G protein

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Phosducin-transducin beta-gamma complex. Beta and gamma subunits of G-protein are shown by blue and red, respectively. 1b9x opm.png
Phosducin-transducin beta-gamma complex. Beta and gamma subunits of G-protein are shown by blue and red, respectively.
Guanosine diphosphate Guanosindiphosphat protoniert.svg
Guanosine diphosphate
Guanosine triphosphate Guanosintriphosphat protoniert.svg
Guanosine triphosphate

G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell to its interior. Their activity is regulated by factors that control their ability to bind to and hydrolyze guanosine triphosphate (GTP) to guanosine diphosphate (GDP). When they are bound to GTP, they are 'on', and, when they are bound to GDP, they are 'off'. G proteins belong to the larger group of enzymes called GTPases.

Contents

There are two classes of G proteins. The first function as monomeric small GTPases (small G-proteins), while the second function as heterotrimeric G protein complexes. The latter class of complexes is made up of alpha (Gα), beta (Gβ) and gamma (Gγ) subunits. [1] In addition, the beta and gamma subunits can form a stable dimeric complex referred to as the beta-gamma complex . [2]

Heterotrimeric G proteins located within the cell are activated by G protein-coupled receptors (GPCRs) that span the cell membrane. [3] Signaling molecules bind to a domain of the GPCR located outside the cell, and an intracellular GPCR domain then in turn activates a particular G protein. Some active-state GPCRs have also been shown to be "pre-coupled" with G proteins, whereas in other cases a collision coupling mechanism is thought to occur. [4] [5] [6] The G protein triggers a cascade of further signaling events that finally results in a change in cell function. G protein-coupled receptors and G proteins working together transmit signals from many hormones, neurotransmitters, and other signaling factors. [7] G proteins regulate metabolic enzymes, ion channels, transporter proteins, and other parts of the cell machinery, controlling transcription, motility, contractility, and secretion, which in turn regulate diverse systemic functions such as embryonic development, learning and memory, and homeostasis. [8]

History

G proteins were discovered in 1980 when Alfred G. Gilman and Martin Rodbell investigated stimulation of cells by adrenaline. They found that when adrenaline binds to a receptor, the receptor does not stimulate enzymes (inside the cell) directly. Instead, the receptor stimulates a G protein, which then stimulates an enzyme. An example is adenylate cyclase, which produces the second messenger cyclic AMP. [9] For this discovery, they won the 1994 Nobel Prize in Physiology or Medicine. [10]

Nobel prizes have been awarded for many aspects of signaling by G proteins and GPCRs. These include receptor antagonists, neurotransmitters, neurotransmitter reuptake, G protein-coupled receptors, G proteins, second messengers, the enzymes that trigger protein phosphorylation in response to cAMP, and consequent metabolic processes such as glycogenolysis.

Prominent examples include (in chronological order of awarding):

Function

G proteins are important signal transducing molecules in cells. "Malfunction of GPCR [G Protein-Coupled Receptor] signaling pathways are involved in many diseases, such as diabetes, blindness, allergies, depression, cardiovascular defects, and certain forms of cancer. It is estimated that about 30% of the modern drugs' cellular targets are GPCRs." [15] The human genome encodes roughly 800 [16] G protein-coupled receptors, which detect photons of light, hormones, growth factors, drugs, and other endogenous ligands. Approximately 150 of the GPCRs found in the human genome still have unknown functions.

Whereas G proteins are activated by G protein-coupled receptors, they are inactivated by RGS proteins (for "Regulator of G protein signalling"). Receptors stimulate GTP binding (turning the G protein on). RGS proteins stimulate GTP hydrolysis (creating GDP, thus turning the G protein off).

Diversity

Sequence relationship among the 18 human Ga proteins. Human Ga protein phylogeny.png
Sequence relationship among the 18 human Gα proteins.

All eukaryotes use G proteins for signaling and have evolved a large diversity of G proteins. For instance, humans encode 18 different Gα proteins, 5 Gβ proteins, and 12 Gγ proteins. [17]

Signaling

G protein can refer to two distinct families of proteins. Heterotrimeric G proteins, sometimes referred to as the "large" G proteins, are activated by G protein-coupled receptors and are made up of alpha (α), beta (β), and gamma (γ) subunits. "Small" G proteins (20-25kDa) belong to the Ras superfamily of small GTPases. These proteins are homologous to the alpha (α) subunit found in heterotrimers, but are in fact monomeric, consisting of only a single unit. However, like their larger relatives, they also bind GTP and GDP and are involved in signal transduction.

Heterotrimeric

Different types of heterotrimeric G proteins share a common mechanism. They are activated in response to a conformational change in the GPCR, exchanging GDP for GTP, and dissociating in order to activate other proteins in a particular signal transduction pathway. [18] The specific mechanisms, however, differ between protein types.

Mechanism

Activation cycle of G-proteins (pink) by a G-protein-coupled receptor (GPCR, light blue) receiving a ligand (red). Ligand binding to GPCRs (2) induces a conformation change that facilitates the exchange of GDP for GTP on the a subunit of the heterotrimeric complex (3-4). Both GTP-bound Ga in the active form and the released Gbg dimer can then go on to stimulate a number of downstream effectors (5). When the GTP on Ga is hydrolyzed to GDP (6) the original receptor is restored (1). GPCR-Zyklus.png
Activation cycle of G-proteins (pink) by a G-protein-coupled receptor (GPCR, light blue) receiving a ligand (red). Ligand binding to GPCRs (2) induces a conformation change that facilitates the exchange of GDP for GTP on the α subunit of the heterotrimeric complex (3-4). Both GTP-bound Gα in the active form and the released Gβγ dimer can then go on to stimulate a number of downstream effectors (5). When the GTP on Gα is hydrolyzed to GDP (6) the original receptor is restored (1).

Receptor-activated G proteins are bound to the inner surface of the cell membrane. They consist of the Gα and the tightly associated Gβγ subunits. There are four main families of Gα subunits: Gαs (G stimulatory), Gαi (G inhibitory), Gαq/11, and Gα12/13. [20] [21] They behave differently in the recognition of the effector molecule, but share a similar mechanism of activation.

Activation

When a ligand activates the G protein-coupled receptor, it induces a conformational change in the receptor that allows the receptor to function as a guanine nucleotide exchange factor (GEF) that exchanges GDP for GTP. The GTP (or GDP) is bound to the Gα subunit in the traditional view of heterotrimeric GPCR activation. This exchange triggers the dissociation of the Gα subunit (which is bound to GTP) from the Gβγ dimer and the receptor as a whole. However, models which suggest molecular rearrangement, reorganization, and pre-complexing of effector molecules are beginning to be accepted. [4] [22] [23] Both Gα-GTP and Gβγ can then activate different signaling cascades (or second messenger pathways) and effector proteins, while the receptor is able to activate the next G protein. [24]

Termination

The Gα subunit will eventually hydrolyze the attached GTP to GDP by its inherent enzymatic activity, allowing it to re-associate with Gβγ and starting a new cycle. A group of proteins called Regulator of G protein signalling (RGSs), act as GTPase-activating proteins (GAPs), are specific for Gα subunits. These proteins accelerate the hydrolysis of GTP to GDP, thus terminating the transduced signal. In some cases, the effector itself may possess intrinsic GAP activity, which then can help deactivate the pathway. This is true in the case of phospholipase C-beta, which possesses GAP activity within its C-terminal region. This is an alternate form of regulation for the Gα subunit. Such Gα GAPs do not have catalytic residues (specific amino acid sequences) to activate the Gα protein. They work instead by lowering the required activation energy for the reaction to take place. [25]

Specific mechanisms

Gαs

Gαs activates the cAMP-dependent pathway by stimulating the production of cyclic AMP (cAMP) from ATP. This is accomplished by direct stimulation of the membrane-associated enzyme adenylate cyclase. cAMP can then act as a second messenger that goes on to interact with and activate protein kinase A (PKA). PKA can phosphorylate a myriad downstream targets.

The cAMP-dependent pathway is used as a signal transduction pathway for many hormones including:

Gαi

Gαi inhibits the production of cAMP from ATP. e.g. somatostatin, prostaglandins

Gαq/11

Gαq/11 stimulates the membrane-bound phospholipase C beta, which then cleaves PIP2 (a minor membrane phosphoinositol) into two second messengers, IP3 and diacylglycerol (DAG). The Inositol Phospholipid Dependent Pathway is used as a signal transduction pathway for many hormones including:

Gα12/13
  • Gα12/13 are involved in Rho family GTPase signaling (see Rho family of GTPases). This is through the RhoGEF superfamily involving the RhoGEF domain of the proteins' structures). These are involved in control of cell cytoskeleton remodeling, and thus in regulating cell migration.
Gβ, Gγ

Small GTPases

Small GTPases, also known as small G-proteins, bind GTP and GDP likewise, and are involved in signal transduction. These proteins are homologous to the alpha (α) subunit found in heterotrimers, but exist as monomers. They are small (20-kDa to 25-kDa) proteins that bind to guanosine triphosphate (GTP). This family of proteins is homologous to the Ras GTPases and is also called the Ras superfamily GTPases.

Lipidation

In order to associate with the inner leaflet[ clarification needed ] of the plasma membrane, many G proteins and small GTPases are lipidated, that is, covalently modified with lipid extensions. They may be myristoylated, palmitoylated or prenylated.

Related Research Articles

<span class="mw-page-title-main">G protein-coupled receptor</span> Class of cell surface receptors coupled to G-protein-associated intracellular signaling

G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily related proteins that are cell surface receptors that detect molecules outside the cell and activate cellular responses. They are coupled with G proteins. They pass through the cell membrane seven times in the form of six loops of amino acid residues, which is why they are sometimes referred to as seven-transmembrane receptors. Ligands can bind either to the extracellular N-terminus and loops or to the binding site within transmembrane helices. They are all activated by agonists, although a spontaneous auto-activation of an empty receptor has also been observed.

GTPases are a large family of hydrolase enzymes that bind to the nucleotide guanosine triphosphate (GTP) and hydrolyze it to guanosine diphosphate (GDP). The GTP binding and hydrolysis takes place in the highly conserved P-loop "G domain", a protein domain common to many GTPases.

<span class="mw-page-title-main">Protein kinase A</span> Family of enzymes

In cell biology, protein kinase A (PKA) is a family of serine-threonine kinase whose activity is dependent on cellular levels of cyclic AMP (cAMP). PKA is also known as cAMP-dependent protein kinase. PKA has several functions in the cell, including regulation of glycogen, sugar, and lipid metabolism. It should not be confused with 5'-AMP-activated protein kinase.

<span class="mw-page-title-main">Guanosine diphosphate</span> Chemical compound

Guanosine diphosphate, abbreviated GDP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside guanosine. GDP consists of a pyrophosphate group, a pentose sugar ribose, and the nucleobase guanine.

<span class="mw-page-title-main">Opioid receptor</span> Group of biological receptors

Opioid receptors are a group of inhibitory G protein-coupled receptors with opioids as ligands. The endogenous opioids are dynorphins, enkephalins, endorphins, endomorphins and nociceptin. The opioid receptors are ~40% identical to somatostatin receptors (SSTRs). Opioid receptors are distributed widely in the brain, in the spinal cord, on peripheral neurons, and digestive tract.

Second messengers are intracellular signaling molecules released by the cell in response to exposure to extracellular signaling molecules—the first messengers. Second messengers trigger physiological changes at cellular level such as proliferation, differentiation, migration, survival, apoptosis and depolarization.

<span class="mw-page-title-main">G protein-gated ion channel</span>

G protein-gated ion channels are a family of transmembrane ion channels in neurons and atrial myocytes that are directly gated by G proteins.

<span class="mw-page-title-main">Heterotrimeric G protein</span> Class of enzymes

Heterotrimeric G protein, also sometimes referred to as the "large" G proteins are membrane-associated G proteins that form a heterotrimeric complex. The biggest non-structural difference between heterotrimeric and monomeric G protein is that heterotrimeric proteins bind to their cell-surface receptors, called G protein-coupled receptors (GPCR), directly. These G proteins are made up of alpha (α), beta (β) and gamma (γ) subunits. The alpha subunit is attached to either a GTP or GDP, which serves as an on-off switch for the activation of G-protein.

<span class="mw-page-title-main">Gustducin</span> G protein

Gustducin is a G protein associated with taste and the gustatory system, found in some taste receptor cells. Research on the discovery and isolation of gustducin is recent. It is known to play a large role in the transduction of bitter, sweet and umami stimuli. Its pathways are many and diverse.

G<sub>s</sub> alpha subunit Mammalian protein found in Homo sapiens

The Gs alpha subunit is a subunit of the heterotrimeric G protein Gs that stimulates the cAMP-dependent pathway by activating adenylyl cyclase. Gsα is a GTPase that functions as a cellular signaling protein. Gsα is the founding member of one of the four families of heterotrimeric G proteins, defined by the alpha subunits they contain: the Gαs family, Gαi/Gαo family, Gαq family, and Gα12/Gα13 family. The Gs-family has only two members: the other member is Golf, named for its predominant expression in the olfactory system. In humans, Gsα is encoded by the GNAS complex locus, while Golfα is encoded by the GNAL gene.

Gq protein alpha subunit is a family of heterotrimeric G protein alpha subunits. This family is also commonly called the Gq/11 (Gq/G11) family or Gq/11/14/15 family to include closely related family members. G alpha subunits may be referred to as Gq alpha, Gαq, or Gqα. Gq proteins couple to G protein-coupled receptors to activate beta-type phospholipase C (PLC-β) enzymes. PLC-β in turn hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) to diacyl glycerol (DAG) and inositol trisphosphate (IP3). IP3 acts as a second messenger to release stored calcium into the cytoplasm, while DAG acts as a second messenger that activates protein kinase C (PKC).

Gi protein alpha subunit is a family of heterotrimeric G protein alpha subunits. This family is also commonly called the Gi/o family or Gi/o/z/t family to include closely related family members. G alpha subunits may be referred to as Gi alpha, Gαi, or Giα.

G12/G13 alpha subunits are alpha subunits of heterotrimeric G proteins that link cell surface G protein-coupled receptors primarily to guanine nucleotide exchange factors for the Rho small GTPases to regulate the actin cytoskeleton. Together, these two proteins comprise one of the four classes of G protein alpha subunits. G protein alpha subunits bind to guanine nucleotides and function in a regulatory cycle, and are active when bound to GTP but inactive and associated with the G beta-gamma complex when bound to GDP. G12/G13 are not targets of pertussis toxin or cholera toxin, as are other classes of G protein alpha subunits.

<span class="mw-page-title-main">G alpha subunit</span>

G alpha subunits are one of the three types of subunit of guanine nucleotide binding proteins, which are membrane-associated, heterotrimeric G proteins.

<span class="mw-page-title-main">Regulator of G protein signaling</span>

Regulators of G protein signaling (RGS) are protein structural domains or the proteins that contain these domains, that function to activate the GTPase activity of heterotrimeric G-protein α-subunits.

<span class="mw-page-title-main">GNA12</span> Protein-coding gene in the species Homo sapiens

Guanine nucleotide-binding protein subunit alpha-12 is a protein that in humans is encoded by the GNA12 gene.

In the field of molecular biology, the cAMP-dependent pathway, also known as the adenylyl cyclase pathway, is a G protein-coupled receptor-triggered signaling cascade used in cell communication.

<span class="mw-page-title-main">G beta-gamma complex</span>

The G beta-gamma complex (Gβγ) is a tightly bound dimeric protein complex, composed of one Gβ and one Gγ subunit, and is a component of heterotrimeric G proteins. Heterotrimeric G proteins, also called guanosine nucleotide-binding proteins, consist of three subunits, called alpha, beta, and gamma subunits, or Gα, Gβ, and Gγ. When a G protein-coupled receptor (GPCR) is activated, Gα dissociates from Gβγ, allowing both subunits to perform their respective downstream signaling effects. One of the major functions of Gβγ is the inhibition of the Gα subunit.

<span class="mw-page-title-main">GoLoco motif</span> Protein structural motif

GoLoco motif is a protein structural motif. In heterotrimeric G-protein signalling, cell surface receptors (GPCRs) are coupled to membrane-associated heterotrimers comprising a GTP-hydrolyzing subunit G-alpha and a G-beta/G-gamma dimer. The inactive form contains the alpha subunit bound to GDP and complexes with the beta and gamma subunit. When the ligand is associated to the receptor, GDP is displaced from G-alpha and GTP is bound. The GTP/G-alpha complex dissociates from the trimer and associates to an effector until the intrinsic GTPase activity of G-alpha returns the protein to GDP bound form. Reassociation of GDP-bound G-alpha with G-beta/G-gamma dimer terminates the signal. Several mechanisms regulate the signal output at different stage of the G-protein cascade. Two classes of intracellular proteins act as inhibitors of G protein activation: GTPase activating proteins (GAPs), which enhance GTP hydrolysis, and guanine dissociation inhibitors (GDIs), which inhibit GDP dissociation. The GoLoco or G-protein regulatory (GPR) motif found in various G-protein regulators. acts as a GDI on G-alpha(i).

<span class="mw-page-title-main">PLEKHG2</span> Protein-coding gene in the species Homo sapiens

Pleckstrin homology domain containing, family G member 2 (PLEKHG2) is a protein that in humans is encoded by the PLEKHG2 gene. It is sometimes written as ARHGEF42, FLJ00018.

References

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  3. ↑ "Seven Transmembrane Receptors: Robert Lefkowitz". 9 September 2012. Retrieved 11 July 2016.
  4. 1 2 Qin K, Dong C, Wu G, Lambert NA (August 2011). "Inactive-state preassembly of G(q)-coupled receptors and G(q) heterotrimers". Nature Chemical Biology. 7 (10): 740–7. doi:10.1038/nchembio.642. PMC   3177959 . PMID   21873996.
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  9. 1 2 The Nobel Prize in Physiology or Medicine 1994, Illustrated Lecture.
  10. ↑ Press Release: The Nobel Assembly at the Karolinska Institute decided to award the Nobel Prize in Physiology or Medicine for 1994 jointly to Alfred G. Gilman and Martin Rodbell for their discovery of "G-proteins and the role of these proteins in signal transduction in cells". 10 October 1994
  11. ↑ "The Nobel Prize in Physiology or Medicine 1992 Press Release". Nobel Assembly at Karolinska Institutet . Retrieved 21 August 2013.
  12. ↑ Press Release
  13. ↑ "Press Release: The 2004 Nobel Prize in Physiology or Medicine". Nobelprize.org. Retrieved 8 November 2012.
  14. ↑ Royal Swedish Academy of Sciences (10 October 2012). "The Nobel Prize in Chemistry 2012 Robert J. Lefkowitz, Brian K. Kobilka" . Retrieved 10 October 2012.
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  16. ↑ Baltoumas FA, Theodoropoulou MC, Hamodrakas SJ (June 2013). "Interactions of the α-subunits of heterotrimeric G-proteins with GPCRs, effectors and RGS proteins: a critical review and analysis of interacting surfaces, conformational shifts, structural diversity and electrostatic potentials". Journal of Structural Biology. 182 (3): 209–18. doi:10.1016/j.jsb.2013.03.004. PMID   23523730.
  17. 1 2 Syrovatkina V, Alegre KO, Dey R, Huang XY (September 2016). "Regulation, Signaling, and Physiological Functions of G-Proteins". Journal of Molecular Biology. 428 (19): 3850–68. doi:10.1016/j.jmb.2016.08.002. PMC   5023507 . PMID   27515397.
  18. ↑ Lim, Wendell (2015). Cell signaling : principles and mechanisms. Bruce Mayer, T. Pawson. New York. ISBN   978-0-8153-4244-1. OCLC   868641565.{{cite book}}: CS1 maint: location missing publisher (link)
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