Ras proteins were identified through their association with cell transformation. Since then they have been shown to regulate cell growth, differentiation and apoptosis, as well as influencing processes such as cell migration and neuronal activity. Ras regulates a number of signalling molecules by translocating them to the plasma membrane for activation.
H-Ras, K-Ras, and N-Ras are the founding members of a superfamily of monomeric small GTPases that are characterized by the ability to bind and hydrolyze guanine nucleotides. The Ras superfamily is comprised of more than 150 proteins that can be subdivided into five subfamilies (Ras, Rho, Rab, Arf, Ran) based on similarity in both sequence and function. In their active GTP-bound state, members of the Ras superfamily regulate diverse cellular functions, including gene expression, actin organization and vesicular trafficking.
Cellular regulation of Ras
Upstream signaling to Ras
Ras propagates extracellular signals into the cell by responding to stimuli such as soluble growth factors. The classic example of growth factor signaling to Ras is by epidermal growth factor (EGF). Upon binding of EGF to the EGF receptor (EGFR), the receptor undergoes a conformational change, dimerizes, and activates its tyrosine kinase activity. This kinase activity leads to phosphorylation of its cytoplasmic domain, which in turn recruits adaptor molecules such as Grb. Grb links the receptor to the Ras exchange factor, Sos, which activates Ras. Ras GAPs such as pi20-RasGAP and neurofibromanin (NFl) conversely function to downregulate Ras activity. Ultimately, EGFR provides such a critical mitogenic signal to cells that many human tumors are associated with constitutively active or overexpressed EGFR. Germline loss of function of NFl also leads to the tumor syndrome neurofibromatosis type I, also known as Von Recklinghausen disease.
Ras signaling to downstream effectors
Raf kinases: After activation by Ras, B-Raf phosphorylates and activates the dual specificity kinases, MEK1/2. MEK1/2 then phosphorylate and activate their only known substrates, ERK1/2. Active ERK1/2 signals to substrates in both the nucleus and cytoplasm. Interestingly, ERK1/2 does not have a nuclear localization sequence, instead translocation from the cytoplasm to the nucleus is thought to involve direct binding to components of the nuclear pore complex. ERK1/2 substrates include a wide variety of transcription factors, kinases and phosphatases, cytoskeletal proteins, signaling proteins, and apoptotic proteins and proteinases. Feedback inhibition of ERK1/2 signaling is achieved by induction of dual-specificity phosphatases (DUSPs) that dephosphorylate both threonine and tyrosine residues on ERK1/2.
Phosphatidylinositol 3-kinases: There are three classes of PI3Ks. Although both Class I and II PI3Ks have Ras-binding domains, Ras has only been shown to signal to Class I PI3Ks. Of class I PI3Ks, subclass IA (PI3KCA, PI3KCB, PI3KCD) binds to the p85 regulatory subunit while subclass IB (PI3KCD) binds to the plOl or p87 regulatory subunits. Class I PI3Ks, from here on referred to as PI3Ks, can signal directly downstream of receptors or indirectly through Ras. In order for Ras to activate subclass IA PI3Ks, for example, tyrosine kinase receptors must be activated, presenting phospho-tyrosine residues that bind Src homology 2 (SH2) domains on p85. Thus, growth factor signaling is required for Ras to access to PI3Ks and induce the conformational changes in PI3Ks needed for their activation.
RalGDS family of exchange factors: The third effector group found to bind to Ras is the RalGDS protein family. This family includes four members, RalGDS, Rgl, Rgl2, and Rgl3, which function as GEFs for the RalA and RalB small GTPases. The RalGDS proteins bind to active Ras through their C-terminal Ras-binding domain. This interaction results in the translocation of RalGDS proteins to the plasma membrane, which is sufficient to activate RalA and RalB, proteins that have been shown to regulate endocytosis, exocytosis, actin organization and gene expression. Unlike Raf and PI3K, however, somatic mutations in RalGDS proteins or RalA/B are extremely rare.
Kirti Magudia. K-RAS AND B-RAF ONCOGENES INHIBIT POLARITY ESTABLISHMENT THROUGH ERK-MEDIATED REGULATION OF C-MYC IN COLON EPITHELIAL CELLS