GTPase
Encyclopedia
GTPases are a large family of hydrolase
enzyme
s that can bind and hydrolyze
guanosine triphosphate (GTP)
. The GTP binding and hydrolysis takes place in the highly conserved
G domain common to all GTPases.
of GTP into guanosine diphosphate
(GDP) and Pi, inorganic phosphate, occurs by the SN2 mechanism (see nucleophilic substitution
) via a pentavalent intermediate state and is dependent on the magnesium
ion
Mg2+.
is imparted by the base-recognition motif, which has the consensus sequence [N/T]KXD.
processes. Most prominent among the regulatory GTPases are the G proteins.
chain on and off. Toggling the switch is performed by the unidirectional change of the GTPase from the active, GTP
-bound form to the inactive, GDP
-bound form by hydrolysis
of the GTP through intrinsic GTPase-activity, effectively switching the GTPase off. This reaction is initiated by GTPase-activating proteins (GAPs), coming from another signal transduction pathway. It can be reverted (switching the GTPase on again) by Guanine nucleotide exchange factor
s (GEFs), which cause the GDP to dissociate from the GTPase, leading to its association with a new GTP. This closes the cycle to the active state of the GTPase; the irreversible hydrolysis of the GTP to GDP forces the cycle to run only in one direction. Only the active state of the GTPase can transduce a signal to a reaction chain.
with kdiss.GDP being the dissociation constant of GDP, and kcat.GTP the hydrolysis constant of GTP for the specific GTPase. Both constants can be modified by special regulatory proteins.
The amount of active GTPase can be changed in several ways :
s. The GTPase is normally inactive. Upon receptor activation, the intracellular receptor domain activates the GTPase, which in turn activates other molecules of the signal transduction chain, either via the α unit or the βγ complex. Among the target molecules of the active GTPase are adenylate cyclase
and ion channel
s. The heterotrimeric G proteins can be classified by sequence homology of the α unit into four families:
By combination of different α, β and γ subunits, a great variety (>1000) G proteins can be produced.
GDP is not needed for GTP.
for the G protein - GDP complex. On binding with the complex, GDP dissociates from the complex; the receptor works as a GEF - GDP-GTP Exchange Factor; the free complex has a high affinity for GTP. Upon GTP binding, both Gα-GTP and Gβγ separate from both the receptor and from each other. Depending on the lifetime of the active state of the receptor, it can activate more G proteins this way.
Both Gα-GTP and Gβγ can now activate separate and/or the same effector molecules, thus sending the signal further down the signal reaction chain. Once the intrinsic GTPase activity of the α unit has hydrolyzed the GTP to GDP, and then the two parts associate to the original, inactive state. The speed of the hydrolysis reaction works as an internal clock for the length of the signal.
s. Small GTPases have a molecular weight of about 21 kilodaltons and generally serve as molecular switches for a variety of cellular signaling events. According to their primary amino acid sequences and biochemical properties, the Ras superfamily is further divided into five subfamilies: Ras, Rho, Rab
, Arf
and Ran. The Rho subfamily is further divided into RHOA, RAC1, and CDC42.
, elongation
and termination of protein biosynthesis
.
Hydrolase
In biochemistry, a hydrolase is an enzyme that catalyzes the hydrolysis of a chemical bond. For example, an enzyme that catalyzed the following reaction is a hydrolase:-Nomenclature:...
enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s that can bind and hydrolyze
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
guanosine triphosphate (GTP)
Guanosine triphosphate
Guanosine-5'-triphosphate is a purine nucleoside triphosphate. It can act as a substrate for the synthesis of RNA during the transcription process...
. The GTP binding and hydrolysis takes place in the highly conserved
Conserved sequence
In biology, conserved sequences are similar or identical sequences that occur within nucleic acid sequences , protein sequences, protein structures or polymeric carbohydrates across species or within different molecules produced by the same organism...
G domain common to all GTPases.
Functions
GTPases play an important role in:- Signal transductionSignal transductionSignal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
at the intracellular domain of transmembrane receptorG protein-coupled receptorG protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...
s, including recognition of taste, smell and light. - Protein biosynthesisProtein biosynthesisProtein biosynthesis is the process in which cells build or manufacture proteins. The term is sometimes used to refer only to protein translation but more often it refers to a multi-step process, beginning with amino acid synthesis and transcription of nuclear DNA into messenger RNA, which is then...
(aka translation) at the ribosomeRibosomeA ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
. - Control and differentiationCellular differentiationIn developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...
during cell divisionCell divisionCell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
. - TranslocationTranslocationTranslocation may refer to:* Chromosomal translocation, in genetics* Translocation in plants, transport of food or pesticides through phloem or xylem* Protein translocation or protein targeting, a process in protein biosynthesis...
of proteinProteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s through membraneCell membraneThe cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
s. - Transport of vesicleVesicle (biology)A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
s within the cellCell (biology)The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
. (GTPases control assembly of vesicle coats).
Mechanism
The hydrolysis of the γ phosphatePhosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
of GTP into guanosine diphosphate
Guanosine diphosphate
Guanosine diphosphate, abbreviated GDP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside guanosine. GDP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase guanine....
(GDP) and Pi, inorganic phosphate, occurs by the SN2 mechanism (see nucleophilic substitution
Nucleophilic substitution
In organic and inorganic chemistry, nucleophilic substitution is a fundamental class of reactions in which an electron nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms called the leaving group; the positive or partially positive...
) via a pentavalent intermediate state and is dependent on the magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
Mg2+.
Major motifs
In most GTPases, the specificity for the base guanineGuanine
Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...
is imparted by the base-recognition motif, which has the consensus sequence [N/T]KXD.
Regulatory GTPases
Regulatory GTPases, also called the GTPase superfamily, are GTPases used for regulation of other biochemicalBiochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
processes. Most prominent among the regulatory GTPases are the G proteins.
GTP switch
All regulatory GTPases have a common mechanism that enables them to switch a signal transductionSignal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
chain on and off. Toggling the switch is performed by the unidirectional change of the GTPase from the active, GTP
Guanosine triphosphate
Guanosine-5'-triphosphate is a purine nucleoside triphosphate. It can act as a substrate for the synthesis of RNA during the transcription process...
-bound form to the inactive, GDP
Guanosine diphosphate
Guanosine diphosphate, abbreviated GDP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside guanosine. GDP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase guanine....
-bound form by hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
of the GTP through intrinsic GTPase-activity, effectively switching the GTPase off. This reaction is initiated by GTPase-activating proteins (GAPs), coming from another signal transduction pathway. It can be reverted (switching the GTPase on again) by Guanine nucleotide exchange factor
Guanine nucleotide exchange factor
Guanine nucleotide exchange factors activate monomeric GTPases by stimulating the release of guanosine diphosphate to allow binding of guanosine triphosphate . A variety of unrelated structural domains have been shown to exhibit guanine nucleotide exchange activity...
s (GEFs), which cause the GDP to dissociate from the GTPase, leading to its association with a new GTP. This closes the cycle to the active state of the GTPase; the irreversible hydrolysis of the GTP to GDP forces the cycle to run only in one direction. Only the active state of the GTPase can transduce a signal to a reaction chain.
Switch regulation
The efficiency of the signal transduction via a GTPase depends on the ratio of active to inactive GTPase. That equals:with kdiss.GDP being the dissociation constant of GDP, and kcat.GTP the hydrolysis constant of GTP for the specific GTPase. Both constants can be modified by special regulatory proteins.
The amount of active GTPase can be changed in several ways :
- Acceleration of GDP dissociation by GEFs speeds up the building of active GTPase.
- Inhibition of GDP dissociation by Guanine nucleotide dissociation inhibitors (GDIs) slows down the building of active GTPase.
- Acceleration of GTP hydrolysis by GAPs reduces the amount of active GTPase.
- GTP analogues like γ-S-GTP, β,γ-methylene-GTP, and β,γ-imino-GTP that cannot be hydrolyzed fixate the GTPase in its active state.
Heterotrimeric G proteins
These G proteins are made from three subunits, with the G domain located on the largest one (the α unit); together with the two smaller subunits (β and γ units), they form a tightly associated protein complex. α and γ unit are associated with the membrane by lipid anchors. Heterotrimeric G proteins act as the specific reaction partners of G protein-coupled receptorG protein-coupled receptor
G protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...
s. The GTPase is normally inactive. Upon receptor activation, the intracellular receptor domain activates the GTPase, which in turn activates other molecules of the signal transduction chain, either via the α unit or the βγ complex. Among the target molecules of the active GTPase are adenylate cyclase
Adenylate cyclase
Adenylate cyclase is part of the G protein signalling cascade, which transmits chemical signals from outside the cell across the membrane to the inside of the cell ....
and ion channel
Ion channel
Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells...
s. The heterotrimeric G proteins can be classified by sequence homology of the α unit into four families:
- Gs family. These G proteins are used in the signal transduction of taste and smell. They always use the activation of adenylate cyclaseAdenylate cyclaseAdenylate cyclase is part of the G protein signalling cascade, which transmits chemical signals from outside the cell across the membrane to the inside of the cell ....
as the next step in the signal chain. The s stands for stimulation. Their function is permanently activated by the cholera toxinCholera toxinCholera toxin is a protein complex secreted by the bacterium Vibrio cholerae. CTX is responsible for the massive, watery diarrhea characteristic of cholera infection.- Structure :...
, which is the cause of the fatal effects of infection with Vibrio choleraeVibrio choleraeVibrio cholerae is a Gram-negative, comma-shaped bacterium. Some strains of V. cholerae cause the disease cholera. V. cholerae is facultatively anaerobic and has a flagella at one cell pole. V...
. - Gi family. The i stands for inhibition of the adenylate cyclaseAdenylate cyclaseAdenylate cyclase is part of the G protein signalling cascade, which transmits chemical signals from outside the cell across the membrane to the inside of the cell ....
; another effector molecule for this protein family is phospholipasePhospholipaseA phospholipase is an enzyme that hydrolyzes phospholipids into fatty acids and other lipophilic substances. There are four major classes, termed A, B, C and D, distinguished by the type of reaction which they catalyze:*Phospholipase A...
C. Also, Gt and Gg proteins are summarized under this label due to sequence homologies. Gt proteins, aka transducinTransducinTransducin is a heterotrimeric G protein that is naturally expressed in vertebrate retina rods and cones .- Mechanism of action :...
, is used in the light recognition pathway in retina cells. Gg protein occurs in the taste recognition for bitter. Most Gi protein family members can be inhibited by the pertussis toxinPertussis toxinPertussis toxin is a protein-based AB5-type exotoxin produced by the bacterium Bordetella pertussis, which causes whooping cough. PT is involved in the colonization of the respiratory tract and the establishment of infection...
of Bordetella pertussisBordetella pertussisBordetella pertussis is a Gram-negative, aerobic coccobacillus of the genus Bordetella, and the causative agent of pertussis or whooping cough. Unlike B. bronchiseptica, B. pertussis is non-motile. Its virulence factors include pertussis toxin, filamentous hæmagglutinin, pertactin, fimbria, and...
. - Gq family. These proteins usually have phospholipasePhospholipaseA phospholipase is an enzyme that hydrolyzes phospholipids into fatty acids and other lipophilic substances. There are four major classes, termed A, B, C and D, distinguished by the type of reaction which they catalyze:*Phospholipase A...
C as effector protein. - G12 family. These G proteins can be activated by thromboxan receptors and thrombin receptorThrombin receptorThere are three known thrombin receptors termed PAR1, PAR3 and PAR4 .These receptors are members of the 7 transmembrane g protein-coupled family of receptors, however, their method of activation is unique. Thrombin, a serine protease, binds to and cleaves the extracellular N-terminal domain of the...
s. Their effector proteins are unknown.
By combination of different α, β and γ subunits, a great variety (>1000) G proteins can be produced.
GDP is not needed for GTP.
Activation cycle of heterotrimeric G proteins
In the basic state, the Gα-GDP-Gβγ complex and the receptor that can activate it are separately associated with the membrane. On receptor activation, the receptor becomes highly affineElectron affinity
The Electron affinity of an atom or molecule is defined as the amount of energy released when an electron is added to a neutral atom or molecule to form a negative ion....
for the G protein - GDP complex. On binding with the complex, GDP dissociates from the complex; the receptor works as a GEF - GDP-GTP Exchange Factor; the free complex has a high affinity for GTP. Upon GTP binding, both Gα-GTP and Gβγ separate from both the receptor and from each other. Depending on the lifetime of the active state of the receptor, it can activate more G proteins this way.
Both Gα-GTP and Gβγ can now activate separate and/or the same effector molecules, thus sending the signal further down the signal reaction chain. Once the intrinsic GTPase activity of the α unit has hydrolyzed the GTP to GDP, and then the two parts associate to the original, inactive state. The speed of the hydrolysis reaction works as an internal clock for the length of the signal.
The Ras GTPase superfamily
These are small monomeric proteins homologous to Ras. They are also called small GTPaseSmall GTPase
Small GTPases are a family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate . They are a form of G-proteins found in the cytosol which are homologous to the alpha subunit of heterotrimeric G-proteins, but unlike the alpha subunit of G proteins, a small GTPase can function...
s. Small GTPases have a molecular weight of about 21 kilodaltons and generally serve as molecular switches for a variety of cellular signaling events. According to their primary amino acid sequences and biochemical properties, the Ras superfamily is further divided into five subfamilies: Ras, Rho, Rab
Rab (G-protein)
The Rab family of proteins is a member of the Ras superfamily of monomeric G proteins. Approximately 70 types of Rabs have now been identified in humans. Rab GTPases regulate many steps of membrane traffic, including vesicle formation, vesicle movement along actin and tubulin networks, and membrane...
, Arf
ADP ribosylation factor
ADP Ribosylation Factors are members of the ARF family of GTP-binding proteins of the Ras superfamily. ARF family proteins are ubiquitous in eukaryotic cells, and six highly conserved members of the family have been identified in mammalian cells. Although ARFs are soluble, they generally...
and Ran. The Rho subfamily is further divided into RHOA, RAC1, and CDC42.
Translation factor family
These GTPases play an important role in initiationInitiation
Initiation is a rite of passage ceremony marking entrance or acceptance into a group or society. It could also be a formal admission to adulthood in a community or one of its formal components...
, elongation
Elongation
In astronomy, a planet's elongation is the angle between the Sun and the planet, as viewed from Earth. Elongations occur when an inner planet’s position, in its orbital path, is at tangent to the view from Earth. Because these inner planets are inside the Earth’s orbits their positions as viewed...
and termination of protein biosynthesis
Protein biosynthesis
Protein biosynthesis is the process in which cells build or manufacture proteins. The term is sometimes used to refer only to protein translation but more often it refers to a multi-step process, beginning with amino acid synthesis and transcription of nuclear DNA into messenger RNA, which is then...
.
External links
- Neurofibromin 1 from GeneCardsGeneCardsGeneCards is a database of human genes that provides genomic, proteomic, transcriptomic, genetic and functional information on all known and predicted human genes. It is being developed and maintained by the Crown Human Genome Center at the Weizmann Institute of Science.- References :*Stelzer G,...
- GTPases: From The Bench (blog)