Cell Signaling (Lectures 3 and 4) Flashcards
What are the 4 types of signaling receptors?
- ) ligand-activated ion channels
- ) G protein coupled receptors
- ) ligand-activated transcription factors
- ) tyrosine kinase receptors
About 40% of drugs target what type of receptor?
G protein coupled receptor
When can ligand-activated ion channels be found?
- ) plasma membrane
2. ) organells
Describe plasma membrane ligand-activatd ion channels
- signaling in excitable cells
- multi-subunit proteins
- produce graded potential (effects are local)
Describe depolarization vs hyperpolarization in ligand-activated ion channels
depolarization: lets in Na+ (usually not particularly selective)
- acetylcholine, serotonin, glutamate
hyperpolarization: lets in Cl-
- glycine, GABA (gamma amino butyric acid)
Activating and inhibiting ligand activated plasma membrane ion channels is the basis of?
synaptic transmission of nerve impulses
note: plenty of drugs target here
What are some ways drugs affect the synaptic transmission of nerve impulses?
- ) mimicking or blocking the action of neurotransmitters
- ) blocking the ion channel
- ) acting on transmitter re-uptake
- ) acting on transmitter metabolism
note: this was in the section of ligand-activated ion channels in the plasma membrane
What is an example of ligand activated ion channels in an organelle?
IP3 mediated Ca+ release from the ER
note: this is the receptor mediated Ca2+ release from the ER internal source. IP3 sensitive Ca2+ chanel. (IP3 is the ligand)
Most signaling by Ca++ is mediated by?
Ca++/Calmodulin
Describe Ca2+/Calmodulin signaling
IP3 –> Ca2+
[Ca2+] in the cytoplasm of a cell greatly increases after IP3 mediated release
- ) The increase in [Ca2+] increases the binding of Ca2+ to calmodulin, altering the structure of the Ca/calmodulin complex
- ) The Ca2+/calmodulin complex is able to interact and activate numerous effector proteins
What are 2 examples of proteins activated by Ca2+/calmodulin
- ) calcium calmodulin dependent protein kinase
- endothelial
- many substrates for this kinase
- ) nitric oxide synthase
- neuronal
- leads to synthesis of cGMP
How do agonists activate G-protein coupled receptors (GPCRs)?
Agonists bind to the GPCR’s, alter the proteins’s conformation, leading to the activation of a trimeric G protein
Describe the G protein cycle
- ) Ligand bound GPCR stimulates GDP-GTP exchange
- ) The GPCR complex separates into two pieces
- G alpha and G beta G gamma - ) G alpha has intrinsic GTPase activity. Eventually the GTP is hydrolyzed to GDP and Pi. RCS proteins accelerate this process
- ) After GTP hydrolysis the complex reassembles
important note: Both G alpha and G beta G gamma participate in cell signaling events
What are the components of the GPCR complex and what do they do?
major component: G alpha
minor component: G beta G gamma
both components bind to and activate other proteins in the cell
What are the major effector of G alpha’s?
- ) Adenylate cyclase (10 isoforms)
- ) Phospholipase A2
note: major does not mean only.
also : he lists phospholipase C as a minor effector and phospholipase Cbeta as a major effector later
Describe the mechanism of Adenylate cyclase
1.) along with ATP adenylate cyclase it forms cAMP and PPi
2.) cAMP acts on (activates) protein kinase A (PKA)
3.) PKA driven phosphorylation of a large number of proteins produces a large number of biological effects
-PKA alters the activity of target proteins (literally
hundreds of targets)
- phosphorylation of transcription factors by PKA and
other protein kinases can lead to gene expression in
cells
-the whole process is pretty complicated
note: his notes say to know this
Where are signaling molecules often embedded and how to GPCRs liberate them?
-they’re embedded within the phospholipids in the
membrane
-GPCRs liberate them by activating phospholipases
Why do we care about arachidonic acid?
its the source of eicosinoid signaling molecules
What is the substrate for phospholipase C?
- PIP2 (phosphatidyllinositol-4,5 bis phosphate)
- it is a phospholipid (hence phosphatidyl)
Cleavage of PIP2 by phospholipase C (PLC beta) yields?
- DAG
- IP3
note: they are important signaling molecules. Remember IP3 increases [Ca2+] in the cytoplasm
What does DAG activate?
-PKC
note : PKC is Ca2+ dependent. DAG allows PKC to bind Ca2+ tighter so the enzyme is able to function at lower Ca2+ concentrations
What critical role does G beta G gamma play?
it helps turn of the GPCR
The trimeric G protein turns itself off but the agonist GPCR complex is free to activate other trimeric G proteins that it comes in contact with. Does this signal last for as long as the agonist is present?
No. The receptor gets turned off by desensitization.
Describe the process of desensitization and endocytosis of GPCRs.
1.) The agonist GPCR complex is phosphorylated by a GRK
(recruited by G beta G gamma)
2.) B-arrestin binds
3.) A clathrin-coated pit forms
4.) EndocytosisWhat are the possible fates of an endocytosed receptor?
- ) recycling
2. ) degradation
Why should you care about receptor desensitization?
Sometimes drugs are administered chronically. This can lead to a loss of responsiveness to the drug and results that would seem paradoxical if one failed to consider desensitization.
Describe signaling by receptor tyrosine kinases (RYK).
1.) receptors contain either and intrinsic or extrinsic tyrosine
kinase enzyme
2.) binding of a ligand leads to dimerization of the receptor
- some form homodimers, heterodimers or both
3.) dimerization leads to transphosphorylation of receptors
4.) tyrosine phosphorylation leads to docking of signaling
molecules
5.) the positioning of proteins near the membrane leads to
different cascades of signaling
When protein kinases (PKA, PKA, MAP kinase) phosphorylate transcription factors this can lead to ?
Changes in gene expression in cells
note: this is just the basic concept. Something is phosphorylated , it activates transcription factors (usually via phosphorylation), the transcription factors bind to target genes and the gene expression changes.
Activation of Akt kinase is an example of what and why is Akt important?
- a signaling cascade
- it pushes cells towards growth and proliferation. It inhibits apoptosis
note: RYK recruits P13K to form PIP3. PIP3 recruits PDK. PDK phosphorylates Akt
Describe JAK-STAT signaling
1.) binding of ligand (cytokine) leads to phosphorylation of
receptor by JAK kinase
2.) one of six STAT proteins bind to the phosphorylated
receptor.
3.) JAK kinase then phosphorylates the STAT protein
4.) the STAT protein dimerizes
5.) the STAT protein is translocated to the nucleus where
transcriptional activation of panels of genes occurs
What is the significance of JAK-STAT signaling?
-Different cytokines recruit different STAT proteins.
- Different STATs have different target sites which leads to
the transcription of different genes
Cross talk amongst different signaling pathways creates?
Complex signaling possibilities
Nuclear hormone receptors are receptors for?
steroids, hormones and other molecules (vitamin D, retinoic acid, etc)
Nuclear hormone receptors are classified as either?
class I or class II
How are class I and class II nuclear receptors different?
Class I receptors
-ligand binding causes the receptor complex to bind to
target DNA leading to transcription of certain genes
Class II receptors
-the receptor is already bound to the target DNA and binds
co-repressors in the absence of ligand (represses
transcription)
-binding of the ligand causes the receptor complex to bind
co-activators which leads to transcription
