RTK-Ras-signaling Flashcards
How do receptor tyrosine kinases promote RAS activity?
Ligand binding to RTK causes the RTK to dimerize, promoting activation of both cytoplasmic kinase domains by phosphorylation. Ras is activated by RTK through adapter protein, linking activated RTK to Ras-GEF. Which promotes Ras activation by stimulating dissociation of GDP and uptake of GTP.
How does signaling through PI 3-kinase promote cell survival?
Activation of PI3K by activation of Ras causes the Pi3K to phosphorylate and activate Akt. Akt enhances the survival of cells by by blocking the function of proapoptotic processes and proteins. For example by promoting the degradation of p53 by phosphorylation of MDM2. P53 promotes apoptosis.
How is the EGF receptor kinase activated?
EGF binding causes conformational change, promoting dimerization of the external domains of the receptor. Dimerization orients internal kinase domains into an asymmetric dimer. One kinase domain (activator) pushes against the other (receiver). Causing an activating conformational change in receiver. Active receiver domain phosphorylates multiple tyrosine’s in C-terminals of both receptors which become docking sites for intracellular signaling proteins.
Name three important modular binding domains involved in RTK-signaling pathways. Specify which domains they bind to.
SH2/PTB: can recognize phosphorylated tyrosine residues
SH3: can recognize scaffold proteins and binds to the proline-rich region
PH: Pleckstrin homology domain, this recognizes PIP3 proteins on the plasma membrane
How does dimerization cause activation of RTKs? How is this different from GPCRs?
Binding of a ligand brings two RTK monomers together to form a dimer. The close proximity in the dimer leads the two kinase domains to phosphorylate each other. Phosphorylation at some tyrosines int he kinase domains promotes the complete activation of the domains. And, phosphorylation at tyrosine’s in other parts of the receptors generates docking sites for intracellular signaling proteins.
After binding of a ligand to GPCR, the GPCR udergoes a conformational change, causing it to act as GEF. Stimulating GDP release from Gprotein and uptake of GTP, resulting in activation
How does activation of the EGF receptor vary from the general RTK activation by other external ligands?
Activation of the EGF receptor the kinase is not activated by phosphorylation (such as in general RTK activation), but by conformational changes brought about by interactions between the two kinase domains outside their active sites.
Is Ras active or inactive while bound to GTP?
Active
In the Ras-pathway, Ras associates with Sos. Which proteins are necessary for the activation and inactivation of monomeric GTPases? Which of these functions relate to Sos?
Ras-GEFs stimulate the dissociation of GDP and the uptake of GTP from the cytosol, activating Ras (Sos is a Ras-GEF). Ras inactivation is done by Ras-GAPs, which increase the rate of hydrolysis of bound GTP by Ras.
How is the signal of Ras relayed downstream to change gene expression and/or protein activation?
Activated Ras recruits Raf (MAPKKK) to the plasma membrane and helps activate it.
Raf activates Mek (MAPKK), which activates Erk (MAPK). Erk phosphorylates a variety of downstream proteins, including other protein kinases and transcription factors.
Define which conserved modular binding domains are involved in RTK-signaling and what they recognize.
SH2 domain on the Grb2 adaptor protein binds to the activated RTK. SH2 recognizes phosphorylated-tyrosine.
SH3 domains on the Grb2 adaptor protein bind to Sos. SH3 recognizes proline-rich motifs on intracellular proteins.
ERK functions to phosphorylate nuclear transcription factors, but also plays a role in desensitization of the Ras-pathway. How is this realized?
Aside from phosphorylating downstream proteins, Erk also phosphorylates and inactivates Raf, providing a negative feedback loop that helps shut off the MAP kinase module.
What is incorrect with hyperactive mutants of Ras? Explain two mechanisms causing hyperactive Ras in cells.
Hyperactive mutants of Ras are resistant to GAP stimulation and are permanently locked in an active state.
1. Mutations impairing GTPase activity, making Ras resistant to GAPs, causing Ras to accumulate permanently in active, GTP bound state.
2. Mutations in GAPs (such as NF1 mutations) causing the GAPs to not work and keep Ras in an active, GTP-bound state.
