Transcript A G-protein-coupled receptor

G-protein-Couped receptor
Liu Ningsheng
12/3/2010
Cell structure
The binding of extracellular signal molecules to
either cell-surface or intracellular receptors.
(A) Most signal molecules are hydrophilic, they
bind to cell-surface receptors, which in turn
generate signals inside the target cell.
(B) Some small,signal molecules, by contrast,
diffuse across the plasma membrane and bind
to receptor proteins inside the target cell—
either in the cytosol or in the nucleus
A simple intracellular signaling pathway activated
by an extracellular signal molecule.
The signal molecule
usually binds to a
receptor protein that is
embedded in the
plasma membrane of
the target cell and
activates one or more
intracellular signaling
pathways mediated by
a series of signaling
proteins. Finally, one
or more of the
intracellular signaling
proteins alters the
activity of effector
proteins and thereby
the behavior of the
cell.
A hypothetical
intracellular
signaling pathway from
a cell-surface receptor
to the nucleus.
In this example, a series of
signaling proteins and small
intracellular mediators relay the
extracellular signal into the
nucleus, causing a change in gene
expression.
Three classes of cellsurface receptors.
Overview of seven major classes of cellsurface
receptors
Two types of intracellular signaling proteins that act as
molecular switches.
Although one type is activated by phosphorylation and the other by GTP binding, in both cases the addition
of a phosphate group switches the activation state of the protein and the removal of the phosphate switches it
back again.
A G-protein-coupled receptor (GPCR).
GPCRs that bind protein
ligands have a large
extracellular domain
formed by the part of the
polypeptide chain shown
in light green. This domain,
together with some of the
transmembrane segments,
binds the protein ligand.
Receptors for small ligands
such as adrenaline have
small extracellular
domains, and the ligand
usually binds deep within
the plane of the membrane
to a site that is formed by
amino acids from several
transmembrane segments.
Schematic diagram of the general structure of G
protein–coupled receptors.
All receptors of this type have the same orientation in the membrane and contain seven
transmembrane -helical regions (H1–H7), four extracellular segments (E1–E4), and four
cytosolic segments (C1–C4). The carboxyl-terminal segment (C4), the C3 loop, and, in some
receptors, also the C2 loop are involved in interactions with a coupled trimeric G protein.
The structure of an inactive G protein.
(A) Note that both the a and the g subunits have covalently attached lipid molecules (red) that
help bind them to the plasma membrane, and the a subunit has GDP bound. (B) The
three-dimensional structure of an inactive G protein. the G protein that operates in visual
transduction. The a subunit contains the GTPase domain and binds to one side of the b subunit,
which locks the GTPase domain in an inactive conformation that binds GDP. The g subunit binds
to the opposite side of the b subunit, and the b and g subunits together form a single functional
unit. (B)
Activation of a G protein by an activated GPCR.
Binding of an extracellular signal to
a GPCR changes the conformation
of the receptor, which in turn alters
the conformation of the G protein.
The alteration of the a subunit of the
G protein allows it to exchange its
GDP for GTP, activating both the
a subunit and the bg complex, both
of which can regulate the activity of
target proteins in the plasma
membrane. The receptor stays
active while the external signal
molecule is bound to it, and it can
therefore catalyze the activation of
many molecules of G protein, which
dissociate from the receptor once
activated (not shown). In some
cases, the a subunit and the bg
complex dissociate from each other
when the G protein is activated.
model for ligand-induced activation of G protein–
coupled receptors.
The G and G subunits of trimeric G proteins are tethered to the membrane by covalently attached
lipid molecules. Following ligand binding, dissociation of the G protein, and exchange of GDP
with GTP (steps 1 – 3 ), the free G·GTP binds to and activates an effector protein (step 4 ).
Hydrolysis of GTP terminates signaling and leads to reassembly of the trimeric form, returning
the system to the resting state (step 5 ). Binding of another ligand molecule causes repetition of
the cycle. In some pathways, the effector protein is activated by the free G subunit.
Moives
http://www.celanphy.science.ru.nl/Bruce%20web/Flash%20Movies.htm
Four Major Families of Trimeric G Proteins*
Structure of mammalian adenylyl cyclases and their
interaction with Gs·GTP.
(a) Schematic diagram of mammalian adenylyl
cyclases. The membrane-bound enzyme
contains two similar catalytic domains on
the cytosolic face of the membrane and two
integral membrane domains, each of which
is thought to contain six transmembrane
helices.
(b) Three-dimensional structure of Gs·GTP
complexed with two fragments
encompassing the catalytic domain of
adenylyl cyclase determined by x-ray
crystallography.
The activation of cyclic-AMP-dependent protein
kinase (PKA).
The binding of cyclic AMP to the regulatory subunits of the PKA tetramer induces a
conformational change, causing these subunits to dissociate from the catalytic subunits,
thereby activating the kinase activity of the catalytic subunits.
Effect on
adenylyl cyclase
Hormone-induced activation and inhibition of
adenylyl cyclase in adipose cells.
Ligand binding to Gs-coupled receptors causes activation of adenylyl cyclase, whereas
ligand binding to Gi-coupled receptors causes inhibition of the enzyme. The G subunit
in both stimulatory and inhibitory G proteins is identical; the G subunits and their
corresponding receptors differ. Ligand-stimulated formation of active G·GTP
complexes occurs by the same mechanism in both Gs and Gi proteins. Gs·GTP and
Gi·GTP interact differently with adenylyl cyclase, so that one stimulates and the other
inhibits its catalytic activity.
How a rise in
intracellular cyclic
AMP concentration can
Alter gene transcription.
The binding of an extracellular signal
molecule to its GPCR activates
adenylyl cyclase via Gs and thereby
increases cyclic AMP concentration in
the cytosol..
Activation of gene expression following ligand
binding to Gs protein–coupled receptors.
Receptor stimulation ( 1 ) leads to
activation of PKA ( 2 ). Catalytic
subunits of PKA translocate to the
nucleus ( 3 ) and there phosphorylate
and activate the transcription factor
CREB ( 4 ). Phosphorylated CREB
associates with the co-activator
CBP/P300 ( 5 ) to stimulate various
target genes controlled by the CRE
regulatory element. See the text for
details.
Amplification of an external signal downstream
from a cell-surface receptor.
The more steps in such a cascade, the greater the signal amplification possible.
How GPCRs increase cytosolic Ca2+ and activate
PKC.
IP3/DAG pathway & the elevation of cytosolic Ca2.
Activation of the Tubby transcription factor
following ligand binding to receptors coupled to Go
or Gq.
In resting cells, Tubby is bound
tightly to PIP2 in the plasma
membrane. Receptor stimulation
leads to activation of
phospholipase C, hydrolysis of
PIP2, and release of Tubby into
the cytosol ( 1 ). Directed by two
functional nuclear localization
sequences (NLS) in its N-terminal
domain, Tubby translocates into
the nucleus ( 2 ) and activates
transcription of target genes ( 3 ).
The roles of GPCR kinases (GRKs) and arrestins in
GPCR desensitization.
A GRK phosphorylates only activated receptors because it is the activated GPCR that
activates the GRK. The binding of an arrestin to the phosphorylated receptor prevents
the receptor from binding to its G protein and also directs its endocytosis (not shown).
Mice that are deficient in one form of arrestin fail to desensitize in response to
morphine, for example, attesting to the importance of arrestins for desensitization.
Further Reading
1. Molecular Biology Of The Cell.
Alberts. (Chapter 15)
2. Molecular Cell Biology. Lodish
(Chapter 13)
Take-home Message
Extracellular signaling molecules regulate interactions between unicellular organisms
and are critical regulators of physiology and development in multicellular organisms.
Binding of extracellular signaling molecules to cell-surface receptors triggers
intracellular signal-transduction pathways that ultimately modulate cellular metabolism,
function, or gene expression
Receptors bind ligands with considerable specificity,which is determined by
noncovalent interactions between a ligand and specific amino acids in the receptor
protein
The level of second messengers, such as Ca2, cAMP,and IP3, increases or
occasionally decreases in response to binding of ligand to cell-surface receptors. These
nonprotein intracellular signaling molecules, in turn, regulate the activities of enzymes
and nonenzymatic proteins.
Trimeric G proteins transduce signals from coupled cellsurface receptors to associated
effector proteins, which are either enzymes that form second messengers or cation
channel proteins.
Signals most commonly are transduced by G, a GTPase switch protein that
alternates between an active (“on”) state with bound GTP and inactive (“off”)
state with GDP. The and subunits, which remain bound together,
occasionally transduce signals.
Gs, which is activated by multiple types of GPCRs, binds to and activates
adenylyl cyclase, enhancing the synthesis of 3,5-cyclic AMP (cAMP). cAMPdependent activation of protein kinase A (PKA) mediates the diverse effects
of cAMP in different cells. The substrates for PKA and thus the cellular
response to hormone-induced activation of PKA vary among cell types.
Simulation of some GPCRs and other cell-surface receptors leads to
activation of phospholipase C, which generates two second messengers:
diffusible IP3 and membrane-bound DAG
IP3 triggers opening of IP3-gated Ca2 channels in the endoplasmic
reticulum and elevation of cytosolic free Ca2. In response to elevated
cytosolic Ca2, protein kinase C is recruited to the plasma membrane, where
it is activated by DAG