11.3 Signal Transduction III Flashcards
What does the binding of ligands to enzyme-linked receptors lead to?
Leads to the activation of enzymatic activity built into the receptor itself OR leads to the activation of an associated kinase.
This binding does not require intermediate complexes, like G proteins or second messengers
What is a difference in the transmembrane pass between G proteins and enzyme linked cell surface receptors?
Enzyme-linked receptors only span the membrane a single time and the G protein receptors area seven pass transmembrane receptors & coupled receptors
Enzyme linked receptors don’t require a g-protein or secondary messenger and g-proteins do.
What are the characteristics of receptor tyrosine kinases?
- highly variable extracellular domain w/wide variety of ligands
- single pass transmembrane protein w/a single alpha helical hydrophilic domain between the extracellular and intracellular domains.
- on the cytoplasmic side of the membrane, these receptors have a conserved domain that has kinase activity, which phosphorylates the amino acid tyrosine on other proteins (another copy of the receptor or a different protein entirely)
What is the structure of the insulin receptor?
A tetramer of four proteins held together by disulfide bonds.
The receptor tyrosine kinase family includes receptors for many hormones important for?
important for growth, including:
- epidermal growth factor
- nerve growth factor
- vascular endothelial growth factor
- fibroblast growth factor
These receptors receive signals that the cell uses to understand whether or not its supposed to grow and divide and the activation of the kinase domain is what lets the cell know that a signal has been received.
Mutations of the receptor tyrosine kinases family can lead to what?
leads to situations where the cell is always receiving a signal to grow and these mutations are an important cause of uncontrolled cell growth in some types of cancers.
What happens if binding of a ligand to a receptor w/a single alpha helical transmembrane occurs in a Receptor Tyrosine Kinase?
it is unlikely that this binding will generate a structural change which can be received by the cytoplasmic side of the membrane. Therefore, these receptors have a different strategy for transferring information which is generated by the ligand binding into the cytoplasm.
STRATEGY: Dimerization of receptor tyrosine kinases, followed by cross -phosphorylation. Phosphorylation w/in kinase domain on cytoplasmic side alters conformation and increases kinase activity. Phospho-tyrosines create high-affinity docking sites for binding of signaling proteins.
What is the receptor activation mechanism for receptor tyrosine kinases? What is the name of this mechanism?
1) Receptor tyrosine kinase (2 of them) form a dimer in the presence of the ligand. the intracellular domain of these receptors has a kinase-active region.
2) The kinase domain phosphorylates identical domains on the receptor that it’s dimerized with.
3) the phosphorylation of the kinase domain on opposing halves of the receptor dimer leads to further phosphorylation of additional tyrosines
4) additional phosphorylated sites can serve as activated docking sites for downstream signaling proteins
NAME: cross-phosphorylation
What happens when a mutant receptor protein has a non-functional kinase domain?
When ligands bind to these receptors, dimers are formed but no phosphorylation or cross-phosphorylation occurs.
B/c the mutant can prevent the activation of the normal receptor it is said to have a dominant, negative effect.
What is meant by a dominant negative effect ?
When a mutant receptor tyrosine kinase prevents the activation of a normal receptor tyrosine kinase.
Insulin is what type of receptor? How does it differ from most receptor tyrosine kinases and how does the insulin receptor become active? What other proteins are similar to insulin?
Insulin receptor is a receptor tyrosine kinase but doesn’t follow the receptor activation mechanism b/c insulin does not have to couple w/another copy of itself to become active.
INSTEAD, ligand binding simply rearranges the cytoplasmic domains, resulting in phosphorylation of the two intracellular kinase domains that are part of the receptor. A coupling protein called the insulin receptor substrate 1 aka IRS1 binds to phosphorylated phosphoinositides found in the cell membrane and then serve as a scaffold for the recruitment of downstream signaling proteins.
OTHER PROTEINS LIKE INSULIN:
-insulin-like growth factors (IGFs)
Where are most of the phosphorylated docking sites on an insulin receptor found?
-docking sites are not found on the receptor itself. Instead they are found on a coupling protein called the insulin receptor substrate 1 (IRS1).
What is IRS1? What does IRS1 do?
Insulin Receptor Substrate 1 (IRS1) homes the phosphorylation docking sites for insulin receptors
IRS1 also binds to phosphorylated phosphoinositides found in the cell membrane and then serves as a scaffold for the recruitment of downstream signaling proteins.
Phosphorylated tyrosines serve as docking sites for what proteins?
proteins with SH2 and SH3 domains.
Give an example of how receptor tyrosine kinases act as scaffold proteins. Why is this arrangement favorable?
Receptor for PDGF (platelet derived growth factor) is an example.
The cytoplasmic portion of the receptor has docking sites for three different types of signaling molecules ( phospholipase C, PI 3-Kinase, and GTPase-activating protein).
This arrangement allows a single activated receptor to control a wide variety of cellular events.
How do receptors tyrosine kinases act like scaffold proteins?
on most receptor tyrosine kinases, the multiple phosphorylation sites created by the active kinase domains serve as docking sites for downstream signaling proteins and so the receptor acts like a scaffold protein.
In addition, the binding of a signaling molecule to the phosphorylated receptor can serve as a signal that activates other proteins.
what part of the family of receptor tyrosine kinase proteins is highly conserved?
the sites that are used to bind to the receptors are highly conserved.
The functionality of these proteins is not conserved as they differ for each receptor tyrosine kinase.
What is the best known protein that interacts w/a receptor tyrosine kinase in conserved manner? How does it do it?
sarc or SRC. SRC is named after the Rous Sarcoma Virus which contains a gene for a mutated tyrosine kinase that causes cancer
The receptor binding domain found in SRC is conserved in many other proteins that interact with receptor tyrosine kinases and these domains are known as SRC homology domains.
What is the most common SRC homology domain? What other SRC homology domain is found on some of the downstream proteins?
SRC homology 2 domain (SH2) is the most common
SRC homology 3 (SH3) domain is found on some downstream proteins.
SH2 and SH3 domains are SRC homology domains that are used by?
signaling proteins to bind to phosphorylated tyrosines on receptor tyrosine kinases
What are the two receptor tyrosine kinase signaling pathways?
1) Ras
2) Pi-3 Kinase or AKT pathway
What is Ras?
Ras is a major proto-oncogene, which means that it is a normal protein that is often found to be mutated in cancer cells.
Ras is also a membrane-associated G protein that acts a lot like G proteins but has two key differences
What’s the key difference between Ras membrane associated G protein and G proteins?
1) Ras is not a trimer of proteins instead it is a monomer
2) Ras is not activated by receptors instead it is activated through a series of other proteins beginning w/an adaptor protein containing SH2 and SH3 domains.
What are the names of the adaptor proteins used by drosophila and mammals in the Ras pathway?
Drosophila: Drk
Mammals: Grb-2
How does Drk adaptor protein behave in Ras pathway of drosophila?
Drk recruits a guanine exchange factor called SoS, which stands for son of sevenless. This exchange factor catalyzes the replacement of GDP on Ras w/GTP, and this is what activates the Ras protein.
SoS is always active, but it needs to be recruited to the membrane for it to efficiently activate Ras.
On the Ras pathway, how to Drk and Grb-2 bind to phosphorylated tyrosines?
via its SH2 domain and to SoS via its SH3 domain
