GTPase

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GTPases are a large family of hydrolase enzymes that bind to the nucleotide guanosine triphosphate (GTP) and hydrolyze it to guanosine diphosphate (GDP). [1] The GTP binding and hydrolysis takes place in the highly conserved P-loop "G domain", a protein domain common to many GTPases. [1]

Contents

Functions

GTPases function as molecular switches or timers in many fundamental cellular processes. [2]

Examples of these roles include:

GTPases are active when bound to GTP and inactive when bound to GDP. [2] [3] In the generalized receptor-transducer-effector signaling model of Martin Rodbell, signaling GTPases act as transducers to regulate the activity of effector proteins. [3] This inactive-active switch is due to conformational changes in the protein distinguishing these two forms, particularly of the "switch" regions that in the active state are able to make protein-protein contacts with partner proteins that alter the function of these effectors. [1]

Mechanism

Hydrolysis of GTP bound to an (active) G domain-GTPase leads to deactivation of the signaling/timer function of the enzyme. [2] [3] The hydrolysis of the third (γ) phosphate of GTP to create guanosine diphosphate (GDP) and Pi, inorganic phosphate, occurs by the SN2 mechanism (see nucleophilic substitution) via a pentacoordinate transition state and is dependent on the presence of a magnesium ion Mg2+. [2] [3]

GTPase activity serves as the shutoff mechanism for the signaling roles of GTPases by returning the active, GTP-bound protein to the inactive, GDP-bound state. [2] [3] Most "GTPases" have functional GTPase activity, allowing them to remain active (that is, bound to GTP) only for a short time before deactivating themselves by converting bound GTP to bound GDP. [2] [3] However, many GTPases also use accessory proteins named GTPase-activating proteins or GAPs to accelerate their GTPase activity. This further limits the active lifetime of signaling GTPases. [4] Some GTPases have little to no intrinsic GTPase activity, and are entirely dependent on GAP proteins for deactivation (such as the ADP-ribosylation factor or ARF family of small GTP-binding proteins that are involved in vesicle-mediated transport within cells). [5]

To become activated, GTPases must bind to GTP. Since mechanisms to convert bound GDP directly into GTP are unknown, the inactive GTPases are induced to release bound GDP by the action of distinct regulatory proteins called guanine nucleotide exchange factors or GEFs. [2] [3] The nucleotide-free GTPase protein quickly rebinds GTP, which is in far excess in healthy cells over GDP, allowing the GTPase to enter the active conformation state and promote its effects on the cell. [2] [3] For many GTPases, activation of GEFs is the primary control mechanism in the stimulation of the GTPase signaling functions, although GAPs also play an important role. For heterotrimeric G proteins and many small GTP-binding proteins, GEF activity is stimulated by cell surface receptors in response to signals outside the cell (for heterotrimeric G proteins, the G protein-coupled receptors are themselves GEFs, while for receptor-activated small GTPases their GEFs are distinct from cell surface receptors).

Some GTPases also bind to accessory proteins called guanine nucleotide dissociation inhibitors or GDIs that stabilize the inactive, GDP-bound state. [6]

The amount of active GTPase can be changed in several ways:

  1. Acceleration of GDP dissociation by GEFs speeds up the accumulation of active GTPase.
  2. Inhibition of GDP dissociation by guanine nucleotide dissociation inhibitors (GDIs) slows down accumulation of active GTPase.
  3. Acceleration of GTP hydrolysis by GAPs reduces the amount of active GTPase.
  4. Artificial GTP analogues like GTP-γ-S, β,γ-methylene-GTP, and β,γ-imino-GTP that cannot be hydrolyzed can lock the GTPase in its active state.
  5. Mutations (such as those that reduce the intrinsic GTP hydrolysis rate) can lock the GTPase in the active state, and such mutations in the small GTPase Ras are particularly common in some forms of cancer. [7]

G domain GTPases

In most GTPases, the specificity for the base guanine versus other nucleotides is imparted by the base-recognition motif, which has the consensus sequence [N/T]KXD. The following classification is based on shared features; some examples have mutations in the base-recognition motif that shift their substrate specificity, most commonly to ATP. [8]

TRAFAC class

The TRAFAC class of G domain proteins is named after the prototypical member, the translation factor G proteins. They play roles in translation, signal transduction, and cell motility. [8]

Translation factor superfamily

Multiple classical translation factor family GTPases play important roles in initiation, elongation and termination of protein biosynthesis. Sharing a similar mode of ribosome binding due to the β-EI domain following the GTPase, the most well-known members of the family are EF-1A/EF-Tu, EF-2/EF-G, [9] and class 2 release factors. Other members include EF-4 (LepA), BipA (TypA), [10] SelB (bacterial selenocysteinyl-tRNA EF-Tu paralog), Tet (tetracycline resistance by ribosomal protection), [11] and HBS1L (eukaryotic ribosome rescue protein similar to release factors).

The superfamily also includes the Bms1 family from yeast. [8]

Ras-like superfamily

Heterotrimeric G proteins

Heterotrimeric G protein complexes are composed of three distinct protein subunits named alpha (α), beta (β) and gamma (γ) subunits. [12] The alpha subunits contain the GTP binding/GTPase domain flanked by long regulatory regions, while the beta and gamma subunits form a stable dimeric complex referred to as the beta-gamma complex. [13] When activated, a heterotrimeric G protein dissociates into activated, GTP-bound alpha subunit and separate beta-gamma subunit, each of which can perform distinct signaling roles. [2] [3] The α and γ subunit are modified by lipid anchors to increase their association with the inner leaflet of the plasma membrane. [14]

Heterotrimeric G proteins act as the transducers of G protein-coupled receptors, coupling receptor activation to downstream signaling effectors and second messengers. [2] [3] [15] In unstimulated cells, heterotrimeric G proteins are assembled as the GDP bound, inactive trimer (Gα-GDP-Gβγ complex). [2] [3] Upon receptor activation, the activated receptor intracellular domain acts as GEF to release GDP from the G protein complex and to promote binding of GTP in its place. [2] [3] The GTP-bound complex undergoes an activating conformation shift that dissociates it from the receptor and also breaks the complex into its component G protein alpha and beta-gamma subunit components. [2] [3] While these activated G protein subunits are now free to activate their effectors, the active receptor is likewise free to activate additional G proteins – this allows catalytic activation and amplification where one receptor may activate many G proteins. [2] [3]

G protein signaling is terminated by hydrolysis of bound GTP to bound GDP. [2] [3] This can occur through the intrinsic GTPase activity of the α subunit, or be accelerated by separate regulatory proteins that act as GTPase-activating proteins (GAPs), such as members of the Regulator of G protein signaling (RGS) family). [4] The speed of the hydrolysis reaction works as an internal clock limiting the length of the signal. Once Gα is returned to being GDP bound, the two parts of the heterotrimer re-associate to the original, inactive state. [2] [3]

The heterotrimeric G proteins can be classified by sequence homology of the α unit and by their functional targets into four families: Gs family, Gi family, Gq family and G12 family. [12] Each of these Gα protein families contains multiple members, such that the mammals have 16 distinct α-subunit genes. [12] The Gβ and Gγ are likewise composed of many members, increasing heterotrimer structural and functional diversity. [12] Among the target molecules of the specific G proteins are the second messenger-generating enzymes adenylyl cyclase and phospholipase C, as well as various ion channels. [16]

Small GTPases

Small GTPases function as monomers and have a molecular weight of about 21 kilodaltons that consists primarily of the GTPase domain. [17] They are also called small or monomeric guanine nucleotide-binding regulatory proteins, small or monomeric GTP-binding proteins, or small or monomeric G-proteins, and because they have significant homology with the first-identified such protein, named Ras, they are also referred to as Ras superfamily GTPases. Small GTPases generally serve as molecular switches and signal transducers for a wide variety of cellular signaling events, often involving membranes, vesicles or cytoskeleton. [18] [17] According to their primary amino acid sequences and biochemical properties, the many Ras superfamily small GTPases are further divided into five subfamilies with distinct functions: Ras, Rho ("Ras-homology"), Rab, Arf and Ran. [17] While many small GTPases are activated by their GEFs in response to intracellular signals emanating from cell surface receptors (particularly growth factor receptors), regulatory GEFs for many other small GTPases are activated in response to intrinsic cell signals, not cell surface (external) signals.

Myosin-kinesin superfamily

This class is defined by loss of two beta-strands and additional N-terminal strands. Both namesakes of this superfamily, myosin and kinesin, have shifted to use ATP. [8]

Large GTPases

See dynamin as a prototype for large monomeric GTPases.

SIMIBI class

Much of the SIMIBI class of GTPases is activated by dimerization. [8] Named after the signal recognition particle (SRP), MinD, and BioD, the class is involved in protein localization, chromosome partitioning, and membrane transport. Several members of this class, including MinD and Get3, has shifted in substrate specificity to become ATPases. [19]

Translocation factors

For a discussion of Translocation factors and the role of GTP, see signal recognition particle (SRP).

Other GTPases

While tubulin and related structural proteins also bind and hydrolyze GTP as part of their function to form intracellular tubules, these proteins utilize a distinct tubulin domain that is unrelated to the G domain used by signaling GTPases. [20]

There are also GTP-hydrolyzing proteins that use a P-loop from a superclass other than the G-domain-containg one. Examples include the NACHT proteins of its own superclass and McrB protein of the AAA+ superclass. [8]

See also

Related Research Articles

<span class="mw-page-title-main">G protein</span> Type of proteins

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.

<span class="mw-page-title-main">Ras GTPase</span> GTP-binding proteins functioning on cell-cycle regulation

Ras, from "Rat sarcoma virus", is a family of related proteins that are expressed in all animal cell lineages and organs. All Ras protein family members belong to a class of protein called small GTPase, and are involved in transmitting signals within cells. Ras is the prototypical member of the Ras superfamily of proteins, which are all related in three-dimensional structure and regulate diverse cell behaviours.

Small GTPases, also known as small G-proteins, are a family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate (GTP). They are a type of G-protein found in the cytosol that are homologous to the alpha subunit of heterotrimeric G-proteins, but unlike the alpha subunit of G proteins, a small GTPase can function independently as a hydrolase enzyme to bind to and hydrolyze a guanosine triphosphate (GTP) to form guanosine diphosphate (GDP). The best-known members are the Ras GTPases and hence they are sometimes called Ras subfamily GTPases.

<span class="mw-page-title-main">Transducin</span>

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.

GTPase-activating proteins or GTPase-accelerating proteins (GAPs) are a family of regulatory proteins whose members can bind to activated G proteins and stimulate their GTPase activity, with the result of terminating the signaling event. GAPs are also known as RGS protein, or RGS proteins, and these proteins are crucial in controlling the activity of G proteins. Regulation of G proteins is important because these proteins are involved in a variety of important cellular processes. The large G proteins, for example, are involved in transduction of signaling from the G protein-coupled receptor for a variety of signaling processes like hormonal signaling, and small G proteins are involved in processes like cellular trafficking and cell cycling. GAP's role in this function is to turn the G protein's activity off. In this sense, GAPs function is opposite to that of guanine nucleotide exchange factors (GEFs), which serve to enhance G protein signaling.

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">Ran (protein)</span> GTPase functioning in nuclear transport

Ran also known as GTP-binding nuclear protein Ran is a protein that in humans is encoded by the RAN gene. Ran is a small 25 kDa protein that is involved in transport into and out of the cell nucleus during interphase and also involved in mitosis. It is a member of the Ras superfamily.

<span class="mw-page-title-main">Guanine nucleotide exchange factor</span> Proteins which remove GDP from GTPases

Guanine nucleotide exchange factors (GEFs) are proteins or protein domains that activate monomeric GTPases by stimulating the release of guanosine diphosphate (GDP) to allow binding of guanosine triphosphate (GTP). A variety of unrelated structural domains have been shown to exhibit guanine nucleotide exchange activity. Some GEFs can activate multiple GTPases while others are specific to a single GTPase.

<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, 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.

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">AKAP13</span> Protein-coding gene in the species Homo sapiens

A-kinase anchor protein 13 is a protein that in humans, is encoded by the AKAP13 gene. This protein is also called AKAP-Lbc because it encodes the lymphocyte blast crisis (Lbc) oncogene, and ARHGEF13/RhoGEF13 because it contains a guanine nucleotide exchange factor (GEF) domain for the RhoA small GTP-binding protein.

<span class="mw-page-title-main">ARHGEF1</span> Protein-coding gene in the species Homo sapiens

Rho guanine nucleotide exchange factor 1 is a protein that in humans is encoded by the ARHGEF1 gene. This protein is also called RhoGEF1 or p115-RhoGEF.

<span class="mw-page-title-main">ARHGEF12</span> Protein-coding gene in the species Homo sapiens

Rho guanine nucleotide exchange factor 12 is a protein that in humans is encoded by the ARHGEF12 gene. This protein is also called RhoGEF12 or Leukemia-associated Rho guanine nucleotide exchange factor (LARG).

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

RhoG is a small monomeric GTP-binding protein, and is an important component of many intracellular signalling pathways. It is a member of the Rac subfamily of the Rho family of small G proteins and is encoded by the gene RHOG.

<span class="mw-page-title-main">GNA13</span> Protein-coding gene in the species Homo sapiens

Guanine nucleotide-binding protein subunit alpha-13 is a protein that in humans is encoded by the GNA13 gene.

<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.

<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).

References

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