G- Protein Coupled Signaling Flashcards
Cell signal transduction
Once receptors in membrane stimulated -> protein-protein interactions -> changes in cell -> biological responses
signaling systems important in
responses, growth, development, and metabolism
G-protein coupled receptors
largest family of membrane receptors; 40% prescription drugs target these; their hallmark is 7 transmembrane domains spanning membrane as alpha helices containing 20-25 amino acids (hydrophobic) which are close together, this alignment is critical
Building blocks G protein coupled receptor
Cell surface receptor (GPCR) which initaites the signal, a GTP binding protein (G protein) which acts as molecular switch to transduce the signal, and biological effector (in many cases this produces a cellular second messenger like cAMP which is directly responsible for the signal)
Steps of G protein coupled receptor activation
- binding of extracellular factor to cell surface receptor
- Activation of G protein (GDP -> GTP)
- Stimulation or inhibition of effector activity
Tenants of signal transduction
- Any signaling pathway can only start when it is supposed to, agonist must bind
- Signaling systems can only signal for a defined length of time; checks and balances on g coupled protein receptor
checks and balances of G coupled protein receptor
- at level of G protein binding effector, typically w/ in 30 sec to 1 min GTP hydrolyzed back to GDP which comes off effector beta and game subunits
- At level of receptor checks and balances = desensitization, what ultimately happens is protein kinase phosphorylates receptor (GRK;G protein coupled receptor kinase) at which point arrestins bind to phosphorylated site getting in way of G protein binding (ex odorant receptor desensitization)
Arrestin
Arrestins bind to phosphorylated site getting in way of G protein binding in desensitization; also plays role of getting receptors into clatharin coated receptors, once receptors in endosome those receptors can lead to second wave of signaling via arrestin; in endosomes arresting recruits at 3 ubiquitin ligase to target receptor for lysosomal degradation
Epinephrine
Beta-adrenergic receptor example of GPCR family; binds epinephrine and stimulates signaling pathway that can control contraction of heart; insertion of cytoplasmic tail of fatty acids of cysteine is critical for proper juxtamembrane location of residues to help create binding site for G protein
N-linked glycosylation
provides important landmarks to know which way goes out of membrane and which goes in
General mechanism of G protein coupled receptor signaling
Ligand binds -> conformational change of receptor -> helices moving relative to each other -> change in configuration propogated to intracellular loops -> binding of G protein -> G protein binds from GDP -> GTP bc subunit opens, closes when GTP binds; alpha structure changes structure and dissociates from beta and gamma subunits looks for effector -> cellular outcome (alpha subunit can be inhibitory or excitatory; in some cases beta/gama subunit complex works together with alpha GTP species to regulate the effector)
G protein subunits
alpha, beta, gama
What do all GPCRs have in common
7-transmembrane span architecture and ability to activate heterotrimeric G protein that transduce signal of extracellular ligand receptor interaction to various intracellular effectors
Examples of GPCRs
Hormonal regulation of cardiac contraction, vertebrate vision
GPCRs in signaling pathway of cardiac cells
epinephrine dependent B-adrenergic receptor signaling pathway in cardiac cells results in stimulation of adenylyl cyclase activity and generation of cAMP -> activation of A- Kinase -> phosphorylation of cardiac Ca2+ channels -> action potential for cardiac contraction
GPCRs in vertebrate vision
Light activated rhodopsoin-coupled phototransduction system allows us to see in dim light; light-activated rhodopsin promotes exchange GDP for GTP on alpha subunit of G protein transducin -> stimulation cGMP phosphodiesterase (PDE) -> hydrolysis of cyclic GMP -> Na+ channel closure -> hyper polarization of retinal rod membranes -> signal to optic nerve
mutations GPCRs
mutant receptors can be incapable of ligand binding or generating normal signals, or might constitutively generate signals or are not appropriately expressed on cell surface
GCPRs structure
have extracellular amino-terminal segment, seven transmembrane domains (20-27 amino acids), 3 exoloops, 3 cytoloops, and carboxyl-terminal segment that includes fatty acid tail; N and C terminal segments at opposite sides of membrane surface
- N terminal segment- glycosylation
- C terminal segment- phosphorylation and palmitoylaiton
For desensitization and internalization
Ligand binding and receptor activation modes
- biding to transmembrane domain core exclusively
- binding to both the core and exollops
- binding to exoloops and N-terminal segment
- Binding exclusively to N terminal segment
Hormone binding to B-adrenergic receptor
Amine of catecholamines pairs with carboxyl group of aspartic acid 113 of transmembrane helix 3; catechol ring (aromatic ring with 2 hydroxyls) fits in pocket comprising transmembrane helices 5 and 6, meta and para hydroxyl groups hydrogen bond with serine residues -> adjustment in structure and packing of transmembrane helices -> G protein activation
why don’t antagonists lead to signaling when they bind
they do not have a catechol ring (lack 2 hydroxyl groups on the ring) and do not induce necessary changes in B-adrenergic receptor to allow it to bind G protein and stimulate signal propagation
Catecholamine examples
epinephrine and isoproternol
antagonists (B blockers) examples
Propranolol and P-azido benzyl carazolol
Family of heterotrimeric G proteins
4 subfamilies
Gs- mediate stimulatory effects of hormones
Gi- includes G proteins that mediate hormonal inhibition of adenylyl cyclase, visual phototransduction response, and various hormonal effect ion channels
Gq- mainly involved in regulation of phospholipase C
G12- influence cell cycle progression and cell growth
hydrolyses of GTP
all G proteins have ability to hydrolyze GTP but sometimes it is stimulated by distinct regulatory proteins called GTPase-activiating proteins (GAPS) or by members of RGS (regulators of G protein signaling) family
desensitization
occurs bc of persistent exposure of GPCR to its hormone/ ligand; receptor-stimulated signal progressively decreases
hormone dependent phosphorylation of receptor
one type of desensitization; phosphorylation via G protein coupled receptor kinase; phosphorylation leads to binding of arrestin; arrestin binds and GPCR no longer able to interact with G protein preventing G protein mediated signal propagation (desensitization)
PKA
cyclic AMP-dependent protein kinase; comes in in some cases which can phosphorylate calcium channels, can phosphorylate 3rd cytoplasmic loop and keep it from moving
Biased agonists
ligand that stabilizes a particular conformation of the receptor stimulating some responses but not others; can lead to stimulation of exclusively arrestin or G protein mediated siglaning
arresting-biased angiotensin receptor ligand
that is incapable of triggering G-protein mediated signaling while at same time stimulating beneficial B arrestin mediated effects COULD represent a novel and effective therapeutic agent (this doesn’t exist but would be cool if it did) (this would turn off vasoconstriction but leave cell survival cascades on)
signal amplification vision
1 photon activates 100 G proteins activates 1000 cGMP -> GMP in vision
