Cell Signaling - Lectures 3 - 4 Flashcards
What are the 4 types of signaling receptors?
Ligand-activated ion channels
G-protein coupled receptors
Tyrosine kinase receptors
Ligand-activated transcription factors
What are the two sites of ligand-activated ion channels?
Plasma membrane
Organelle membranes
Ligand-activated ion channels
Multisubunit proteins that produce graded potentials
How does a ligand-activated channel depolarize, and what molecules trigger this event?
Let in Na
Acetylcholine, Glutamate, Serotonin
How does a ligand-activated channel hyperpolarize, and what molecules trigger this event?
Let in Cl
Glycine, GABA
Activating and inhibiting ligand activated plasma membrane ion channels is the basis of what?
Synaptic transmission of nerve impulses
What are some mechanisms of drugs that target ligand activated ion channels?
- Mimic or block the actions of neurotransmitters
- Block the channel
- Affect transmitter reuptake
- Affect transmitter metabolism
IP3
Mediated Ca release from the ER internal store
What is the concentration of Ca in the cytoplasm at rest?
10-100 nM
What is the concentration of Ca in the cytoplasm following IP3-mediated release?
500-1000 nM
What happens when the Ca concentration of the cytoplasm increases?
A transient increase in A increases the binding of Ca to Calmodulin, altering the structure of the Ca-Calmodulin complex
The Ca-Calmodulin complex is able to activate numerous effector proteins
How many Ca ions bond with Calmodulin?
4
What are two examples of proteins activated by Ca/calmodulin?
Ca/Calmodulin dependent protein kinase
Nitric Oxide synthase
G-protein coupled receptors
A single polypeptide consisting of 7 transmembrane a-helices
40% of drugs target these receptors
How do agonist activate GPCRs?
When an agonist binds to a GPCR, it alters the protein conformation, leading to the activation of a trimeric G-protein
Describe the G-protein cycle
1) Ligand binds to GPCR, which stimulates GDP-GTP exchange
2) This causes the complex to dissociate and fall apart in two pieces - Ga and GBGy (both can participate in cell signaling events)
3) The Ga has an intrinsic GTPase activity and eventuall the GTP is hydrolyzed to GDP and Pi (this process is accelerated by RGS proteins)
4) After GTP hydrolysis, the complex reassembles (Ga binds to GBGY, which once again binds to GPCR)
Which compound of the G-protein cycle is the major signaler, and which is the minor signaler?
Major: Ga
Minor: GBGy
What is the course of action for adenylate cyclase?
It hydrolyzes ATP to cAMP and Pi.
cAMP activates protein kinase A
PKA alters the activity of hundreds of target proteins
How can PKA alter gene expression?
PKA can phosphorylate transcription factors that bind to target genes and up-regulate their expression
What effect does Ga have on Phospholipase A2?
It activates PLA-2, which liberates arachadonic acid
What is the importance of arachadonic acid?
It is the source of eicosinoid signaling molecules
What is the importance of Phospholipase CB?
Activated Ga comes in and hydrolyzes PLCB, which cleaves PIP2 (a minor phospholipid in the membrane), producing DAG and IP3, which are important signaling molecules.
What is the importance of GBGy?
Helps localize trimeric G protein to the membrane
Plays a critical role in turning off GPCR (desensitization)
Describe the process of GPCR desensitization
1) Phosphorylation of GRK (recruited by GBGy)
2) Binding by B-arrestin
3) Formation of clathrin-coated pit
4) Receptor is endocytosed
What are the two possible fates of an endocytosed GCPR?
1) Recycling
2) Degradation
When can receptor desensitization occur?
When drugs are administered chronically
What types of tyrosine kinases can be associated with YRKs?
Extrinsic or intrinsic tyrosine kinases
What happens to Tyrosine Kinase receptors when bound to a ligand?
They dimerize - can either be a heterodimer or a homodimer
What are possible ligands for Tyrosine Kinase receptors?
Growth factors
Differentiation factors
Cytokines
What happens when a tyrosine kinase receptor dimerizes?
It leads to transphosphorylation of receptors
What happens when dimerized tyrosine kinase receptors transphosphorylate?
Signaling molecules are recruited and dock on the RYKs
What leads to different signaling cascades of RYKs?
The positioning of proteins near the membrane
Describe the JAK-STAT signaling cascade
1) Ligand (cytokine) binding leads to phosphorylation of receptor by JAK kinase
2) This leads to the binging of a STAT protein to the phosphorylated receptor
3) The STAT protein gets phosphorylated by JAK kinase
4) This leads to the dimerization of STAT protein
5) This leads to translocation of the STAT dimer to the nucleus and transcriptional activation of panels of genes
What are the characterizations of Ligand activated nuclear receptors?
Class 1
Class 2
Class 1 nuclear receptor mechanism
Ligand binding induces the complex to fall apart, form a homodimer, and bind to specific DNA response elements
Once bound to DNA response elements, it binds to coactivators to induce transcription
Class 2 nuclear receptor mechanism
Binds to specific DNA response elements in the absence of ligands, and is bound to corepressors
When a ligand binds, it switches form corepressors to coactivators
