GPCRs Flashcards
Components of Trimeric G protein
- complex of 3 subunits (alpha, beta and gamma)
- Alpha subunit - GTPase activity
G alpha s Mechanism of Action
- Ligand binds GPCR —> conformational change in receptor which inc affinity for G alpha s —> causes Galpha s to dock on receptor—> docking causes switch from GDP to GTP —> triggers dissociation from receptor and breaks from beta and gamma subunits —> enhances interaction w/downstream effector (adenylyl cyclase)–> inc cAMP
- G alpha s - ATP = more affinity for effector
- G alpha s -ADP = more affinity for activated receptors and beta/gamma units
G alpha q Mechanism of Action
Same as Galpha s but diff effector (PLC-beta)
G alpha i Mechanism of Action
Galpha i -bound to GTP INHIBITS adenylyl cyclase (pop of Galpha s)
What does activated PLC-Beta do?
- Activated PLC-beta —> hydrolysis of plasma membrane phospholipid —> IP3 (sugar) and diacylglycerol (lipid)
- IP3 —> binds/opens certain Ca++ channels —> inc intracellular Ca++
- Diacylglycerol —> activates protein kinase C
Transducin (Gt) Mechanism of Action
Rhodopsin is a GPCR activated by light rather than ligand —> conformational change —> activated Gt —> when GTP-bound Gt activates cGMP phosphodiesterase —> breakdown of gCMP —> close gCMP-gated Na+ channels —> hyper polarization
G olf Mechanism of Action
Coupled to olfactory receptors —> when GTP-bound activates adenylylcyclase —> inc cAMP —> open cAMP-gated cation channels —> depolarization (action potential)
How are cAMP and PKA related and both restricted?
cAMP diffusion is limited kinetically and specially by phosphodiesterase (breakdown cAMP —> AMP)
cAMP binds R (regulatory) subunit of PKA -> release of activate C (catalytic) subunit —> phosphorylation in cell
- BUT…PKAs can be specially restricted by AKAPs (anchor PKAs by binding R subunits)
CREB
Active PKA –> phosphorylation of CREBS which are transcription factors SO alters gene expression
CDP Kinase II
Ca++ binds calmodulin —> triggers auto-phosphrylation of CaM Kinase II —> this kinase is in active/phosphorylated state even AFTER intracellular Ca++ levels come down
Gbeta/Ggamma Signaling in Heart Muscle
- Once liberated, open K+ channels in heart —> K+ exits heart muscle cell —> harder to depolarize/contract
- Liberated when acetylcholine binds muscarinic receptors in heart muscle cells…SO acetylcholine has an inhibitory effect on heart muscle contraction
Cholera Toxin
enzyme that catalyzes transfer of ADP ribose to Galpha s proteins —> can no longer hydrolyze GTP —> remains in active state —> amplified levels of cAMP —> act CFTR —> watery diarrhea
Pertussis Toxin
catalyzes ADP-ribosylation of Galpha i —> stabilizes Galpha i in GDP-bound form —> blocks its ability to inhibit adenylyl cyclase -> inc cAMP
RGS Proteins
- G alpha s and G alpha q termination
- RGS (regulator of G protein signaling) stimulate the otherwise weak GTPase activity of the alpha subunit
- Leads to hydrolysis —> Galpha s -GTP conformation which has LESS affinity for effector
GPCRs in General
- family of receptors; all have 7 transmembrane units and extracellular and cytoplasmic domains
- On plasma membranes
- Mult GPCRs can bind same G protein —> extra complexity and tissue-specificity
- Can hetero-dimerize
- Ex) Gi and Gq; normally Gi»_space;Gq but inc in Gq in psychosis
Beta-Arrestins
- proteins promote internalization of GPCRs via clathrin-coated vesicles
- Once endocytosed…GPCRs can be recycled, degraded OR have distinct signal pathways inside cell (paths that are INDEP of G protein complexes)
Biased Ligand
- drugs that can be created to purposely favor the beta-arresting path OR the Galpha path (unique therapy)
PARs
type of GPCR
protease cleaves part of the receptor which then acts as the ligand —> activation —> coupled to G proteins whose effectors lead to cell growth
3 Ways That Abberant GPCR Regulation Can Lead to Cancer
- 1-overexpression of protease activated receptors (PARs -type of GPCR)
- 2-Mutations that make GCPR constitutively active - TSH receptor
- 3-persistent sim of receptors by agonists made by tumor cells
Homologous Desensitization
Heterologous Desensitization
loss of responsiveness after repeated exposure to ligand
when activation of 1 receptor can desensitize another receptor
- Involves second messengers
Nicotinic v Beta-Adrenergic Receptor Desensitization
Nicotinic-
- Intrinsic
- Fast (w/in fraction of second)
- Homologous
- Uncouples the binding of ligand to ion channel opening (all w/in the receptor protein)
- REG by must kinases (phosphorylation inc desensitization)
Muscarinic-
- Not intrinsic (happens via kinases/second mess)
- Slower (seconds or minutes)
- Homologous and heterologous
- Uncouples ligand binding from G protein activation
- MEDIATED by must kinases (necessary)
Bark
- Beta adrenergic receptor kinase (Bark)
- Bark recognizes ligand-bound receptor and phosphorylates the C terminal —> causes arresting to bind the receptor —> endocytosis
Denervation Supersensitivity
- Denervation/nerve injury —> turn on genes for embryo isoforms in addition to still making adult isoforms + OVERALL HIGHER # ACTIVE NUCLEI—> supersensitivity
- Denervation only occurs when loss of activity of nerve; so if you stimulate extracellularly this process will not occur (AKA activity-dependent)
