IV.Signaling Mechanisms and Drug Action Recent research has revealed in considerable detail the molecular processes that transduce extracellular signals

IV. Signaling Mechanisms and Drug Action

Recent research has revealed in considerable detail the molecular processes that transduce extracellular signals into intracellular messages that control cell function.

cellintracellularextracellular

NOTE: not drawn to scale

Messages

Messages

Signal Transduction

CellularFunction

Transmembranereceptors

Intracellularreceptors


Understanding cellular signaling gives answers to important clinical questions:

Why do some drugs produce effects that persist for minutes, hours or even days after the drug is no longer present?

Why do chemically-similar drugs often exhibit extraordinary selectivity in their actions?

Do these mechanisms provide targets for developing new drugs?


Most transmembrane signaling is accomplished by only a few different molecular mechanisms:

each type of mechanism has been adapted, through the evolution of distinctive protein families, to transduce many different signals.

These protein families include:*receptors on the cell surface*intracellular receptors (that are not enzymes)*enzymes


Let’s first discuss mechanisms for carrying chemical information across the

plasma membrane.

Five basic mechanisms of transmembrane signaling are well understood.

Each uses a different strategy to circumvent the barrier posed by the lipid bilayer of the plasma membrane.

While the five established mechanisms DO NOT account for all the chemical signals conveyed across cell membranes, they do transduce many of the most important signals pertaining to pharmacotherapeutic strategies.


1. A lipid-soluble ligand that crosses the membrane and acts on an intracellular receptor.

2. a transmembrane receptor protein whose intracellular enzymatic activity is allosterically regulated by a ligand that binds to a site on the protein’s extracellular domain.

3. a transmembrane receptor that binds and stimulates a protein tyrosine kinase


4.a ligand-gated transmembrane ion channel that can be induced to open or close

by the binding of a ligand

5.a transmembrane receptor protein that stimulates a GTP-binding signal transducer

protein (G protein), which in turn generates an intracellular second messenger.

Let’s take a closer look.

IVA. Intracellular Receptors for Lipid-Soluble Agents

CELL MEMBRANE

exterior= extracellular

Lipid-soluble chemical signal crossesthe plasma membrane and acts onan intracellular receptor -which may be an enzyme or

a regulator of gene transcription


CELL MEMBRANE

exterior= extracellularNO

One example… nitric oxide (NO) is a freely diffusable gas which acts by crossing the membrane and stimulating an intracellular enzyme Guanylyl Cyclase

ENZYMEGuanylyl Cyclase

NOGTP

cGMP

2nd messengerCell Signaling

Cyclic GMP


CELL MEMBRANE


Intracellular receptors for these agents bind their ligands and undergo a conformational change that allows them to enter the nucleus and help regulate gene expression.

Lipid-soluble ligands such as:Corticosteroids, mineralocorticoids, sex steroids,Vitamin D and thyroid hormone

Cell nucleus

Nuclear signaling;Transcription of genes


CELL MEMBRANE


Structural features common to these receptors suggest that they belong to a protein family that evolved from a common precursor

Lipid-soluble ligands such as:Corticosteroids, mineralocorticoids, sex steroids,Vitamin D and thyroid hormone

Cell nucleus



Cell nucleus

HSP90

Binding of glucocorticoid hormone to its normal receptor relieves an inhibitory constraint, HSP90– usually keeps the receptor in the cytoplasm.

Binding of hormone to the ligand-binding domain triggers release of hsp90.

This allows the folding of the DNA-binding domains and transcription-activating domains of the receptor to fold into their proper conformations

so that the activated receptor can initiate transcription of target genes in the nucleus.

HSP90



Cell nucleus

HSP90

The Mechanism Used by Hormones that act by regulating gene expression has two therapeutically important consequences:

1. Hormones produce their effects after a characteristic lag period of 30 minutes to several hours

the time required for the synthesis of new proteins

2. Effects of these agents can persist for hours or days after the agonist has been removed.

persistence of effect is primarily due to the relatively slow dissociation of the agonist from the hormone receptor.

can remain active in cells for hours or days after they’ve been synthesized.

HSP90

HSP90 fallsoff after ligandbinding

IVB. Ligand-Regulated Transmembrane Enzymes (Including Receptor Tyrosine Kinases)

This class of receptor molecules:

polypeptides consisting of an extracellular hormone-binding domain

cytoplasmic enzyme domain*protein tyrosine

kinase*protein ser/thr kinase*guanylyl cyclase

The two domains are connected by a hydrophobic segment of the polypeptide that crosses the lipid bilayer of the plasma membrane.

extracellular

Hormone-bindingdomain

Cytoplasmic enzyme domain


This class of receptor molecules mediates the first steps in signaling by:

insulin

epidermal growth factor (EGF)

platelet-derived growth factor

atrial natriuretic factor (ANF)

transforming growth factor-beta

extracellular

Hormone-bindingdomain



Example: receptor tyrosine-kinase signaling pathway.

Hormone binds to receptor’s extracellular domain.

Resulting change in receptor conformation causes receptor molecules to bind to one another– in effect bringing the protein tyrosine kinase domains together.

Tyrosine residues in both cytoplasmic domains become phosphorylated (each most likely by the other).NOTE: at this point, it is necessary to realize that lipid-soluble

hormones activate cytosolic receptors (eg. glucocorticoids) and other hormones activate extracellular receptors (insulin; EGF).


extracellular

Hormone-binding domain


HORMONE (turquoise spheres) will bind to the extracellular hormone binding domains resulting in a conformational change in the receptors which will cause them to associate in pairs (dimerize).


extracellular

Hormone-binding domain


HORMONE (turquoise spheres) will bind to the extracellular hormone binding domains resulting in a conformational change in the receptors which will cause them to associate in pairs (dimerize).


extracellular

TYROSINE residues (Y) in both cytoplasmic domains become phosphorylated (Yellow spheres). Each domain is believed to be phosphorylated by the other in a cross-phosphorylation event. This is called AUTOphosphorylation.

Y Y


extracellular

After autophosphorylation, receptors catalyze phosphorylation of tyrosine residues on different downstream signaling proteins, but only a few of these substrate proteins have been identified.

Y Y PP


extracellular

Y Y PP

Substrate SubstratePATP

+ ADP

NOTE: The phosphate placed onto the substrate comes from ATP. The phosphates on the catalytic domains do not donate phosphate groups. NO. They are phosphorylated and then are able to act as a phosphorylating enzyme… and use ATP as the source for the phosphate group.


extracellular

Y Y PP

Substrate Substrate

PATP

+ ADP

One example of the complexity:Each of the growth factors initiates in its specific target cells a complex program of cellular events ranging from: altered membrane transport of protons, other ions and

metabolites to characteristic changes in the expression of many genes.

Different receptors catalyze phosphorylation of tyrosine residues on different downstream signaling proteins, and only a few of these downstream signaling proteins have been identified.


Substrate

PAltered membrane transport ofprotons, other ions and metabolites;

altered gene expression

Insulin, for example, uses a single class of receptors *to trigger increased uptake of glucose and

amino acids *and to regulate metabolism of glycogen and

triglycerides in the cell.


The tyrosine kinase receptors provide attractive targets for drug development.

it is easy to envision therapeutic uses for specific inhibitors of growth factor receptors, especially in neoplastic disorders.

Y Y

Substrate

+ ADP

Cell signalingCell growth/proliferation

No dimerization

No receptor phosphorylation

No phosphorylation of downstreamsubstrates


The intensity and duration of action of EGF,PDGF and other agents that act via this class of receptors are limited by receptor down-regulation.

Ligand binding induces accelerated endocytosis of receptors from the cell surface

Followed by the degradation of the bound ligands and receptors (in some instances, receptors are not degraded, but brought back to the cell surface– a receptor-recycling).

when the rate of:

endocytosis/degradation >> de novo synthesis of rcptrs

The TOTAL number of cell-surface receptors is reduced (down regulated) and the cell’s responsiveness to ligand is diminisheddiminished


Receptor Binding & Activation: Ligand binds to its receptor

Coated Pit Formation: Clathrin forms cage around forming endosome

Clathrin Vesicle Formation: Vesiculation occurs

Vesicle Uncoating: Clathrin coat is removed

CURL Endosome Forms: Compartment of unbinding of receptor and ligand.

Receptor recycles to the surface or is sent to a digestive vacuole

Ligand may also return to the surface or to a digestive vacuole


http://www.erin.utoronto.ca/~w3bio315/RME_frameset.htm

IVB. What Happens to Receptors After They Are Internalized?

What happens to those receptors which are internalized?

eg. EGF receptor

Endosome fuses with lysosome.Receptor degradation.

Unbinding of ligand and Receptor-recycling to the membrane.

Can lead to receptor downregulation

IVB. Ligand-Regulated Transmembrane Enzymes (Including Receptor Serine Kinases)

Certain regulators of growth and differentiation act through transmembrane receptors that have cytoplasmic domains with serine/thr kinase activity (eg. transforming growth factor-beta)

IVB. Ligand-Regulated Transmembrane Enzymes (Including Receptor Ser/Thr Kinases)

http://www.sigmaaldrich.com/suite7/Area_of_Interest/Life_Science/Cell_Signaling/Pathway_Slides_and_Charts/Signaling_Pathway_of_TGF___223_.html

IVB. Ligand-Regulated Transmembrane Enzymes (Including Receptor Ser/Thr Kinases)

http://www.sigmaaldrich.com/suite7/Area_of_Interest/Life_Science/Cell_Signaling/Pathway_Slides_and_Charts/Signaling_Pathway_of_TGF___223_.html

Signaling Pathway of TGF-b TGF-b regulates growth and proliferation of cells, blocking the growth of many different cell types.

The TGF-b receptor includes Type I and Type II subunits that are serine-threonine kinases thatsignal through the Smad family of proteins.

Binding of transforming growth factor b (TGF-b) to its cell surface receptor Type II leads to the phosphorylation of the Type I receptor by Type II.

The Type I receptor is then able to phosphorylate and activate the Smad2 protein.

Smad2, in combination with Smad4, is translocated to the nucleus where the activated Smad complex recruits other transcription factors (TF) that together activate the expression of target genes that mediate the biological effects of TGF-b.

IVB. Ligand-Regulated Transmembrane Enzymes (Guanylyl cyclases )

Atrial natriuretic factor (ANF), an important regulator of blood volume and vascular tone, acts on a transmembrane receptor whose intracellular domain has a guanylyl cylase activity (GC). Like receptor tyrosine kinases, and receptor serine kinases, ANF receptors are active in their dimeric forms.

GC GC

GTP cGMP Signaling

IVC. Cytokine Receptors

Cytokines include:A vast array of relatively low molecular

weight, pharmacologically active proteins that are secreted by the cell for the purpose of altering either its own functions (autocrine effect) or those of adjacent cells (paracrine effect).

Cytokine receptors respond to a heterogeneous group of peptide ligands that includes:

growth hormoneerythropoietininterferonsother regulators of growth and regulation


These receptors are activated by ligand to form a dimer.

extracellular

Cytokine-binding domain

Cytoplasmic domain

JAK JAKJanus Kinases (JAK)Bind noncovalenty to the cytoplasmic side of the cytokine receptor

IVC. Cytokine Receptors These receptors are activated by ligand to form a dimer.

extracellular

JAK JAK

The JAK kinases now become active and phosphorylate tyrosine residues on the receptor. This is followed by recruitment of another types of protein called STATs (signal transducers and activators of transcription)

YY PP


extracellular

JAK JAK

Recruitment of the STATs occurs at the phosphorylated tyrosines.

YY PP STATSTATYY


extracellular

JAK JAK

JAKs then perform a second phosphorylation on tyrosines, this time on the STATs.

YY PP STATSTATP

P The phosphorylated STATs dissociate from the cytoplasmic side of the receptor and form a dimer with each other.

YY


extracellular

JAK JAK

YY PP

STAT

STATPP

The phosphorylated STATs dissociate from the cytoplasmic side of the receptor and form a dimer with each other.

The STAT dimer migrates into the nucleus… modulates gene transcription

Y

Y


extracellular

JAK JAK

YY PP

STAT

STATPP

The multistep signaling process provides several attractive targets for potential drugs:

*cytokine binding site*1st phosphorylation *2nd phosphorylation*Nuclear translocation

The STAT dimer migrates into the nucleus… modulates gene transcription

Y

Y

IVD. Ligand-Gated Channels Many of the useful drugs in clinical medicine act by:

mimicking or blocking the actions of endogenous ligands that regulate the flow of ions across the plasma membrane

The natural ligands of these receptors include:acetylcholinegamma-aminobutyric acidexcitatory amino acids (glycine, aspartate,

glutamate)

Each of these receptors transmits its signal across the plasma membrane by increasing transmembrane conductance of the relevant ion

thereby altering the electrical potential across the membrane

IVD. Ligand-Gated Channels

Acetylcholine binding (represented by the white spheres on the alpha subunits), induces a conformational change in the receptor. A channel opens allowing passage of the ion. This particular receptor regulates passage of sodium ions.

Na+ can now flow into the cell.

Na+

TOP VIEW of the Nicotinic Acetylcholine Receptor– a look from the extracellular space. The channel opens into the cytoplasm.

In the absence of ligand, the pentameric nictotinic acetylcholine receptor is closed and does not allow the charged sodium ion passage into the cytoplasm.Na+



In the absence of ligand, the pentameric nictotinic acetylcholine receptor is closed and does not allow the charged sodium ion passage into the cytoplasm.

The nictotinic acetylcholine receptor (AChR) is one of the best-characterized cell-surface receptors.

A pentamer-composed of 5 subunitstwo alpha subunits, plus one beta, one gamma and one

delta subunit

Mw of each subunit ranges from 43,000 to 50,000each polypeptide subunit crosses the lipid bilayer 4

times.



The time elapsed between the binding of the agonist to a ligand-gated channel and the cellular response can often be measured in milliseconds.

timing is crucially important for moment-to-moment transfer of information across synapses.

this is a sharp contrast from other molecular signaling mechanisms that may require seconds, minutes or even

hours, as we’ve seen with hormones that activate gene expression (eg. the glucocorticoids)

IVE. G proteins and Second Messengers Many extracellular ligands

act by increasing the intracellular concentrations of 2nd messengers

such as:

*adenosine-3’,5’-monophosphate (cAMP)

*calcium ion*phosphoinositides

In most cases, these ligands use a transmembrane signaling system with three separate components:

1. extracellular ligand is specifically detected by acell surface receptor

2. the receptor activates a G protein (cytoplasmic face)

3. activated G protein changes the activity of an effector element (eg. enzyme like adenylyl cyclase)

IVE. G proteins and Second Messengers

1. extracellular ligand is specifically detected by acell surface receptor

2. the receptor activates a G protein (cytoplasmic face)

3. activated G protein changes the activity of an effector element (eg. enzyme like adenylyl cyclase)

extracellular

ACTIVATED

Increased Effector Activity(eg. increased adenylyl cyclase activity… increased cAMP in cell)


The G protein that activates adenylyl cyclase is Gs.

Gs is activated by a host of hormones and neurotransmitters, each of which acts via a specific receptor.

This is a partial list of endogenous ligands that increase intracellular cAMP levels via Gs:

stimulatory

Adrenocorticotropic hormone

Catecholamines (Beta adrenoceptors)

Follicle Stimulating Hormone

Glucagon

Histamine (H2 receptors)

cAMP


Gs and other G proteins use a molecular mechanism that involves binding and hydrolysis of GTP

GDP

GDP= guanosine diphosphate; GTP= guanosine triphosphate

extracellular When the G-protein coupled receptor is not activated by ligand, the G-protein is associated with GDP.

GTP


Gs and other G proteins use a molecular mechanism that involves binding and hydrolysis of GTP

GDP

extracellular

When the G-protein coupled receptor is activated by ligand, GDP is released and GTP enters into the nucleotide binding site of the G protein.

GTP


extracellular

Activated G-protein (bound to GTP) migrates from the receptor and regulates the activity of an effector enzyme or ion channel (E)

GTP

E

Eactive


extracellular

The signal is terminated by the hydrolysis of GTP, after which the effector returns to its unstimulated state.

GTP

E

Eactive

Hydrolysi

s of GTP


extracellular

G protein, bound again to GDP, couples with the receptor and awaits activation.

GDP

E

Pi

Pi = inorganic phosphate

inactive


This mechanism SEPARATES ligand excitation of the receptor from G-protein-mediated activation of the effector.

extracellular

GTP

GDP

extracellular

extracellular

GTPE

Eactive

extracellular

GTPE

Eactive

Bound receptor? Unbound receptor?Once the ligand activates the G-protein to release its GDP and bind to GTP, it does not matter whether or not the receptor ligand is bound or free. The rate limiting step in the termination of effector activation is HYDROLYSIS OF GTP to GDP.


extracellular

GTPE

Eactive

Once the G-protein is activated by GTP binding, it does not matter whether or not the receptor ligand is bound or free. The rate limiting step in the termination of effector activation is HYDROLYSIS OF GTP to GDP.

extracellular

GTPE

Eactive

extracellular

GTPE

Eactive

E

Eactive

E

Eactive

E

Eactive

Hydrolysis of GTP

Hydrolysis of GTP

Hydrolysis of GTP


Effector activation G-protein/GTP complex and how long that lasts:EXAMPLE: A neurotransmitter like norepinephrine (NE) may encounter its membrane receptor for a short time- a few milliseconds.

When the Gs protein coupled to the NE receptor binds GTP and becomes activated, it may remain active for tens of seconds, enormously amplifying the original signal.

extracellular

GTPE

Eactive

E

Eactive

E

Eactive

Hydrolysis of GTP


This mechanism helps explain how signaling by G-proteins produces maximal effects at concentrations below receptor saturation :

extracellular

GTPE

Eactive

E

Eactive

E

Eactive

Hydrolysis of GTP

GTP

GTP

GTP

GTP

millisec Tens of seconds

[Ligand] = KD (half of the receptors are bound.

After receptor activation, these G-proteins will activate effector molecules until the GTP is hydrolyzed.

Here, 4 G-proteins are activated.


GTP

GTP

GTP

GTP

In this example, there are nine receptors… but only four G-proteins available for activation. Maximal response will occur when the four G proteins are all activated.

ie. In this situation, the concentration or amount of a cellular component other than the receptor limits the coupling of receptor occupancy to response.


GTP

GTP

GTP

GTP

GTP

GTP

GTP

GTP

GTP

Long duration of G protein activation allows the maximum response to be achieved at doses in which the full complement of receptors is not saturated by ligand.

Because of this, many G-protein coupled receptors produce the phenomenon of spare receptors.

GTP

GDP

GDP

GDP

GDP

GDPGDP

Big effect without Big binding.


The family of G proteins contains several functionally diverse subfamilies:

Each subfamily mediates effects of a particular set of receptors to a distinctive group of effectors

G protein Receptors for: Effector/Signaling Path

Gs Beta-adrenergic amines Adenylyl Cyclase (activity)

glucagon increased cAMPhistamineserotoninother hormones


G protein Receptors for:Effector/Signaling Path

Gi (1-3) alpha2-adrenergic amines Several, including:acetylcholine (muscarinic) Adenylyl Cyclase opioids (activity);serotonin decreased cAMPothers

Open cardiac K+ channels; dec

heart rate

Golf odorants (olfactory epithelium) Adenylyl Cyclase

increased cAMP

Go certain neurotransmitters in Not yet clear the brain


G protein Receptors for:Effector/Signaling Path

Gq acetylcholine (muscarinic) Phospholipase C (activity)

IP3serotonin (5-HT1C) DAGmany others cytoplasmic Ca+

+

Gt (1-2) photons cGMP phosphodiesterase

rhodopsin- retinal rods decreased cGMP

color opsins- cone cells(phototransduction)


extracellular

I II III IV V VI VII

G-protein coupled receptors are structurally-related.

Called ‘Serpentine’ receptors because the receptor polypeptide chain resembles a snake.

Often called 7 transmembrane spanning receptors.


extracellular


G-protein coupled receptors include receptors for:

adrenergic amines serotoninacetylcholine (muscarinic) many peptide hormonesodorants photons (rods and cones)

Extracellular N-terminus.

Intracellular carboxy terminus.


extracellular


Similarities between amino acid sequences of serpentine receptors suggest that

all of these receptors share a common evolutionary precursor

Extracellular N-terminus.

Intracellular carboxy terminus.

IVE. G proteins and Second Messengers extracellular


Most Serpentine receptors transduce signals across the plasma membrane in essentially the same way.

IVIII

III

V

VI

VII

NH2

COOH

White lines = extracellular loops Red lines = intracellular loops


The agonist ligand (turquoise sphere) binds within the pocket enclosed by the 7 transmembrane regions of the receptor.

eg. catecholamine; acetylcholine

IVIII

III

V

VI

VII

NH2

COOH

Solid White lines = extracellular loops Red lines = intracellular loops

3rd intracellular loop

GDP


After ligand binding, conformational change in the receptor is sensed by the cytoplasmic loops and G-protein is activated (GDP exchanged for GTP).

IVIII

IIV

NH2

COOH



GTP

VI

VIII E

Eactive

Hydrolysis of GTP


After hydrolysis of GTP, G protein/GDP is inactive and couples again with the receptor

IVIII

IIV

NH2

COOH



GDP

VI

VIII

GTPE

Eactive

Hydrolysis of GTP


Cytoplasmic tail of the G-protein coupled receptor contains numerous serine and threonine residues.

These residues can be phosphorylated.Phosphorylation may be associated with diminished receptor/G-protein interaction.

IVIII

IIV

NH2

COOH



GDP

VI

VIII

IVF. Receptor Regulation

Receptor-mediated responses to drugs and hormonal agonists often desensitize with time.

After reaching an initially high level, the response (eg. intracellular cAMP accumulation, Na+ influx, contractility, etc..) gradually diminishes over seconds or minutes,

even in the continued presence of the agonist.

This ‘desensitization’ is usually reversible.15 min after removal of the agonist, a second

exposure to agonist results in a response similar to the initial response.


Resp

onse

(cA

MP)

Time

AGONIST BINDING AGONIST BINDING

Passage of time in the absence of agonist

The figure above depicts the response to a beta-adrenoceptor agonist versus time. Duration of agonist binding is indicated by the shaded boxes above the graph.

Desensitization- even though the receptors are saturated with agonist, the response gradually decreases.

IVF. Receptor Regulation Although many kinds of receptors undergo desensitization, the mechanism is obscure in most cases

(eg. the nicotinic acetylcholine receptor)

BUT in the case of the beta-adrenoceptor, the mechanism has been worked out in some detail.

Ligand binding activates Gs-GTP as well as a kinase called BARK (beta-adrenergic receptor kinase).

BARK phosphorylates the cytoplasmic tail of the receptor on serine and threonine residues.

IVF. Receptor Regulation A protein named beta-arrestin then binds to the cytoplasmic tail, which results in a diminished

ability of the receptor to interact with the G protein; reducing the response to agonist.

Upon removal of agonist:

*cellular phosphatases cleave the phosphates from the tail

*BARK activity is decreased (stops phosphorylating the cytoplasmic tail of the receptor)

The receptor is then ready to fully respond to agonist; it has ‘re-set itself..’

BARK

IVF. Receptor Regulation IVIII

IIV

NH2

COOH



GTP

VI

VIII

E

Eactive

G-protein coupled receptor is activated by ligand.The G protein has become activated by GTP binding.

In addition to G-protein activation, receptor activation also stimulates the BARK kinase to bind to the cytoplasmic tail of the receptor.

BARK

Active

beta-arrestin


IIV

NH2

COOH



GTP

VI

VIII

E

Eactive

Activated BARK kinase phosphorylates serine and threonine residues in the tail.

BARK

Active

beta-arrestin


IIV

NH2

COOH



GTP

VI

VIII

E

Eactive

Phosphorylation of the tail increases the affinity for beta-arrestin, which then binds to the tail.

BARK

Active

PP

Pbeta-arrestin


IIV

NH2

COOH



GDP

VI

VIII

E

Eactive

Hydrolysis of the G protein GTP inhibits activation of effector (E). BUT.. The G protein/GDP cannot couple with the third intracellular loop because the site is hindered by beta-arrestin.

So.. Even though the ligand is bound, the response will decrease because the Gprotein-GDP cannot be activated.

BARK

Active

PP

P

beta-arrestin


IIV

NH2

COOH


GDP

VI

VIII

When the agonist dissociates from receptor, *BARK is inactivated *the phosphates on the cytoplasmic tail are

cleaved by cellular phosphatases

PP

P

beta-arrestin

BARKX

XX

PHOSPHATASES

IVF. Receptor Regulation Phosphatases are enzymes which cleave phosphate groups off of proteins.

P P P

P

This represents a protein with a phosphorylated residue (yellow sphere). A phosphatase will act on this protein to cleave the phosphate group.

Note: This is an artistic rendition. Proteins do not bleed when cleaved.

PHOSPHATASES

In this example, the “guillotine” represents cellular phosphatase enzymes .


IIV

NH2

COOH



GDP

VI

VIII

(agonist not bound to receptor) *Inorganic phosphates are released (Pi) *Beta-arrestin dissociates from the

cytoplasmic tail*G-protein/GDP can once again couple with

the receptor– the receptor has ‘re-set’ itself!

P

PP

beta-arrestin

BARK


receptor desensitization differs from the slower onset and more prolonged effect of receptor downregulation (discussed earlier).

An agonist- induced decrease in receptor number through internalization and degradation

Documents

IV.Signaling Mechanisms and Drug Action Recent research has revealed in considerable detail the molecular processes that transduce extracellular signals