Flashcards in Cell to Cell Communication Deck (15):
Describe the ligand receptor interaction
The ligand is a molecule or protein that triggers a signal by binding to a receptor like protein.
The receptor is usually in the inactive state until it received a signal from the environment
Once stimulated by ligand binding, receptors are only temporarily activated. Some receptors self inactivate.
Specificity is dictated by tertiary structure and non covalent bonds.
What are the four major classes of receptors?
Ion channels, steroid hormone receptors, protein kinase receptors and seven alpha helix receptor
Describe ion channels and a disease state related to it.
They are pore forming proteins that allow the flow of ions across the membrane down its electrochemical gradient. They are present on the cell surface and intracellular organelles,
Their gating is dependent on gating, voltage, mechanical, phosphorylation
Cystic fibrosis is a loss of mutation in the cystic fibrosis transmembrane conductance regulator (an ABC chloride channel)
The ligand is ATP and the abnormal salt transport leads to thick mucus build up in the respiratory epithelial cells.
What is tetrodotoxin?
Its a sodium channel blocker found in puffer fish. It blocks action potentials in the nerves.
Describe nuclear steroid hormone receptors
Derivatives of cholesterol cross the membrane by themselves, they bind receptors in the cytosol or the nucleus. Then these complexes usually form dimers, bind to hormone response elements on DNA in the nucleus and regulate gene expression.
Describe estrogen and its interaction with receptors + its significance.
So this is an example of a nuclear steroid hormone receptor. The estrogen receptor is associated to a chaperone in its inactive state. When an estrogen binds, a conformational change causes the two dissociates. This then reveals a nuclear import signal which allows the homodimerization and then entry to the nucleus.
They then bind estrogen response elements and activate gene expression
Estrogen receptors are over expressed in cancel cells for 70% of cancer cases.
How is estrogen sensitive breast cancer being treated?
Tamoxifen is a competitive estrogen receptor antagonist. IN breast tissue it gets metabolized into hydroxytamoxifen that binds ER and prevents binding of estrogen. The ER/H-tam complex functions to repress as opposed to activate estrogen target genes by recruiting co-repressors rather than co activators.
This is an example of two fold inhibition.
Describe protein kinase receptor and phosphorylation.
They have three domains: extracellular which binds ligand, transmembrane, and a cytoplasmic protein kinase domain that either has kinase ability or recruits protein kinase proteins.
To be active they usually form dimers,
and there are two major families,
tyrosine kinases and serine/threonine kinases.
A ligand binds to each of the dimer subunits, this will bring the two domains in proximity to one another, they then activate and phosphorylate each other at multiple positions. This causes the domains to be substrates to cytosolic proteins and the pathway begins.
Phosphorylation in general is reversible via phosphatases, and this will regulate the on or off of the receptors. It can also turn a hydrophobic portion of the protein to hydrophilic
It can give rise to a conformational change which faciliates interactions with other parts of the protein or with other molecules.
Describe the Ras-MAPK pathway.
First Grb binds to phosphorylated receptors, (G protein receptor binding protein)
It has an SH2-domain which recognizes phosphorylated tyrosines.
SoS (the ras-GEF) Son of sevenless binds to Grb and activates small G-protein Ras by catalyzing the exchange of GDP for GTP
GAPS (GTPase activating proteins) regulate the activation by stimulating GTPase, converting GTP to GDP by hydrolysis (Ras inherently has this property as well).
This goes on to activate transcription factor. Ras activates MAPKKK, MAPKK, etc. These stimulate the production of proteins involved in cell division. Phosphorylated MAPK enters the nucleus and phosphorylates transcription factors in the nucleus.
That is one pathway, Ras is also involved in cell growth, anti-apoptosis, cytoskeleton, etc.. basic metabolic pathways.
How does Ras cause cancer.
Also neurofibromatosis Type 1
and Noonan syndrome
Ras is activated by many metabolic pathways. Oncogenic mutations in ras turns all of these. on. Most oncogenic mutations are amino acid substitutions at gly-12/13 or gln-61 which affects the structure of ras preventing its ability to hydrolyze GTP into GDP so always active.
Neurofibromatosis Type 1: a mutation in a GAP gene so overexpression
Noonan: mutation in the SHP2 protein which leads to gain of function mutation.
Describe 7 alpha helix receptors.
They are the most abundant class and mostly sensory purposes. Odor, light, taste, hormones, neurotransmitters etc.
Opsins, olfactory receptors, neurotransmitter receptors for mood, autonomic nervous system,
7-alpha helix receptors are coupled with trimeric "large G proteins" with a alpha, beta and gamma subunit. Alpha subunit binds GTP. Ligand binding causes exchange of GDP by GTP by the receptor (an intrinsic GEF) initiating signaling.
The alpha dissociates from the beta+gamma. The alpha subunits can change.
1. If its Gs-a it activates adenylate cyclase which phosphorylates PKA
2. If its Gi-a, it inhibits adenylate cyclase with will inhibit PKA.
3. If it is Gq-a, it activates PLC (phospholipase C) which activates PKC.
Describe desensitization and the beta-adrenergic receptor:
The beta-adrenergic receptor binds epinephrine and neurotransmitter norepinephrine. The ligand bound receptor becomes a substrate for beta-adrenergic receptor kinase (BARK)
Phosphorylated receptor will then be bound by beta arrestin and this blocks the interaction of the receptor with Gs even if the ligand is bound.
How does Gs-alpha and Gi-alpha regulate calcium levels.
Gs-a will activate adenyl cyclase. This will convert ATP into cAMP which activates PKA. This will lead to calcium influx. Gi-alpha will inhibit adenyl cyclase.
Calcium is normally pumped out of the cell so once PKA is activated, it will open a calcium channel leading to rapid influx of calcium ions, important in muscle contraction, regulated secretion and cell division.
Norepinephrine and Epinephrine bind a Gs-a
Acetylcholine would bind a Gi-alpha.
Describe the Gq-pathway.
It activates PKC via calcium and PLC.
A Gq-alpha-GTP will activate phospholipase C,
PLC hydrolyzes PiP2 into iP3 and DAG.
iP3 triggers release of Ca2+ from lumen of smooth ER into the cytoplasm, DAG and Ca2+ activate PKC
PKC is usually inactivated because its active sites are filled with terminal pseudosubstrate peptide held in its C1 and C2 domains.
C1 binds DAG in membranes and in the presence of calcium C2 binds PS. This removes the pseudosubstrate from the PKC so its active.
Then PKC is cleaved from C1 and C2, and its permanently active after cleavage.