GPCRs Flashcards
What is the role of intracellular receptors?
Signaling molecules for these receptors typically enter the cell by simple diffusion across the plasma membrane or facilitated diffusion
What is the role of cell-surface receptors?
Bind signaling molecules that are too large or too hydrophilic to pass through the plasma membrane
What are three ways a cell surface receptor can transmit the signal to the cell?
- Altering the flux of ions across the membrane
- Turning on enzymatic activity present on the receptor or activating a cytoplasmic enzyme coupled to the receptor
- regulating the activity of plasma membrane G proteins that are coupled to the receptor
What are three ways ligand-gated ion channels are opened and closed?
- By binding a specific site on the ligand-binding domain
- intracellular signaling molecules such as cyclic nucleotides and G proteins
- Phosphorylation by kinases
What are Enzyme-linked receptors?
Cell surface receptors that are directly linked to enzymes
Many have a domain that functions as an enzymes
What is the largest family of enzyme-linked receptors that have activity in the cytoplasmic domain?
Receptor tyrosine kinases
What are G Protein-coupled receptors?
GPCRs are transmembrane proteins that transmit signals by means of intermediary proteins called G proteins
What is the largest family of cell surface receptors in the human genome?
GPCRs
What three things do all G protein coupled receptors contain?
A membrane-spanning region composed of 7 a-helices
An N-terminal segment and 3 loops on the external surface of the plasma membrane
A cytoplasmic domain that binds to and activates a GTP-binding regulatory protein
Why are G proteins referred to as heterotrimeric proteins?
They are composed of three subunits: alpha, beta, gamma
Which two subunits of the heterotrimeric G protein are coupled to the membrane?
alpha and gamma
Which two subunits remain coupled during signaling?
beta and gamma
How do G proteins act as molecular switches?
They are active when GTP occupies the alpha subunit and inactive when it is hydrolyzed to GDP
What are regulator of G protein signaling proteins (RGS)?
Proteins that accelerate the intrinsic GTPase activity of the alpha subunit.
Responsible for the rapid inactivation of G protein-coupled signaling
What happens when the alpha subunit is activated?
The Ga subunit dissociates from the By subunit and modulates the activity of specific membrane-bound effector proteins
What is the role of the dissociated By subunit after a activation?
In some cell types can also interact with effector proteins to transduce a signal
What is adenylyl cyclase?
a plasma membrane associated enzyme that catalyzes the conversion of ATP to cyclic AMP (cAMP)
What are Gs and Gi?
Gs - stimulatory G protein
Gi - inhibitory G protein
How is the cyclic AMP signaling pathway activated?
- Extracellular signaling molecule binds to GPCR
- GPCR GEF activity activates Gsa by replacing GDP with GTP
- active Gsa associates with and activated adenylyl cyclase
- AC catalyzes conversion of ATP to cAMP
How is the cAMP signaling pathway inactivated?
RGS stimulates the intrinsic GTPase activity of Gsa, inactivating
Gsa dissociates from adenyly cylcase, turning it off
What is Protein Kinase A and how is it activated?
Enzyme composed of 2 regulatory subunits and 2 catalytic subunits.
Activated when cAMP binds the R subunits, causing dissociation and activationof the C subunits
What is phosphodiesterase?
Metabolizes cAMP to 5’ AMP
This allows the subunits of PKA to reassociate
What is the action of PKA?
Activates or inhibits enzymes via phosphorylation of serine or threonine
What kinds of proteins can PKA activate?
Other kinases
Ion channels
Transcription factors
What is CREB?
cAMP regulatory element binding protein
Once phosphorylated by PKA, binds as a dimer to the cAMP response element (CRE) in the regulatory region of cAMP-inducible genes
What is CBP/300?
A coactivator protein that links CREB with the basal transcription machinery once CREB is phosphorylated
What are A-Kinase Anchoring Proteins (AKAP)?
Scaffolding proteins that have one domain that binds to the regulatory subunit of PKA and another that tethers it to a specific subsellular compartment
What are two functionsof AKAPs?
To limit cAMP-dependent response to a specific subcellular location by tethering PKA to that location.
Can also integrate PKA phosphorylation with other signaling events.
E.g. PDE in the heart muscle
What is responsible for inactivating proteins activated by PKA?
Protein phosphatases
E.g. Protein Phosphatase 1
What is the level of PKA-phosphorylated proteins within a cell determined by?
The balance between PKA and protein phosphatase activities
Besides PKA, what else can cAMP affect?
cAMP can also have a direct effect on ion channels
E.g. gated cation channels in olfactory response
What is the effect of forskolin?
direct stimulatory effect on adenylyl cyclase
What is the effect of caffeine?
inhibits phosphodiesterase activity
What is isoproterenol?
B-adrenergic receptor agonist
What is propranolol?
B-adrenergic receptor antagonist
What type of disorders have been associated with loss-of-function or gain-of-function mutations in Gsa subunits?
Rare endocrine disorders
What is the function of inhibitory G protein?
The a subunit has a direct inhibitory effect on adenylyl cyclase
What happens when acetylcholine binds to M2 muscarinic receptor?
The By subunits released by Gia activation bind to and open K+ channels. The efflux of K+ hyperpolarizes the plasma membrane
What is the mechanism of pertussis toxin?
Covalently modifies Gai so that it stays in the G form
What are five mechanisms that terminate or diminish the GPCR/cAMP signaling pathway?
- Dissociation/degredation of ligand
- intrinsic GTPase activity
- Phosphodiesterase activity
- Phosphatase action on PKA-phosphorylated proteins
- GPCR down-regulation
