G PROTEIN COUPLED G PROTEIN COUPLED RECEPTORS RECEPTORS

These are a large family of cell membrane receptors. These are a large family of cell membrane receptors.

These are linked to effectors such as enzyme, channel, carrier These are linked to effectors such as enzyme, channel, carrier proteins for response effectuation. proteins for response effectuation.

All such receptors have a common pattern of structural All such receptors have a common pattern of structural organization. organization.

Molecule has 7 Molecule has 7 αα-helical membrane spanning hydrophobic -helical membrane spanning hydrophobic amino acid segments, which run into 3 extra cellular 3 intra amino acid segments, which run into 3 extra cellular 3 intra cellular loops.cellular loops.

The agonist binding site is located some were between the helices The agonist binding site is located some were between the helices on the extra cellular face, while another recognition site formed on the extra cellular face, while another recognition site formed by cytosolic segments. by cytosolic segments.

In the inactive state GDP is bound to their exposed domainIn the inactive state GDP is bound to their exposed domain

Displacement of GDP by GTP occurs.Displacement of GDP by GTP occurs.

The active The active αα-subunit carring GTP dissociates from the other two -subunit carring GTP dissociates from the other two subunits and either activates or inhibits the effector.subunits and either activates or inhibits the effector.

ββ-subunits also been shown to modulate certain effectors like -subunits also been shown to modulate certain effectors like adenylylcyclase and phospholipase C.adenylylcyclase and phospholipase C.

The The αα-subunit has GTPase activity. -subunit has GTPase activity.

.

There are 3 major effector path ways,through which GPCRs Functions Adenylylcylase (AC) :- cyclic AMP pathway:

Activation of AC

Intracellular accumulation of second messenger cAMP

Protein kinase (PKA)

Alters the functions of many enzymes, ion channels.

Phospholipase C: IP3-DAG pathway :-

Activation of phospholipase C

Hydrolyses of the membrane phopholipid phosphatidyl inositol 4,5-bis-phosphate

Generate second messenger IP3 and DAG

Protein kinase (PKC )Ca 2+

Modulates Contraction, secretion, metabolism

Channel regulation :-

Activated G-proteins can also open or close ionic channelsspecipic for Ca2+ or K+ or Na+ with out intervention of cAMP or IP3

Hyper polarization / depolarization

Changes in Physiological responses

Maribissen & Gutkind, 2001. G-protein coupled receptors and signaling networks: emerging paradigms. Trend Pharm. Sci. 22:368-376.

Luttrel, et al., 1999. Regulation of tyrosine kinase cacades by G protein coupled receptors. Curr. Opin.Cell Biol. 11:177-183.

Schonberg, T, et al., 1999. Structural basis of G protein-coupled receptor function. Mol. Cell. Endocrin. 151:181-193.

Hamm, H. 1998. The many faces of G protein signaling. JBC 273:669-672.

Ji et al., 1998. G protein coupled receptors I. Diversity of receptor-ligand interactions. JBC 273:17299-17302.

Gether and Koblikas, 1998. G protein coupled receptors: II. Mechansim of agonist actiavtion. JBC 273:17979-17982.

Lefkowitz, RJ. 1998. G protein coupled receptors III: New roles for receptor kinases and b-arrestins in receptor signaling and desensitization. JBC 273:18677.

Gutkind, S. 1998. The pathways connecting G protien coupled receptors to the nucleus through divergent mitogen-activated protein kinase cascades. JBC 273:1839.

Fukuhara et al., 2000. Signaling from G p receptors to the nucleus, text.

Gether & Koblikas, 1998. JBC 273:17979-17982.

Palczewski, et al., 2000. Crystal structure of Rhodopsin

Science 289:739-745.

Gutkind, S. 1998. JBC 23:1839-1842.

Hamm, 1998. JBC 273:669

Silver: G subunit

Magenta: bound nucleotides; A, GTPS; B, GDP

G contact sites: Pink, polar residues; yellow, hydrophobic residues; blue, basic residues; red, acidic residues

Orange: loops in receptor that interact with G protein

Blue: Galpha

Pink: G beta

Dark blue: G gamma

Magenta: Bound GDP

Red: receptor contact sites

Hurley, 1999. JBC 274:7599.

Binding sites for:

Gsalpha, black

Gialpha, blue

Gbg, yellow

cAMP activates protein kinase A

cAMP activates

Protein kinase A, which phosphorylates CREB protein and initiates gene transcription.

Fig. 15-29 Inositol phospholipid signaling

PI Phosphoinositide

PIC Phosphoinositidase C

PI3-Kinase (PI3K) Phosphoinositide 3-kinase

PtdIns (PtdI, PI) Phosphatidylinositol

PtdIns(3)P (PtInsP, PIP) Phosphatidylinositol 3-phosphate

PtdIns(4,5)P2 (PI(4,5)P2, PIP2) Phosphatidylinositol (4,5)-bis phosphate

PtdIns(3,4,5) P3 (PI(3,4,5)P3, PIP3) Phosphatidylinositol (3,4,5)-phosphate

Ins(1,4)P2 (IP2) Inositol (1,4) bisphosphate

Ins(1,4,5)P3 (IP3) Inositol (1,4,5) trisphosphate

DAG Diacylglycerol

PLC Phospholipase C

Fig. 15-32

Activation of gene transcription

by PKC

11 PKC isozymes fall into 3 groups:

1.      Conventional ( and ) Activated by DG and Ca2+

2.      Novel ( ) Activated by DG not Ca2+

3. Atypical (, DG and Ca2+ independent

Fig. 15-27 Controls on Cytosolic Calcium

Fukuhara, et al. 2000. Signaling from G protien-coupled receptors to the nucleus. From: signaling networks and cell cycle control: The molecular basis of cancer and other diseases, Ed. JS Gutkind, Humana Press, NJ.

Maribissen & Gutkind, 2001. G-protein coupled receptors and signaling networks: emerging paradigms. Trend Pharm. Sci. 22:368-376.

Miller & Lefkowitz, 2001. Expanding roles for -arrestins as scaffolds and adaptors in GPCR signaling and trafficking. Curr. Opin. Cell Biol. 13:139-145.