway: One of the best studied signaling pathways in biology is the RAS-ERK pathway in which the RAS G-protein activates the MAPKKK RAF, which activates the MAPKK MEK1 (MAPK/ERK kinase 1), which then activates the MAPK ERK. Several scaffold proteins have been identified to be involved in this pathway ...
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