g protein coupled receptor

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G Protein Coupled Receptor (GPCR) kinase in health and disease

G Protein Coupled Receptor (GPCR) kinase in health and disease

Dr.Mayank Agarwal ma.gsvm@gmail.com

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CONTENT

Types of cell surface receptorStructure of G protein coupled receptorGCPRs activation mechanismG protein family classificationMajor type of G protein signaling pathwaysGPCR desensitizationClassification of GPCR kinasesSummary

Cell-surface receptors

ESSENTIAL CELL BIOLOGY 4ED, PG.538 FIG 16-17

Ion-channel-coupled receptors change the permeability of the plasma membrane to selected ions, thereby altering the membrane potential and, if the conditions are right, producing an electrical current

G-protein-coupled receptors activate membrane-bound G protein, which then activate (or inhibit) an enzyme or an ion channel in the plasma membrane, initiating an intracellular signaling cascade3

ESSENTIAL CELL BIOLOGY 4ED, PG.538 FIG 16-17

Enzyme-coupled receptors either act as enzymes or associate with enzymes inside the cell; when stimulated, the enzymes can activate a wide variety of intracellular signalling pathways4

GPCR: An Overview G proteins are intracellular signaling proteins that are named for their ability to bind to guanosine triphosphate (GTP)

They also possess GTPase activity, the ability to hydrolyze GTP to GDP

Two categories of G proteins are described: heterotrimeric G proteins and the homomeric G proteinsCELL AND MOLECULAR BIOLOGY : LIPPINCOTT ILLUSTRATED REVIEW SERIES 1ED, PG.160

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All GPCRs have a similar structure

CELL AND MOLECULAR BIOLOGY : LIPPINCOTT ILLUSTRATED REVIEW SERIES 1ED, PG.161

Alberts molecular biology of the cell 6ed, pg. 832

The signal molecules that act on GPCRs are proteins, small peptides, derivatives of amino acids and fatty acids, photons of light and all the molecules that we can smell or taste.

Alberts molecular biology of the cell 6ed, pg. 832

The same signal molecule can activate many different GPCR family members; for example, adrenaline activates at least 9 distinct GPCRs, acetylcholine another 5, and the neurotransmitter serotonin at least 14. The different receptors for the same signal are usually expressed in different cell types and elicit different responses.

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Heteromeric G Proteins

Note that both the and the subunits have covalently attached lipidmolecules (red tails) that help bind them tothe plasma membrane, and the subunithas GDP bound. subunit contains the GTPase domain alphahelical or AH domain, clamps the nucleotide in placeRas domain provides one face of the nucleotide-binding pocketAlberts molecular biology of the cell 6ed, pg. 833

The and subunits together form a single functional unit. The GTPase domain of the subunit contains two major subdomains: the Ras domain, and the alphahelical or AH domain9

Stimulation of GPCRs Activates G-Protein Subunits

In the unstimulated state, the receptor and the G protein are bothinactive. Although they are shown here as separate entities in the plasma membrane, in some cases at least, they are associated in a preformed complex.Alberts molecular biology of the cell 6ed, pg. 833

Binding of an extracellular signal molecule to a GPCR changes the conformation of the receptor, which allows the receptor to bind and alter the conformation of a trimeric G protein. The AH domain of the G protein subunit moves outward to open the nucleotide-binding site, thereby promoting dissociation of GDPAlberts molecular biology of the cell 6ed, pg. 833

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GTP binding then promotes closure of the nucleotide-binding site, triggering conformational changes that cause dissociation of the subunit from the receptor and from the complex

Alberts molecular biology of the cell 6ed, pg. 833

ESSENTIAL CELL BIOLOGY 4ED, PG.542

When an activated subunit interacts with its target protein, it activates that target protein (or in some cases inactivates it; not shown) for as long as the two remain in contact

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ESSENTIAL CELL BIOLOGY 4ED, PG.542

Normally the subunit hydrolyzes its bound GTP to GDP within seconds. This loss of GTP inactivates the subunit, which dissociates from its target protein andif the subunit had separated from the complex (as shown)re-associates with a complex to re-form an inactive G protein

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When a GPCR is activated, it acts like a guanine nucleotide exchange factor (GEF) GTPase activity is greatly enhanced by the binding of the subunit to a secondprotein, which can be either the target protein or a specific regulator of G protein signaling (RGS). RGS proteins act as -subunit-specific GTPase-activating proteins (GAPs)Alberts molecular biology of the cell 6ed, pg. 832

GAPs help shut off G-protein-mediated responses in all eukaryotes. There are about 25 RGS proteins encoded in the human genome, each of which interacts with a particular set of G proteins15

Cell physiology source book, 4ed, pg 88

Four Major Families of Trimeric G Proteins determined by amino acid sequence relatedness of the subunits

BRS cell biology histology 7ed, pg 9; Alberts molecular biology of the cell 6ed, pg. 846

Heterotrimeric G proteins signaling pathways

Cyclic-AMP-Dependent Protein Kinase (PKA)

Phospholipase C dependent IP3, DAG and calcium

Direct regulation of ion channels

cGMP Phosphodiesterase pathway

The activation of cAMP dependent protein kinaseMammalian cells have at least two types of PKAs: type I is mainly in the cytosol, whereas type II is bound via its regulatory subunits and special anchoringproteins to the plasma membrane, nuclear membrane, mitochondrial outer membrane,and microtubulesAlberts molecular biology of the cell 6ed, pg. 834

The regulatory subunits of PKA (also called A-kinase) are important for localizing the kinase inside the cell: special A-kinase anchoring proteins (AKAPs) bind both to the regulatory subunits and to a component of the cytoskeleton or a membrane of an organelle, thereby tethering the enzyme complex to a particular subcellular compartment. Some AKAPs also bind other signalling proteins, forming a signaling complex. An AKAP located around the nucleus of heart muscle cells, for example, binds both PKA and a phosphodiesterase that hydrolyzes Camp.20

CRE-binding (CREB) protein21

Alberts molecular biology of the cell 6ed, pg. 835

Gs and Gi are targets for medically important bacterial toxinsCholera toxin, is an enzyme that causes ADP ribosylation of the subunit so that it can no longer hydrolyze its bound GTP, causing it to remain in an active state that stimulates adenylyl cyclase indefinitely.

The resulting prolonged elevation in cAMP concentration within intestinal epithelial cells causes a large efflux of Cl and water into the gut, thereby causing the severe diarrhea that characterizes cholera.

Pertussis toxin, catalyzes the ADP ribosylation of the subunit of Gi, so that Gi cannot inhibit adenylyl cyclase. Adenylyl cyclase remains active indefinitely, producing excess cAMPCELL AND MOLECULAR BIOLOGY : LIPPINCOTT ILLUSTRATED REVIEW SERIES 1ED, PG.163

The action of phospholipase C generates two small messenger molecules: inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG)CELL AND MOLECULAR BIOLOGY : LIPPINCOTT ILLUSTRATED REVIEW SERIES 1ED, PG.163

IP3 is a water-soluble sugar phosphate that is released into the cytosol; there it binds to and opens Ca2+ channels that are embedded in the endoplasmic reticulum (ER) membrane

protein kinase C (PKC), so named because it is Ca2+-dependent

Diacylglycerol can be further cleaved to release arachidonic acid, which can either act as a signal in its own right or be used in the synthesis of other small lipid signal molecules called eicosanoids. Most vertebrate cell types make eicosanoids, including prostaglandins, which have many biological activities24

Calmodulin mediates many effects of intracellular calciumCELL AND MOLECULAR BIOLOGY : LIPPINCOTT ILLUSTRATED REVIEW SERIES 1ED, PG.164

Alberts molecular biology of the cell 6ed, pg. 837

Some G Proteins Directly Regulate Ion Channels

Essential cell biology 4ed, pg.543

Binding of the neurotransmitter acetylcholine to its GPCR on the heart cells results in the activation of the G protein, Gi. (B) the activated complex directly opens a K+ channel in the plasma membrane, increasing its permeability to K+ and thereby making the membrane harder to activate and slowing the heart rate(c) Inactivation of the subunit by hydrolysis of its bound GTP returns the G protein to its inactive state, allowing the K+ channel to close.27

cGMP phosphodiesterase pathwayNeuroscience exploring the brain 4ed, 315

In the dark, cGMP gates a sodium channel, causing an inward Na current and depolarization of the cell. The activation of rhodopsin by light energy causes the G-protein (transducin) to exchange guanosine diphosphate for guanosine triphosphate, which in turn activates the enzyme phosphodiesterase (PDE). PDE breaks down cGMP and shuts off the dark current.28

Monomeric G proteinsMonomeric G proteins (=150 different ones) are important intermediaries in numerous types of signal transduction systems and can be subdivided into five general families: Ras, Rho, Rab, Ran and Arf

They are classified under enzyme coupled receptors

Ras G proteins are homologous to the subunits of heterotrimeric G proteins. They do not regulate membrane bound enzymes or induce the production of second messengers. Instead, their activation by GTP allows them to initiate a cytoplasmic phosphorylation cascade that terminates with activation of gene tra

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