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(Redirected from G-protein)
'G proteins,' short for 'guanine nucleotide binding proteins', are a family of proteins involved in second messenger cascades. They are so called because of their signaling mechanism, which uses the exchange of guanosine diphosphate (GDP) for guanosine triphosphate (GTP) as a general molecular "switch" function to regulate cell processes.
G proteins belong to the larger grouping of GTPases.
Alfred Gilman and Martin Rodbell were awarded the Nobel Prize in Physiology or Medicine in 1994 for their discovery of and research on G proteins.
G proteins are perhaps the most important signal transducing molecules in cells. In fact, diseases such as diabetes and certain forms of pituitary cancer, among many others, are thought to have some root in the malfunction of G proteins, and thus a fundamental understanding of their function, signaling pathways, and protein interactions may lead to eventual treatments and possibly the creation of various preventive approaches. G proteins are also central in our urge to urinate and defecate as the second messengers they transduce (stimulate) various receptors in our lower intestine and bladder wall.
"G protein" usually refers to the membrane-associated heterotrimeric G proteins, sometimes referred to as the ''"large" G proteins''. These proteins are activated by G protein-coupled receptors and are made up of alpha (α), beta (β) and gamma (γ) subunits. There are also ''"small" G proteins'' (20-25kDa) that belong to the Ras superfamily of small GTPases. These proteins are homologous to the alpha (α) subunit found in heterotrimers, and are in fact monomeric. However, they also bind GTP and GDP and are involved in signal transduction.
Receptor activated G proteins are bound to the inside surface of the cell membrane. They consist of the Gα and the tightly associated Gβγ subunits. Presently, four main families exist for Gα subunits: Gαs, Gαi, Gαq/11, and Gα12/13. These groups differ primarily in effector recognition, but share a similar mechanism of activation. When a ligand activates the G protein-coupled receptor, it induces a conformation change in the receptor (a change in shape) that allows for the exchange of GDP for GTP on the Gα subunit. In the traditional view of heterotrimeric protein activation, this exchange triggers the dissociation of the Gα subunit from the Gβγ dimer and the receptor. However, both molecular rearrangement and reorganization, as well pre-complexing of effector molecules are beginning to be accepted. Regardless, both, Gα-GTP and Gβγ, can then activate different ''signalling cascades'' (or ''second messenger pathways'') and effector proteins, while the receptor is able to activate the next G protein. The Gα subunit will eventually hydrolyze the attached GTP to GDP by its inherent enzymatic activity, allowing it to reassociate with Gβγ and starting a new cycle. Although, there does exist groups of proteins called RBMs that act as GTPases activating proteins (GAPS) which are specific for Gα subunits, which act to accelerate hydrolysis and terminate the transduced signal.
'Well characterized examples'
★ Gαs stimulates the production of cAMP from ATP. This is accomplished by direct stimulation of the membrane associated enzyme adenylate cyclase. cAMP acts as a second messenger which goes on to interact with and activate protein kinase A (PKA). PKA can then phosphorylate a myriad of downstream targets.
★ Gαi inhibits the production of cAMP from ATP.
★ Gαq/11 stimulates membrane bound phospholipase C which then cleaves PIP2 (a minor membrane phosphoinositol) into two second messengers, IP3 and diacylglycerol (DAG).
★ Gα12/13 are involved in Rho family GTPase signaling (through RhoGEF superfamily) and control cell cytoskeleton remodeling, thus regulating cell migration.
Many G proteins are modified by having specific lipids attached to them, i.e. they are lipidated.
★ Trimeric G proteins may be myristolated, palmitoylated, or prenylated.
★ Small G proteins may be prenylated.
★ Principles of neural science, Eric R. Kandel, James H. Schwartz, Thomas M. Jessell, , , McGraw-Hill, 2000, ISBN 0-8385-7701-6
★ Lodish et al. 2000. ''Molecular Cell Biology'' 4th ed. W.H. Freeman and Company, New York.
★ Voet, Donald and Judith G. Voet. 1995. ''Biochemistry'' 2nd ed. John Wilely & Sons, New York.
★ G proteins: transducers of receptor-generated signals., Gilman A, , , Annu Rev Biochem, 1987
★ G Protein Pathways., Neves S.R., Ram P.T., Iyengar R., , , Science, 2002
★
'G proteins,' short for 'guanine nucleotide binding proteins', are a family of proteins involved in second messenger cascades. They are so called because of their signaling mechanism, which uses the exchange of guanosine diphosphate (GDP) for guanosine triphosphate (GTP) as a general molecular "switch" function to regulate cell processes.
G proteins belong to the larger grouping of GTPases.
| Contents |
| History |
| Function |
| Types of G protein signaling |
| Hetereotrimeric G proteins |
| Lipidation |
| References |
| External links |
History
Alfred Gilman and Martin Rodbell were awarded the Nobel Prize in Physiology or Medicine in 1994 for their discovery of and research on G proteins.
Function
G proteins are perhaps the most important signal transducing molecules in cells. In fact, diseases such as diabetes and certain forms of pituitary cancer, among many others, are thought to have some root in the malfunction of G proteins, and thus a fundamental understanding of their function, signaling pathways, and protein interactions may lead to eventual treatments and possibly the creation of various preventive approaches. G proteins are also central in our urge to urinate and defecate as the second messengers they transduce (stimulate) various receptors in our lower intestine and bladder wall.
Types of G protein signaling
"G protein" usually refers to the membrane-associated heterotrimeric G proteins, sometimes referred to as the ''"large" G proteins''. These proteins are activated by G protein-coupled receptors and are made up of alpha (α), beta (β) and gamma (γ) subunits. There are also ''"small" G proteins'' (20-25kDa) that belong to the Ras superfamily of small GTPases. These proteins are homologous to the alpha (α) subunit found in heterotrimers, and are in fact monomeric. However, they also bind GTP and GDP and are involved in signal transduction.
Hetereotrimeric G proteins
Activation cycle of G-proteins by G-protein-coupled receptors
Receptor activated G proteins are bound to the inside surface of the cell membrane. They consist of the Gα and the tightly associated Gβγ subunits. Presently, four main families exist for Gα subunits: Gαs, Gαi, Gαq/11, and Gα12/13. These groups differ primarily in effector recognition, but share a similar mechanism of activation. When a ligand activates the G protein-coupled receptor, it induces a conformation change in the receptor (a change in shape) that allows for the exchange of GDP for GTP on the Gα subunit. In the traditional view of heterotrimeric protein activation, this exchange triggers the dissociation of the Gα subunit from the Gβγ dimer and the receptor. However, both molecular rearrangement and reorganization, as well pre-complexing of effector molecules are beginning to be accepted. Regardless, both, Gα-GTP and Gβγ, can then activate different ''signalling cascades'' (or ''second messenger pathways'') and effector proteins, while the receptor is able to activate the next G protein. The Gα subunit will eventually hydrolyze the attached GTP to GDP by its inherent enzymatic activity, allowing it to reassociate with Gβγ and starting a new cycle. Although, there does exist groups of proteins called RBMs that act as GTPases activating proteins (GAPS) which are specific for Gα subunits, which act to accelerate hydrolysis and terminate the transduced signal.
'Well characterized examples'
★ Gαs stimulates the production of cAMP from ATP. This is accomplished by direct stimulation of the membrane associated enzyme adenylate cyclase. cAMP acts as a second messenger which goes on to interact with and activate protein kinase A (PKA). PKA can then phosphorylate a myriad of downstream targets.
★ Gαi inhibits the production of cAMP from ATP.
★ Gαq/11 stimulates membrane bound phospholipase C which then cleaves PIP2 (a minor membrane phosphoinositol) into two second messengers, IP3 and diacylglycerol (DAG).
★ Gα12/13 are involved in Rho family GTPase signaling (through RhoGEF superfamily) and control cell cytoskeleton remodeling, thus regulating cell migration.
Lipidation
Many G proteins are modified by having specific lipids attached to them, i.e. they are lipidated.
★ Trimeric G proteins may be myristolated, palmitoylated, or prenylated.
★ Small G proteins may be prenylated.
References
★ Principles of neural science, Eric R. Kandel, James H. Schwartz, Thomas M. Jessell, , , McGraw-Hill, 2000, ISBN 0-8385-7701-6
★ Lodish et al. 2000. ''Molecular Cell Biology'' 4th ed. W.H. Freeman and Company, New York.
★ Voet, Donald and Judith G. Voet. 1995. ''Biochemistry'' 2nd ed. John Wilely & Sons, New York.
★ G proteins: transducers of receptor-generated signals., Gilman A, , , Annu Rev Biochem, 1987
★ G Protein Pathways., Neves S.R., Ram P.T., Iyengar R., , , Science, 2002
External links
★
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