Cell Signaling 2 Flashcards
What are receptor tyrosine kinases?
Transmembrane cell surface receptors that possess tyrosine kinase activity in their cytoplasmic domain
Most are single-chain polypeptides that exist as monomers in the absence of ligand binding
What is each receptor tyrosine kinase monomer composed of?
Extracellular binding domain
Transmembrane a-helix
Cytoplasmic domain with tyrosine kinase activity and several autophosphorylation sites
How are monomeric RTKs activated?
Ligand binding induces dimerization of two monomers
Each monomer phosphorylates a tyrosine residue in the other’s activation loop
Conformational change in activation loops unblocks the tyrosine kinase activity
What two things can fully active catalytic domains of RTKs phosphorylate?
Cytosolic segments of the receptor dimer – act as docking sites
Intracellular signaling molecules
Describe the structure of the insulin and IGF-1 receptors.
Two external alpha subunits, each with a ligand binding site
Two transmembrane beta subunits, the cytoplasmic portion of each contains a tyrosine kinase domain.
What happens after cross phosphorylation of the B subunits in the insulin receptor?
Insulin receptor subustrates (IRS) associates with the activated receptor by means of a phosphotyrosine binding domain (PTB)
IRS is then phosphorylated on specific tyrosine residues by the receptor.
What is the role of IRS in the insulin response?
Serve as a docking site for other intracellular signaling proteins via their SH2 domains
What are phosphotyrosine phosphatases?
important negative regulators of insulin and IFG signaling
Acts by dephosphorylating tyrosine residues on activated insulin and IGF-1 receptors and on IRS proteins
Also act on other RTKs in this manner as a form of inhibition
What purposes do binding of intracellular signaling proteins to the receptor serve?
Localizes them to the plasma membrane
Enables them to interact with other proteins that are associated with the receptor
Activates the signal transuction pathways to which the intracellular signaling proteins are coupled
What are Src homology 2 (SH2) domains?
Highly conserved region that enables an intracellular signaling protein to recognize and bind specific phosphotyrosine residues
What is Phospholipase C-gamma?
A signaling protein that is activated when it binds to an active RTK.
Catalyzes the breakdown of PIP2 to DAG and IP3
How is the Ras/MAPK pathway activated?
A GEF docks with an RTK and activates Ras
Ras phosphorylates the first kinase in the pathway (Raf)
What pathway can virtually all RTKs activate?
The Ras/MAP pathway
What happens when MAPK is phosphorylated?
Dimerizes
Phosphorylates p90RSK
Phosphoryaltes TCF, a transcription factor
What is the function of p90RSK?
Phosphorylated by MAPK
Phosphorylates SRF (serum response factor)
What is the function of SRF and TCF?
Once they are activated, the associate in the promotor region of the regulated gene and stimulate transcription
What are Cytokines?
Family of protein/peptide signaling molecules that play a major role in regulating the proliferation and differentiation of blood cells and cells of the immune system
Activate the JAK/STAT pathway
What are JAK proteins?
Tyrosine kinases that are bound to cytokine receptors
How are JAK proteins activated?
Ligand binding of a cytokine receptor promotes dimerization (or the dimer is preformed), this allows each JAK protein to cross phosphorylate the other’s activation loop.
Active JAKs phosphorylate receptor tyrosine residues that serve as docking sites
What are STAT (Signal Transduction and Activation of Transcription) proteins?
Transcription factors that are the major target of JAKs.
Once phosphorylated, STAT proteins form dimers and translocate to the nucleus where they activate gene transcription
What are four mechanisms of inhibiting tyrosine kinases
Phosphotyrosine phosphatases
PKC - phosphorylates specific serine and threonine residues on the receptor
Desensitization
Certain SH2 signaling proteins with a ubiquitin ligase domain that targets the receptor for degredation
What is SHP1?
Phosphotyrosine phosphatase with two SH2 domians, normally one blocks the phosphatase domain
When a receptor is stimulated, the covering SH2 docks on the receptor.
The exposed phosphatase removes a phosphate in the activation lip of JAK, reducing its activity
What is PI 3-kinase and what is the function of its product?
Catalyzes the addition of a 3-phosphate to phosphoinositide substrates in the PM to form PI 3-phosphates
PI 3-phosphates function as docking sites for certain signal transduction proteins
What is a Plextrin homology (PH) domain?
Domain on certain proteins that binds 3-phosphates with high affinity
How is Akt(PKB) activated?
Stimulation by ligand catalyzes the formation of PI 3-phosphate.
The PHB domain of PKB binds to PI 3-Phosphate, inducing partial activation.
PDK1 and PDK2 also phosphorylate PKB, causing its full activation
What is the role of the PKB/Akt pathway?
Plays important role in promoting cell division and survival and in the regulation of metabolism
What does NF-kB regulate?
The transcription of numerous genes involved in the inflammatory and immune responses
What is NF-kB?
a heterodimer that is sequestered in the cytoplasm by an inhibitory protein called IkBa
Once activated, enters the nucleus and promotes the transcription of target genes
What is the function of I-kB kinase?
Activated by a signaling cascade in response to an immune signaling molecule
Phosphorylates two serine residues on the N-terminal portion of I-kBa
What occurs when I-kBa is phosphorylated?
E3 ubiquitin ligase binds to it and triggers degredation by a proteosome
What protein products does NF-kB transcribe?
Phospholipase A2 and cyclooxygenase
NO synthase
Cytokines (TNFa and interleukin-1)
I-kB
What is the negative feedback loop that inhibits NF-kB activity?
NF-kB transcribes the I-kB gene which is responsible for its inhibition
What is NO synthase?
Creates NO through the deamination of arginine
What is the role of NO?
Released from endothelial cells in response to acetylcholine
Diffuses into underlying vascular smooth muscle cells
Activates guanylyl cyclase
What is cGMP-dependent protein kinase?
Phosphorylates serine or threonine on target proteins when activated by 2 cGMP molecules
Promotes vasorelaxtion via its pathway
Receptors for thyroid hormones, steroid hormones, Vit D and retinoids are what kind of receptors and how do they act?
Intracellular receptors that act as transcription factors when bound
What domains are present in intracellular receptors?
- N-terminal with a ligant-independent transactivation domain
- DNA binding domain - zinc finger
- Hinge region - gives structural flexibility
- Ligand-binding domain
What is bound to a steroid hormone receptor in an inactive state?
Heat shock proteins
What is the mechanism of action of steroid hormones?
When they bind to the receptor, the receptor translocates to the nucleus and binds specific DNA sequences called steroie-response elements (SRE)
They can either recruit HATs or HDACs and activate or repress transcription
What are examples of drugs that block the action of steroid hormones?
Flutamide - androgen receptor antagonist
RU486 - progesterone receptor antagonist
What is the state of an unoccupied Thyroid hormone receptors?
- do not form complexes with Hsp’s
- bind tightly to DNA even in the absence of hormone and repress gene transcription by recruiting proteins and HDAC
What are TREs?
Thyroid response elements
Location of thyroid receptor binding on the DNA
What is the preferred arrangement of a THR?
Heterodimer with the RXR receptor (retinoid X receptor)
What does thyroid hormone binding cause?
Promotes the dissociation of corepressor proteins and the association of coactivator proteins
However, in certain genes it can also cause repression, where the gene is active in the absence of TH
