Chapter 4: Growth factor signaling and oncogenes (Book 4.1) Flashcards
There are four types of proteins involved in the transduction of a growth factor signal. Which are they?
Growth factors, growth factor receptors, intracellular signal transducers and nuclear transcription factors.
What happens when a phosphate is added to a target protein?
It induces a conformational change in the protein, resulting in the activation of inactivation of an enzymatic activity and/or serve as a recognition site for new protein-protein interactions.
There are two (or actually three) types of kinases. What type of kinase receptor is EGFR?
A tyrosine kinase receptor.
HER1-HER4 contain several domains. What are these?
- Extracellular ligand-binding domain - Single transmembrane domain - Cytoplasmic protein tyrosine kinase domain
So we just concluded that HER1-HER4 share the same domains. But HER2 and HER3 have exceptions. What are these exceptions?
HER2 does not bind to a known ligand but acts as a co-receptor for the other members of the family. HER3 has only weak kinase activity.
Name 6 steps that are required to get the signal from a growth factor outside the cell to inside the nucleus where gene expression is regulated.
- Binding of the growth factor to the receptor
- Receptor dimerization
- Autophosphorylation
- Activation of intracellular transducers (including RAS)
- Activation of a cascade of serine/threonine kinases (Raf, MEK, MAPK)
- Regulation of transcription factors for gene expression
The first step in the EGF signal transduction pathway is the binding of EGF to its receptor, EGFR. What extracellular domains of EGFR form a binding pocket for the ligand?
Extracellular domains I and III.
Describe in short dimerization of EGFRs.
It is the process of two EGFR monomers interacting to form a dimer. Binding of one EGF causes conformational change which induces the reveal of an extracellular receptor dimerization domain. This facilitates the binding to a similar domain in another EGF-bound receptor monomer, resulting in a receptor dimer.
What leads to autophosphorylation?
The conformational change upon ligand binding also disrupts intramolecular autoinhibitory interactions, leading to kinase activation.
Does autophosphorylation happen at multiple tyrosine residues of the EGFR? Why (not)?
Yes it does occur at multiple tyrosine residues. This is crucial for the recruitment of cytoplasmic substrate proteins that will pass the signal from the receptor to the signal transducers.
What are mechanisms for termination of kinase activity?
- Additional phosphorylation triggering a conformational change that inhibits extracellular ligand binding and kinase activity. - Dephosphorylation of regulatory phosphorylated tyrosine residues by tyrosine phosphatases. - Binding of negative regulators to the kinase domain - Receptor endocytosis and degradation.
Some phosphorylated tyrosine residues resulting from autophosphorylation create high-affinity binding sites for proteins that contain…
…Src homology 2 (SH2) domains.
What is the function of SH2 and SH3 domains (Src homogology 2 and 3)?
They mediate protein-protein interactions in pathways activated by tyrosine kinases.
What is the main difference between SH2 and SH3?
SH2 domains recognize and bind to amino acid sequences C-terminal to phosphorylated tyrosine residues. SH domains recognize and bind to proline and hydrophobic amino acid residues on partner proteins.
Why is this difference between SH2 and SH3 important to know?
Because Grb2 (intracellular protein with SH2 and SH3 domains) is able to recognize phosphorylated EGFR via its SH2 domain and facilitates the recruitment of specific porteins to the membrane via its SH3 domains.
What protein is recruited through the SH3 domains of Grb2? What does this protein do?
The exchange protein SOS, which will facilitate the activation of the intracellular transducer RAS.
Why are RAS proteins star players in regulating cell growth?
Because of their position in the signal transduction pathway; they act as a pivotal point for the integration of a growth factor signal initiating from the membrane with a number of crucial signaling pathways that carry the signal through the cytoplasm and into the nucleus.
RAS proteins are: N– H- and K-RAS. They are GTP-binding proteins. What does this mean?
This means that when they’re bound to GDP, they are inactive. When they are bound to GTP, they are active.
How do guanine nucleotide exchange factors (such as SOS) mediate the exchange of GDP for GTP?
By catalyzing the release of GDP from the guanine nucleotide binding pocket on RAS. Now GTP is able to bind to RAS.
How is RAS activity terminated?
Through the use of GTPase activating proteins (GAPs), they catalyze the hydrolysis of GTP to GDP to terminate the signal.
How can the negative regulatory intramolecular conformation/association within SRC be disrupted?
Through the use of an EGF receptor (EGFR) and a growth factor. The autophosphorylated receptor can interact with the SH2 domain of SRC, which disrupts the conformation.
