The G beta-gamma complex (G
βγ) is a tightly bound dimeric protein complex, composed of one G
β and one G
γ subunit, and is a component of
heterotrimeric G proteins
Heterotrimeric G protein, also sometimes referred to as the ''"large" G proteins'' (as opposed to the subclass of smaller, monomeric small GTPases) are membrane-associated G proteins that form a heterotrimeric complex. The biggest non-structura ...
. Heterotrimeric G proteins, also called guanosine nucleotide-binding proteins, consist of three subunits, called
alpha
Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', or ell, άλφα, álfa) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter aleph , whic ...
, beta, and
gamma subunits, or G
α, G
β, and G
γ. When a
G protein-coupled receptor
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 p ...
(GPCR) is activated, G
α dissociates from G
βγ, allowing both subunits to perform their respective downstream
signaling
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
effects. One of the major functions of G
βγ is the inhibition of the G
α subunit.
History
The individual subunits of the G protein complex were first identified in 1980 when the regulatory component of
adenylate cyclase
Adenylate cyclase (EC 4.6.1.1, also commonly known as adenyl cyclase and adenylyl cyclase, abbreviated AC) is an enzyme with systematic name ATP diphosphate-lyase (cyclizing; 3′,5′-cyclic-AMP-forming). It catalyzes the following reaction:
:A ...
was successfully purified, yielding three
polypeptides
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
A p ...
of different molecular weights. Initially, it was thought that G
α, the largest subunit, was the major effector regulatory subunit, and that G
βγ was largely responsible for inactivating the G
α subunit and enhancing membrane binding.
However, downstream signalling effects of G
βγ were later discovered when the purified G
βγ complex was found to activate a cardiac muscarinic
K+ channel.
Shortly after, the G
βγ complex associated with a mating factor receptor-coupled G protein in yeast was found to initiate a
pheromone
A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...
response. Although these hypotheses were initially controversial, G
βγ has since been shown to directly regulate as many different protein targets as the G
α subunit.
Recently, possible roles of the G
βγ complex in retinal
rod photoreceptors have been investigated, with some evidence for the maintenance of G
α inactivation. However, these conclusions were drawn from ''
in vitro
''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...
'' experiments under unphysiological conditions, and the physiological role of the G
βγ complex in vision is still unclear. Nevertheless, recent ''
in vivo
Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and ...
'' findings demonstrate the necessity of the
transducin
Transducin (Gt) is a protein naturally expressed in vertebrate retina rods and cones and it is very important in vertebrate phototransduction. It is a type of heterotrimeric G-protein with different α subunits in rod and cone photoreceptors.
L ...
G
βγ complex in the functioning of rod photoreceptors under low light conditions.
Structure
The G
βγ subunit is a dimer composed of two polypeptides, however it acts functionally as a monomer, as the individual subunits do not separate, and have not been found to function independently.
The G
β subunit is a member of the
β-propeller
In structural biology, a beta-propeller (β-propeller) is a type of all-β protein architecture characterized by 4 to 8 highly symmetrical blade-shaped beta sheets arranged toroidally around a central axis. Together the beta-sheets form a funnel ...
family of proteins, which typically possess 4-8 antiparallel β-sheets arranged in the shape of a propeller.
G
β contains a 7 bladed β-propeller, each blade arranged around a central axis and composed of 4 antiparallel
β-sheets
The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected laterally by at least two or three backbone hydrogen bonds, forming a gen ...
.
The amino acid sequence contains 7
WD repeat
The WD40 repeat (also known as the WD or beta-transducin repeat) is a short structural motif of approximately 40 amino acids, often terminating in a tryptophan-aspartic acid (W-D) dipeptide. Protein tandem repeats, Tandem copies of these repeats ...
motifs of about 40 amino acids, each highly conserved and possessing the Trp-Asp dipeptide that gives the repeat its name.
The G
γ subunit is considerably smaller than G
β, and is unstable on its own, requiring interaction with G
β to fold, explaining the close association of the dimer. In the G
βγ dimer, the G
γ subunit wraps around the outside of G
β, interacting through hydrophobic associations, and exhibits no tertiary interactions with itself. The N terminus
helical
Helical may refer to:
* Helix, the mathematical concept for the shape
* Helical engine, a proposed spacecraft propulsion drive
* Helical spring, a coilspring
* Helical plc, a British property company, once a maker of steel bar stock
* Helicoil
A t ...
domains of the two subunits form a
coiled coil
A coiled coil is a structural motif in proteins in which 2–7
alpha-helices are coiled together like the strands of a rope. (Dimers and trimers are the most common types.) Many coiled coil-type proteins are involved in important biological fun ...
with one another that typically extends away from the core of the dimer.
To date, 5 β-subunit and 11 γ-subunit genes and have been identified in mammals.
The G
β genes have very similar sequences, while significantly greater variation is seen in the G
γ genes, indicating that the functional specificity of the G
βγ dimer may be dependent on the type of G
γ subunit involved.
Of additional structural interest is the discovery of a so-called “hotspot” present on the surface of the G
βγ dimer; a specific site of the protein that binds to diverse range of peptides and is thought to be a contributing factor in the ability of G
βγ to interact with a wide variety of effectors.
Synthesis and Modification
Synthesis
Synthesis or synthesize may refer to:
Science Chemistry and biochemistry
*Chemical synthesis, the execution of chemical reactions to form a more complex molecule from chemical precursors
** Organic synthesis, the chemical synthesis of organ ...
of the subunits occurs in the
cytosol
The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells (intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...
. Folding of the β-subunit is thought to be aided by the
chaperone CCT (chaperonin containing tailless-complex polypeptide 1), which also prevents aggregation of folded subunits. A second chaperone, PhLP (phosducin-like protein), binds to the CCT/G
β complex, and is phosphorylated, allowing CCT to dissociate and G
γ to bind. Finally, PhLP is released, exposing the binding site for G
α, allowing for formation of the final trimer at the
endoplasmic reticulum
The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...
, where it is targeted to the
plasma membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...
. G
γ subunits are known to be
prenylated (covalently modified by the addition of lipid moieties) prior to addition to G
β, which itself has not been found to be modified. This prenylation is thought to be involved in directing the interaction of the subunit both with membrane lipids and other proteins.
Function
The G
βγ complex is an essential element in the GPCR signaling cascade. It has two main states for which it performs different functions. When G
βγ is interacting with G
α it functions as a negative regulator. In the heterotrimer form, the G
βγ dimer increases the affinity of G
α for
GDP
Gross domestic product (GDP) is a monetary measure of the market value of all the final goods and services produced and sold (not resold) in a specific time period by countries. Due to its complex and subjective nature this measure is ofte ...
, which causes the G protein to be in an inactive state. For the G
α subunit to become active, the nucleotide exchange must be induced by the GPCR. Studies have shown that it is the G
βγ dimer that demonstrates specificity for the appropriate receptor and that the G
γ subunit actually enhances the interaction of the G
α subunit with the GPCR.
The GPCR is activated by an extracellular
ligand
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
and subsequently activates the G protein heterotrimer by causing a
conformational change
In biochemistry, a conformational change is a change in the shape of a macromolecule, often induced by environmental factors.
A macromolecule is usually flexible and dynamic. Its shape can change in response to changes in its environment or oth ...
in the G
α subunit. This causes the replacement of GDP with GTP as well as the physical dissociation of the G
α and the G
βγ complex.
Once separated, both G
α and G
βγ are free to participate in their own distinct signaling pathways. G
βγ does not go through any conformational changes when it dissociates from G
α and it acts as a signaling molecule as a dimer.
The G
βγ dimer has been found to interact with many different effector molecules by
protein-protein interactions. Different combinations of the G
β and G
γ subtypes can influence different
effector
Effector may refer to:
*Effector (biology), a molecule that binds to a protein and thereby alters the activity of that protein
* ''Effector'' (album), a music album by the Experimental Techno group Download
* ''EFFector'', a publication of the El ...
s and work exclusively or synergistically with the G
α subunit.
G
βγ signaling is diverse, inhibiting or activating many downstream events depending on its interaction with different effectors. Researchers have discovered that G
βγ regulates
ion channel
Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of io ...
s, such as
G protein-gated inward rectifier channels,
as well as
calcium channel
A calcium channel is an ion channel which shows selective permeability to calcium ions. It is sometimes synonymous with voltage-gated calcium channel, although there are also ligand-gated calcium channels.
Comparison tables
The following tables e ...
s.
In human
PBMC, G
βγ complex has been shown to activate phosphorylation of
ERK1/2. Another example of G
βγ signaling is its effect of activating or inhibiting adenylyl cyclase leading to the intracellular increase or decrease of the secondary messenger
cyclic AMP
Cyclic adenosine monophosphate (cAMP, cyclic AMP, or 3',5'-cyclic adenosine monophosphate) is a second messenger important in many biological processes. cAMP is a derivative of adenosine triphosphate (ATP) and used for intracellular signal transd ...
. For more examples of G
βγ signaling see table. However, the full extent of G
βγ signaling has not yet been discovered.
Medical implications
Drug design
The G
βγ subunit plays a variety of roles in cell signalling processes and as such researchers are now examining its potential as a therapeutic drug target for the treatment of many medical conditions. However, it is recognized that there are a number of considerations to keep in mind when designing a drug which targets the G
βγ subunit:
# The G
βγ subunit is essential for the formation of heterotrimeric G protein through its association with the G
α subunit allowing the G proteins coupling to the GPCR. Therefore, any agent inhibiting the G
βγ subunits signalling effects must not interfere with the heterotrimeric G protein formation or G
α subunit signalling.
# G
βγ expression is universal throughout almost all the cells of the body so any agent acting to inhibit this subunit could elicit numerous side effects.
# Small molecule inhibitors that target the coupling of G
βγ to specific effectors and do not interfere with normal G protein cycling/ heterotrimeric formation, have the potential to work as therapeutic agents in treating some specific diseases.
Targeting the Gβγ subunit in treatment
Research has been conducted on how altering the actions of G
βγ subunits could be beneficial for the treatment of certain medical conditions. G
βγ signalling has been examined for its role in a variety of conditions including
heart failure
Heart failure (HF), also known as congestive heart failure (CHF), is a syndrome, a group of signs and symptoms caused by an impairment of the heart's blood pumping function. Symptoms typically include shortness of breath, excessive fatigue, a ...
,
inflammation
Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...
and
leukemia
Leukemia ( also spelled leukaemia and pronounced ) is a group of blood cancers that usually begin in the bone marrow and result in high numbers of abnormal blood cells. These blood cells are not fully developed and are called ''blasts'' or ' ...
.
Heart failure
Heart failure can be characterized by a loss of
β adrenergic receptor (βAR) signalling in heart cells. When the βAR is stimulated by
catecholamines
A catecholamine (; abbreviated CA) is a monoamine neurotransmitter, an organic compound that has a catechol (benzene with two hydroxyl side groups next to each other) and a side-chain amine.
Catechol can be either a free molecule or a substi ...
such as
adrenaline
Adrenaline, also known as epinephrine, is a hormone and medication which is involved in regulating visceral functions (e.g., respiration). It appears as a white microcrystalline granule. Adrenaline is normally produced by the adrenal glands and ...
and
noradrenaline
Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain and body as both a hormone and neurotransmitter. The name "noradrenaline" (from Latin '' ad'', ...
, there is normally an increase in the contractility of the heart. However, in heart failure there are sustained and elevated levels of catecholamines which result in chronic
desensitization of the βAR receptor. This leads to a decrease in the strength of heart contractions. Some research suggests that this chronic desensitization is due to the over activation of a kinase,
G protein-coupled receptor kinase 2
G-protein-coupled receptor kinase 2 (GRK2) is an enzyme that in humans is encoded by the ''ADRBK1'' gene. GRK2 was initially called Beta-adrenergic receptor kinase (βARK or βARK1), and is a member of the G protein-coupled receptor kinase subfam ...
(GRK2), which phosphorylates and deactivates certain G protein coupled receptors .
When the G protein coupled receptor is activated, the G
βγ subunit recruits GRK2 which then phosphorylates and desensitizes GPCRs like the βAR. Preventing the interaction of the βγ subunit with GRK2 has therefore been studied as a potential target for increasing heart contractile function. The developed molecule GRK2ct is a protein inhibitor which inhibits the signalling properties of G
βγ subunit but does not interfere with alpha subunit signalling. The over expression of GRK2ct has been shown to significantly rescue cardiac function in
murine models of heart failure by blocking G
βγ subunit signalling. In another study, biopsies were taken from patients with heart failure and
virally induced overexpression of GRK2ct in the heart
myocytes
A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a muscl ...
. Other tests showed an improvement in cardiac cell contractile function by inhibiting G
βγ.
Inflammation
When particular GPCRs are activated by their specific
chemokines
Chemokines (), or chemotactic cytokines, are a family of small cytokines or signaling proteins secreted by cells that induce directional movement of leukocytes, as well as other cell types, including endothelial and epithelial cells. In addition ...
G
βγ directly activates
PI3K
Phosphoinositide 3-kinases (PI3Ks), also called phosphatidylinositol 3-kinases, are a family of enzymes involved in cellular functions such as cell growth, proliferation, differentiation, motility, survival and intracellular trafficking, which i ...
γ which is involved in the recruitment of
neutrophils
Neutrophils (also known as neutrocytes or heterophils) are the most abundant type of granulocytes and make up 40% to 70% of all white blood cells in humans. They form an essential part of the innate immune system, with their functions varying in ...
that contribute to inflammation.
It has been discovered that the inhibition of PI3Kγ significantly reduces inflammation.
PI3Kγ is the intended target molecule in the prevention of inflammation as it is the common signalling effector of many different chemokine and receptor types involved in promoting inflammation.
Although PI3Kγ is the intended target there are other
isoforms
A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some isof ...
of
PI3 which perform different functions from PI3Kγ. Since PI3Kγ is specifically regulated by G
βγ, while other isoforms of PI3 are largely regulated by other molecules, inhibiting Gβγ signalling would provide the desired specificity of a therapeutic agent designed to treat inflammation.
Leukemia
The G
βγ subunit has been shown to activate a
Rho guanine nucleotide exchange factor (RhoGef) gene
PLEKHG2 which is
upregulated
In the biological context of organisms' production of gene products, downregulation is the process by which a cell decreases the quantity of a cellular component, such as RNA or protein, in response to an external stimulus. The complementary pro ...
in a number of leukemia cell lines and mouse models of leukemia.
Lymphocyte
A lymphocyte is a type of white blood cell (leukocyte) in the immune system of most vertebrates. Lymphocytes include natural killer cells (which function in cell-mediated, cytotoxic innate immunity), T cells (for cell-mediated, cytotoxic ad ...
chemotaxis
Chemotaxis (from '' chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemica ...
as a result of
Rac and
CDC42 activation as well as
actin
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...
polymerization
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer, monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are ...
is believed to be regulated by the G
βγ activated RhoGef. Therefore, a drug inhibiting the G
βγ could play a role in the treatment of leukemia.
References
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G proteins
Protein complexes