Receptor Tyrosine Kinases Flashcards
Functions that RTKs tend to regulate
Growth, proliferation, differentiation, apoptosis
RTK ligands are usually. . .
proteins
GPCR ligands are usually. . .
small molecules
Basic structure of a receptor tyrosine kinase
Extracellular ligand binding domain
Transmembrane helix
Intracellular kinase domain
How is the signal of ligand binding tranduced across the membrane by RTKs?
By dimerization and transphosphorylation of the cytoplasmic kinase domains.
Note that this means, practically by definition, that substantial upregulation of these domains will lead to ligand-independent activation.
RTK dimerization loop
Domains on the extracellular side of the protein that dimerize to facilitate bringing together the intracellular domains of the RTKs.
Insulin receptor family-RTKs are. . .
pre-dimerized on the extracellular side
Binding just moves the cytoplasmic regions closer together so that they may interact by making the angle between them more acute.
FGFR family-RTKs. . .
multimerize along a carbohydrate chain (specifically heparan sulfate)
This heparan sulfate is often attached to a protein forming heparan sulfate proteoglycan (HSPG). This also serves to localize the signal.
Difference between the terms “cross-phosphorylation” and “auto-phosphorylation”
“cross-phosphorylation” refers specifically to the phosphorylation of one receptor by another
Autophosphorylation may refer to self phosphorylation or to cross-phosphorylation.
Activation loops on RTKs
How does cross-phosphorylation lead to downstream effects?
Cross-phosphorylation enables the kinase domains to phosphorylate multiple sites on the juxtamembrane domain of the receptor. These sites, when phosphorylated, serve as docking sites for other proteins, such as GRB2. The active kinase domains may also, of course, phosphorylate other proteins to propagate the signal, and some of this activity is mediated by these docking site which serve a recruitment role for substrates.
Common domains in RTK signal propagation
Although all SH2 domains recognize phosphotyrosine, . . .
they also recognize adjacent amino acids.
This is true for almost all of these binding domains, they are slightly different in order to control specificity.
Ras
Grb2
MAPK Cascade
“Mitagen Associated Protein Kinase”
KSR Scaffold
MAPK Cascade downstream effects
Ras is a _____ G protein.
monomeric
PI3K recruitment
Reaction Catalyzed by PI3K
PI3K Activation of Akt/PKB
PDK1
Recruited to the PM by PIP3
Phosphorylates and stably activates Akt when it is brought into proximity. Akt is then active and may diffuse into the cell to exert its effects.
Major Akt targets
BAD and Caspase 9 are proteins that mediate apoptosis, otherwise known as programmed cell death
FOXO is a transcription factor that upregulates expression of pro-apoptotic genes.
Inhibition of certain GAPs leads to increased GLUT4 insertion into the membrane and thus increased glucose uptake.
mTOR
mTOR is a regulator of protein synthesis. The best characterized way it does that is by phosphorylating a protein called 4E-BP. In its unphosphorylated state, 4E-BP binds and inhibits the initiation factor eIF4E (an initiation factor that binds the mRNA cap). Because 4EBP inhibits eIF4E, and mTOR inhibits 4E-BP, the net effect is that mTOR promotes translation.
AMP Kinase
AMP kinase responds to the AMP/ATP ratio in the cell. When the AMP/ATP ratio is high, that activates AMP kinase, which phosphorylates and activates TSC2.
Since TSC2 is a GAP for Rheb, that inactivates Rheb, and the net effect is to inhibit translation when energy stores are low:
if the cell is running out of energy, it doesn’t want to be using the large amount of energy needed to make a protein.
Note that phosphorylation of TSC2 by AMPK activates it, whereas phosphorylation by AKT at different sites inactivates it, and that’s how these two inputs can have opposite effects on translation.
Turing off RTKs
Protein Tyrosine Phosphatases (PTPs)
PTEN
Dephosphorylates PIP3
HER-family RTKs
Her1 = EGFR
Her2
Her2 has no known ligand. However, even without a ligand Her2 is in a conformation that allows it to dimerize: either homodimerizing with itself, or by forming heterodimers with other members of the Her family.
Its activation loop doesn’t need to be phosphorylated to be in the open state: kinase domain dimerization triggers a conformational change that is sufficient to open up the loop and make the kinase catalytically active
Her3
Her3 has an inactive kinase. It actually does have a full length intracellular region, but the kinase domain lacks catalytic activity
