Ras
Encyclopedia
Ras is the name given to a family of related proteins
found inside cells, including human cells. All Ras protein family members belong to a class of protein called small GTPase
, and are involved in transmitting signals within cells (cellular
signal transduction
). Ras is the prototypical member of the Ras superfamily
of proteins, which are all related in 3D structure and regulate diverse cell behaviours.
The name 'Ras' is an abbreviation of 'Rat sarcoma', reflecting the way the first members of the protein family were discovered. The name ras is also used to refer to the family of gene
s encoding those proteins.
When Ras is 'switched on' by incoming signals, it subsequently switches on other proteins, which ultimately turn on genes involved in cell growth
, differentiation and survival
. As a result, mutations in ras genes can lead to the production of permanently activated Ras proteins. This can cause unintended and overactive signalling inside the cell, even in the absence of incoming signals.
Because these signals result in cell growth and division, overactive Ras signaling can ultimately lead to cancer
. Ras is the most common oncogene in human cancer - mutations that permanently activate Ras are found in 20-25% of all human tumors and up to 90% in certain types of cancer (e.g pancreatic cancer
).
and KRAS
, were first identified from studies of two cancer-causing viruses, the Harvey sarcoma virus and Kirsten sarcoma virus, by Edward M. Scolnick and colleagues at the National Institutes of Health (NIH). These viruses were discovered originally in rats during the 1960s by Jennifer Harvey and Werner Kirsten, respectively, hence the name Rat sarcoma
. In 1982, activated and transforming human ras genes were discovered in human cancer cells by Geoffrey M. Cooper at Harvard, Mariano Barbacid
and Stuart A. Aaronson
at the NIH and by Robert Weinberg
of MIT. A third ras gene was subsequently discovered by researchers at the Institute of Cancer Research
, funded by the Imperial Cancer Research Fund (now Cancer Research UK
), and named NRAS
, for its initial identification in human neuroblastoma cells.
The three human ras genes encode extremely similar proteins made up of chains of 188 to 189 amino acids, designated H-Ras
, N-Ras
and K-Ras4A and K-Ras4B (the two K-Ras
proteins arise from alternative gene splicing).
and 5 alpha helices:
The G domain contains five G motifs that bind GDP/GTP directly
and two switches which are the main parts of the protein that move upon activation by GTP.
Ras also binds a magnesium ion which helps to coordinate nucleotide binding.
Ras proteins function as binary molecular switches that control intracellular signaling networks. Ras-regulated signal pathways
control such processes as actin cytoskeletal integrity, proliferation, differentiation, cell adhesion, apoptosis
, and cell migration. Ras and ras-related proteins are often deregulated in cancers, leading to increased invasion and metastasis
, and decreased apoptosis.
Ras activates several pathways, of which the mitogen-activated protein (MAP) kinase cascade
has been well-studied. This cascade transmits signals downstream and results in the transcription
of genes involved in cell growth and division. There is a separate AKT
pathway that inhibits apoptosis.
, or a guanosine
-nucleotide
-binding protein. Specifically, it is a single-subunit small GTPase
, which is related in structure to the Gα subunit of heterotrimeric G proteins (large GTPases). G proteins function as binary signaling switches with "on" and "off" states. In the "off" state it is bound to the nucleotide guanosine diphosphate
(GDP), while in the "on" state, Ras is bound to guanosine triphosphate
(GTP), which has an extra phosphate
group as compared to GDP. This extra phosphate holds the two switch regions in a "loaded-spring" configuration (specifically the Thr-35 and Gly-60). When released, the switch regions relax which causes a conformational change
into the inactivate state. Hence, activation and deactivation of Ras and other small G proteins are controlled by cycling between the active GTP-bound and inactive GDP-bound forms.
The process of exchanging the bound nucleotide is facilitated by guanine nucleotide exchange factor
s (GEFs) and GTPase activating proteins (GAPs). As per its classification, Ras has an intrinsic GTPase
activity, which means that the protein on its own will hydrolyze a bound GTP molecule into GDP. However this process is too slow for efficient function, and hence the GAP for Ras, RasGAP, may bind to and stabilize the catalytic machinery of Ras, supplying additional catalytic residues ("arginine finger") such that a water molecule is optimally positioned for nucleophilic attack on the gamma-phosphate of GTP. An inorganic phosphate is released and the Ras molecule is now bound to a GDP. Since the GDP-bound form is "off" or "inactive" for signaling, GTPase Activating Protein inactivates Ras by activating its GTPase activity. Thus, GAPs accelerate Ras inactivation.
GEFs catalyze a "push and pull" reaction which releases GDP from Ras. They insert close to the P-loop and magnesium cation binding site and inhibit the interaction of these with the gamma phosphate anion. Acidic (negative) residues in switch II "pull" a lysine in the P-loop away from the GDP which "pushes" switch I away from the guanine. The contacts holding GDP in place are broken and it is released into the cytoplasm. Because intracellular GTP is abundant relative to GDP (approximately 10 fold more) GTP predominantly re-enters the nucleotide binding pocket of Ras and reloads the spring. Thus GEFs facilitate Ras activation. Well known GEFs include Son of Sevenless
(Sos) and cdc25
which include the RasGEF domain
.
The balance between GEF and GAP activity determines the guanine nucleotide status of Ras, thereby regulating Ras activity.
In the GTP-bound conformation, Ras has high affinity for numerous effectors
which allow it to carry out its functions. These include PI3K. Other small GTPases may bind adaptors such as arfaptin
or second messenger systems such as adenylyl cyclase. The Ras binding domain is found in many effectors and invariably binds to one of the switch regions, because these change conformation between the active and inactive forms. However, they may also bind to the rest of the protein surface.
owing to its prenylation
and palmitoylation
(HRAS
and NRAS
) or the combination of prenylation
and a polybasic sequence adjacent to the prenylation site (KRAS
). The C-terminal CaaX box of Ras first gets farnesylated at its Cys residue in the cytosol, allowing Ras to loosely insert into the membrane of the endoplasmatic reticulum and other cellular membranes. The Tripeptide (aaX) is then cleaved from the C-terminus by a specific prenyl-protein specific endoprotease and the new C-terminus is methylated by a methyltransferase
. K-Ras procession is completed at this stage. Dynamic electrostatic interactions between its positively charged basic sequence with negative charges at the inner leaflet of the plasma membrane account for its predominant localization at the cell surface at steady-state. NRAS
and HRAS
are further processed on the surface of the Golgi apparatus
by palmitoylation
of one or two Cys residues, respectively, adjacent to the CaaX box. The proteins thereby become stably membrane anchored and are transported to the plasma membrane on vesicles
of the secretory pathway
. Depalmitoylation eventually releases the proteins from the membrane, allowing them to enter another cycle of palmitoylation and depalmitoylation. This cycle is believed to prevent the leakage of NRAS
and HRAS
to other membranes over time and to maintain their steady-sate localization along the Golgi apparatus
, secretory pathway
, plasma membrane and inter-linked endocytosis
pathway.
, KRAS
and NRAS
, mainly for being implicated in many types of cancer.
However, there are many other members of this subfamily as well:
DIRAS1; DIRAS2; DIRAS3; ERAS; GEM
; MRAS
; NKIRAS1; NKIRAS2
; NRAS
; RALA
; RALB
; RAP1A
; RAP1B
; RAP2A
; RAP2B
; RAP2C; RASD1
; RASD2
; RASL10A; RASL10B; RASL11A; RASL11B
; RASL12; REM1; REM2; RERG; RERGL; RRAD
; RRAS
; RRAS2
Ras inhibitor trans-farnesylthiosalicylic acid (FTS, Salirasib) exhibits profound anti-oncogenic effects in many cancer cell lines.
Mutations in a number of different genes as well as RAS itself can have this effect. Oncogene
s such as p210BCR-ABL or the growth receptor erbB are upstream of Ras, so if they are constitutively activated their signals will transduce through Ras.
The tumour suppressor gene NF1
encodes a Ras-GAP – its mutation in neurofibromatosis
will mean that Ras is less likely to be inactivated. Ras can also be amplified, although this only occurs occasionally in tumours.
Finally, Ras oncogenes can be activated by point mutations so that the GTPase reaction can no longer be stimulated by GAP – this increases the half life of active Ras-GTP mutants.
The most common mutations are found at residue G12 in the P-loop and the catalytic residue Q61.
See also "dominant negative" mutants such as S17N and D119N.
, a formulation of reovirus, is currently in clinical trials for the treatment of various cancers.
Protein family
A protein family is a group of evolutionarily-related proteins, and is often nearly synonymous with gene family. The term protein family should not be confused with family as it is used in taxonomy....
found inside cells, including human cells. All Ras protein family members belong to a class of protein called small GTPase
Small 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...
, and are involved in transmitting signals within cells (cellular
Cell (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....
signal transduction
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
). Ras is the prototypical member of the Ras superfamily
Ras superfamily
The Ras superfamily is a protein superfamily of small GTPases, which are all related, to a degree, to the Ras protein subfamily .There are more than a hundred proteins in the Ras superfamily...
of proteins, which are all related in 3D structure and regulate diverse cell behaviours.
The name 'Ras' is an abbreviation of 'Rat sarcoma', reflecting the way the first members of the protein family were discovered. The name ras is also used to refer to the family of gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s encoding those proteins.
When Ras is 'switched on' by incoming signals, it subsequently switches on other proteins, which ultimately turn on genes involved in cell growth
Cell growth
The term cell growth is used in the contexts of cell development and cell division . When used in the context of cell division, it refers to growth of cell populations, where one cell grows and divides to produce two "daughter cells"...
, differentiation and survival
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
. As a result, mutations in ras genes can lead to the production of permanently activated Ras proteins. This can cause unintended and overactive signalling inside the cell, even in the absence of incoming signals.
Because these signals result in cell growth and division, overactive Ras signaling can ultimately lead to cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
. Ras is the most common oncogene in human cancer - mutations that permanently activate Ras are found in 20-25% of all human tumors and up to 90% in certain types of cancer (e.g pancreatic cancer
Pancreatic cancer
Pancreatic cancer refers to a malignant neoplasm of the pancreas. The most common type of pancreatic cancer, accounting for 95% of these tumors is adenocarcinoma, which arises within the exocrine component of the pancreas. A minority arises from the islet cells and is classified as a...
).
History
The first two ras genes, HRASHRAS
GTPase HRas also known as transforming protein p21 is an enzyme that in humans is encoded by the HRAS gene. The HRAS gene is located on the short arm of chromosome 11 at position 15.5, from base pair 522,241 to base pair 525,549.- Function :...
and KRAS
KRAS
GTPase KRas also known as V-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog and KRAS, is a protein that in humans is encoded by the KRAS gene. Like other members of the Ras family, the KRAS protein is a GTPase and is an early player in many signal transduction pathways...
, were first identified from studies of two cancer-causing viruses, the Harvey sarcoma virus and Kirsten sarcoma virus, by Edward M. Scolnick and colleagues at the National Institutes of Health (NIH). These viruses were discovered originally in rats during the 1960s by Jennifer Harvey and Werner Kirsten, respectively, hence the name Rat sarcoma
Sarcoma
A sarcoma is a cancer that arises from transformed cells in one of a number of tissues that develop from embryonic mesoderm. Thus, sarcomas include tumors of bone, cartilage, fat, muscle, vascular, and hematopoietic tissues...
. In 1982, activated and transforming human ras genes were discovered in human cancer cells by Geoffrey M. Cooper at Harvard, Mariano Barbacid
Mariano Barbacid
Mariano Barbacid is a Spanish molecular biologist who discovered the first oncogene....
and Stuart A. Aaronson
Stuart A. Aaronson
Stuart A. Aaronson, M.D. , is an American author and internationally recognized cancer biologist. He has authored more than 500 publications and holds over 50 patents, and is currently the Jane B. and Jack R...
at the NIH and by Robert Weinberg
Robert Weinberg
Robert Allan Weinberg is a Daniel K. Ludwig Professor for Cancer Research at MIT and American Cancer Society Research Professor; his research is in the area of oncogenes and the genetic basis of human cancer. Weinberg is also affiliated with the Broad Institute and is a founding member of the...
of MIT. A third ras gene was subsequently discovered by researchers at the Institute of Cancer Research
Institute of Cancer Research
The Institute of Cancer Research is a cancer research institute located in London, United Kingdom and a constituent college of the federal University of London. The ICR was founded in 1909 as a research department of the Royal Marsden Hospital and joined the University of London in 2003...
, funded by the Imperial Cancer Research Fund (now Cancer Research UK
Cancer Research UK
Cancer Research UK is a cancer research and awareness charity in the United Kingdom, formed on 4 February 2002 by the merger of The Cancer Research Campaign and the Imperial Cancer Research Fund. Its aim is to reduce the number of deaths from cancer. As the world's largest independent cancer...
), and named NRAS
Neuroblastoma RAS viral oncogene homolog
NRAS is an enzyme that in humans is encoded by the NRAS gene. It was discovered by researchers at the Institute of Cancer Research, funded by the Imperial Cancer Research Fund , and named NRAS, for its initial identification in human neuroblastoma cells.-External links:*...
, for its initial identification in human neuroblastoma cells.
The three human ras genes encode extremely similar proteins made up of chains of 188 to 189 amino acids, designated H-Ras
HRAS
GTPase HRas also known as transforming protein p21 is an enzyme that in humans is encoded by the HRAS gene. The HRAS gene is located on the short arm of chromosome 11 at position 15.5, from base pair 522,241 to base pair 525,549.- Function :...
, N-Ras
Neuroblastoma RAS viral oncogene homolog
NRAS is an enzyme that in humans is encoded by the NRAS gene. It was discovered by researchers at the Institute of Cancer Research, funded by the Imperial Cancer Research Fund , and named NRAS, for its initial identification in human neuroblastoma cells.-External links:*...
and K-Ras4A and K-Ras4B (the two K-Ras
KRAS
GTPase KRas also known as V-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog and KRAS, is a protein that in humans is encoded by the KRAS gene. Like other members of the Ras family, the KRAS protein is a GTPase and is an early player in many signal transduction pathways...
proteins arise from alternative gene splicing).
Structure
Ras contains a six-stranded beta sheetBeta sheet
The β sheet is the second form of regular secondary structure in proteins, only somewhat less common than the alpha helix. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet...
and 5 alpha helices:
- G domain (166 amino acids) which binds guanosine nucleotides, about 20kDa.
- C terminal membrane targeting region (CAAX-COOH, also known as CAAX box) which is lipid-modified by farnesyl transferase, RCE1RCE1CAAX prenyl protease 2 is an enzyme that in humans is encoded by the RCE1 gene.-Further reading:...
and ICMTICMTProtein-S-isoprenylcysteine O-methyltransferase is an enzyme that in humans is encoded by the ICMT gene.-Further reading:...
The G domain contains five G motifs that bind GDP/GTP directly
- G1 - P-loop binds the beta phosphate of GDP and GTP
- G2 - threonine-35 also switch 1, binds the terminal phosphate of GTP, but makes no contacts with GDP
- G3 - DXXG motif, aspartate-57 is specific for guanine rather than adenine
- G4 - LVGNKxDL motif
- G5 - SAK consensus sequence, the alanine-146 is specific for guanine rather than adenine
and two switches which are the main parts of the protein that move upon activation by GTP.
- switch I includes threonine-35
- switch II glycine-60 in DXXG motif
Ras also binds a magnesium ion which helps to coordinate nucleotide binding.
Function
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
control such processes as actin cytoskeletal integrity, proliferation, differentiation, cell adhesion, apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
, and cell migration. Ras and ras-related proteins are often deregulated in cancers, leading to increased invasion and metastasis
Metastasis
Metastasis, or metastatic disease , is the spread of a disease from one organ or part to another non-adjacent organ or part. It was previously thought that only malignant tumor cells and infections have the capacity to metastasize; however, this is being reconsidered due to new research...
, and decreased apoptosis.
Ras activates several pathways, of which the mitogen-activated protein (MAP) kinase cascade
MAPK/ERK pathway
The MAPK/ERK pathway is a chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell to the DNA in the nucleus of the cell. The signal starts when a growth factor binds to the receptor on the cell surface and ends when the DNA in the nucleus expresses a...
has been well-studied. This cascade transmits signals downstream and results in the transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
of genes involved in cell growth and division. There is a separate AKT
AKT
Akt, also known as Protein Kinase B , is a serine/threonine protein kinase that plays a key role in multiple cellular processes such as glucose metabolism, cell proliferation, apoptosis, transcription and cell migration.-Family members:...
pathway that inhibits apoptosis.
Activation and deactivation
Ras is a G proteinG protein
G proteins are a family of proteins involved in transmitting chemical signals outside the cell, and causing changes inside the cell. They communicate signals from many hormones, neurotransmitters, and other signaling factors. G protein-coupled receptors are transmembrane receptors...
, or a guanosine
Guanosine
Guanosine is a purine nucleoside comprising guanine attached to a ribose ring via a β-N9-glycosidic bond. Guanosine can be phosphorylated to become guanosine monophosphate , cyclic guanosine monophosphate , guanosine diphosphate , and guanosine triphosphate...
-nucleotide
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
-binding protein. Specifically, it is a single-subunit small GTPase
Small 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...
, which is related in structure to the Gα subunit of heterotrimeric G proteins (large GTPases). G proteins function as binary signaling switches with "on" and "off" states. In the "off" state it is bound to the nucleotide 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), while in the "on" state, Ras is bound to guanosine triphosphate
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...
(GTP), which has an extra phosphate
Phosphate
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...
group as compared to GDP. This extra phosphate holds the two switch regions in a "loaded-spring" configuration (specifically the Thr-35 and Gly-60). When released, the switch regions relax which causes a conformational change
Conformational change
A macromolecule is usually flexible and dynamic. It can change its shape in response to changes in its environment or other factors; each possible shape is called a conformation, and a transition between them is called a conformational change...
into the inactivate state. Hence, activation and deactivation of Ras and other small G proteins are controlled by cycling between the active GTP-bound and inactive GDP-bound forms.
The process of exchanging the bound nucleotide is facilitated 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) and GTPase activating proteins (GAPs). As per its classification, Ras has an intrinsic GTPase
GTPase
GTPases are a large family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate . The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases.-Functions:...
activity, which means that the protein on its own will hydrolyze a bound GTP molecule into GDP. However this process is too slow for efficient function, and hence the GAP for Ras, RasGAP, may bind to and stabilize the catalytic machinery of Ras, supplying additional catalytic residues ("arginine finger") such that a water molecule is optimally positioned for nucleophilic attack on the gamma-phosphate of GTP. An inorganic phosphate is released and the Ras molecule is now bound to a GDP. Since the GDP-bound form is "off" or "inactive" for signaling, GTPase Activating Protein inactivates Ras by activating its GTPase activity. Thus, GAPs accelerate Ras inactivation.
GEFs catalyze a "push and pull" reaction which releases GDP from Ras. They insert close to the P-loop and magnesium cation binding site and inhibit the interaction of these with the gamma phosphate anion. Acidic (negative) residues in switch II "pull" a lysine in the P-loop away from the GDP which "pushes" switch I away from the guanine. The contacts holding GDP in place are broken and it is released into the cytoplasm. Because intracellular GTP is abundant relative to GDP (approximately 10 fold more) GTP predominantly re-enters the nucleotide binding pocket of Ras and reloads the spring. Thus GEFs facilitate Ras activation. Well known GEFs include Son of Sevenless
Son of Sevenless
In cell signalling, Son of Sevenless refers to a set of genes encoding guanine nucleotide exchange factors that act on the Ras subfamily of small GTPases.-History and name:...
(Sos) and cdc25
Cdc25
Cdc25 is a dual-specificity phosphatase first isolated from the yeast Schizosaccharomyces pombe as a cell cycle defective mutant. As with other cell cycle proteins such as Cdc2 and Cdc4, the "cdc" in its name refers to "cell division cycle".Dual-specificity phosphatases are considered a sub-class...
which include the RasGEF domain
RasGEF domain
RasGEF domain is domain found in a family of guanine nucleotide exchange factors for Ras-like small GTPases.Ras proteins are membrane-associated molecular switches that bind GTP and GDP and slowly hydrolyze GTP to GDP...
.
The balance between GEF and GAP activity determines the guanine nucleotide status of Ras, thereby regulating Ras activity.
In the GTP-bound conformation, Ras has high affinity for numerous effectors
Effector (biology)
An effector is a molecule that binds to a protein and thereby alters the activity of that protein...
which allow it to carry out its functions. These include PI3K. Other small GTPases may bind adaptors such as arfaptin
Arfaptin
In molecular biology, the arfaptin domain is a protein domain which interacts with ARF1, a small GTPase involved in vesicle budding at the Golgi complex and immature secretory granules. The structure of arfaptin shows that upon binding to a small GTPase, arfaptin forms an elongated, crescent-shaped...
or second messenger systems such as adenylyl cyclase. The Ras binding domain is found in many effectors and invariably binds to one of the switch regions, because these change conformation between the active and inactive forms. However, they may also bind to the rest of the protein surface.
Membrane attachment
Ras is attached to the cell membraneCell membrane
The 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...
owing to its prenylation
Prenylation
Prenylation, or isoprenylation, or lipidation is the addition of hydrophobic molecules to a protein. It is usually assumed that prenyl groups facilitate attachment to cell membranes, similar to lipid anchor like the GPI anchor, though direct evidence is missing...
and palmitoylation
Palmitoylation
S-Palmitoylation is the covalent attachment of fatty acids, such as palmitic acid, to cysteine residues of membrane proteins. The precise function of palmitoylation depends on the particular protein being considered. Palmitoylation enhances the hydrophobicity of proteins and contributes to their...
(HRAS
HRAS
GTPase HRas also known as transforming protein p21 is an enzyme that in humans is encoded by the HRAS gene. The HRAS gene is located on the short arm of chromosome 11 at position 15.5, from base pair 522,241 to base pair 525,549.- Function :...
and NRAS
Neuroblastoma RAS viral oncogene homolog
NRAS is an enzyme that in humans is encoded by the NRAS gene. It was discovered by researchers at the Institute of Cancer Research, funded by the Imperial Cancer Research Fund , and named NRAS, for its initial identification in human neuroblastoma cells.-External links:*...
) or the combination of prenylation
Prenylation
Prenylation, or isoprenylation, or lipidation is the addition of hydrophobic molecules to a protein. It is usually assumed that prenyl groups facilitate attachment to cell membranes, similar to lipid anchor like the GPI anchor, though direct evidence is missing...
and a polybasic sequence adjacent to the prenylation site (KRAS
KRAS
GTPase KRas also known as V-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog and KRAS, is a protein that in humans is encoded by the KRAS gene. Like other members of the Ras family, the KRAS protein is a GTPase and is an early player in many signal transduction pathways...
). The C-terminal CaaX box of Ras first gets farnesylated at its Cys residue in the cytosol, allowing Ras to loosely insert into the membrane of the endoplasmatic reticulum and other cellular membranes. The Tripeptide (aaX) is then cleaved from the C-terminus by a specific prenyl-protein specific endoprotease and the new C-terminus is methylated by a methyltransferase
Methyltransferase
A methyltransferase is a type of transferase enzyme that transfers a methyl group from a donor to an acceptor.Methylation often occurs on nucleic bases in DNA or amino acids in protein structures...
. K-Ras procession is completed at this stage. Dynamic electrostatic interactions between its positively charged basic sequence with negative charges at the inner leaflet of the plasma membrane account for its predominant localization at the cell surface at steady-state. NRAS
Neuroblastoma RAS viral oncogene homolog
NRAS is an enzyme that in humans is encoded by the NRAS gene. It was discovered by researchers at the Institute of Cancer Research, funded by the Imperial Cancer Research Fund , and named NRAS, for its initial identification in human neuroblastoma cells.-External links:*...
and HRAS
HRAS
GTPase HRas also known as transforming protein p21 is an enzyme that in humans is encoded by the HRAS gene. The HRAS gene is located on the short arm of chromosome 11 at position 15.5, from base pair 522,241 to base pair 525,549.- Function :...
are further processed on the surface of the Golgi apparatus
Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
by palmitoylation
Palmitoylation
S-Palmitoylation is the covalent attachment of fatty acids, such as palmitic acid, to cysteine residues of membrane proteins. The precise function of palmitoylation depends on the particular protein being considered. Palmitoylation enhances the hydrophobicity of proteins and contributes to their...
of one or two Cys residues, respectively, adjacent to the CaaX box. The proteins thereby become stably membrane anchored and are transported to the plasma membrane on vesicles
Vesicle (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...
of the secretory pathway
Secretory pathway
The secretory pathway is a series of steps a cell uses to move proteins out of the cell; a process known as secretion. The path of a protein destined for secretion has its origins in the rough endoplasmic reticulum, a membrane-bound compartment in the cell...
. Depalmitoylation eventually releases the proteins from the membrane, allowing them to enter another cycle of palmitoylation and depalmitoylation. This cycle is believed to prevent the leakage of NRAS
Neuroblastoma RAS viral oncogene homolog
NRAS is an enzyme that in humans is encoded by the NRAS gene. It was discovered by researchers at the Institute of Cancer Research, funded by the Imperial Cancer Research Fund , and named NRAS, for its initial identification in human neuroblastoma cells.-External links:*...
and HRAS
HRAS
GTPase HRas also known as transforming protein p21 is an enzyme that in humans is encoded by the HRAS gene. The HRAS gene is located on the short arm of chromosome 11 at position 15.5, from base pair 522,241 to base pair 525,549.- Function :...
to other membranes over time and to maintain their steady-sate localization along the Golgi apparatus
Golgi apparatus
The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....
, secretory pathway
Secretory pathway
The secretory pathway is a series of steps a cell uses to move proteins out of the cell; a process known as secretion. The path of a protein destined for secretion has its origins in the rough endoplasmic reticulum, a membrane-bound compartment in the cell...
, plasma membrane and inter-linked endocytosis
Endocytosis
Endocytosis is a process by which cells absorb molecules by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane...
pathway.
Members
The clinically most notable members of the Ras subfamily are HRASHRAS
GTPase HRas also known as transforming protein p21 is an enzyme that in humans is encoded by the HRAS gene. The HRAS gene is located on the short arm of chromosome 11 at position 15.5, from base pair 522,241 to base pair 525,549.- Function :...
, KRAS
KRAS
GTPase KRas also known as V-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog and KRAS, is a protein that in humans is encoded by the KRAS gene. Like other members of the Ras family, the KRAS protein is a GTPase and is an early player in many signal transduction pathways...
and NRAS
Neuroblastoma RAS viral oncogene homolog
NRAS is an enzyme that in humans is encoded by the NRAS gene. It was discovered by researchers at the Institute of Cancer Research, funded by the Imperial Cancer Research Fund , and named NRAS, for its initial identification in human neuroblastoma cells.-External links:*...
, mainly for being implicated in many types of cancer.
However, there are many other members of this subfamily as well:
DIRAS1; DIRAS2; DIRAS3; ERAS; GEM
GEM (gene)
GTP-binding protein GEM is a protein that in humans is encoded by the GEM gene.-Further reading:...
; MRAS
MRAS
Ras-related protein M-Ras is a protein that in humans is encoded by the MRAS gene.-Interactions:MRAS has been shown to interact with RASSF5 and RALGDS.-Further reading:...
; NKIRAS1; NKIRAS2
NKIRAS2
NFKB inhibitor interacting Ras-like 2 is a protein that in humans is encoded by the NKIRAS2 gene....
; NRAS
Neuroblastoma RAS viral oncogene homolog
NRAS is an enzyme that in humans is encoded by the NRAS gene. It was discovered by researchers at the Institute of Cancer Research, funded by the Imperial Cancer Research Fund , and named NRAS, for its initial identification in human neuroblastoma cells.-External links:*...
; RALA
RALA
Ras-related protein Ral-A is a protein that in humans is encoded by the RALA gene.-Interactions:RALA has been shown to interact with Filamin, Phospholipase D1 and RALBP1.-Further reading:...
; RALB
RALB
Ras-related protein Ral-B is a protein that in humans is encoded by the RALB gene.-Interactions:RALB has been shown to interact with EXOC8 and RALBP1.-Further reading:...
; RAP1A
RAP1A
Ras-related protein Rap-1A is a protein that in humans is encoded by the RAP1A gene.- Function :The product of this gene belongs to the family of Ras-related proteins. These proteins share approximately 50% amino acid identity with the classical RAS proteins and have numerous structural features in...
; RAP1B
RAP1B
Ras-related protein Rap-1b, also known as GTP-binding protein smg p21B, is a protein that in humans is encoded by the RAP1B gene....
; RAP2A
RAP2A
Ras-related protein Rap-2a is a protein that in humans is encoded by the RAP2A gene. RAP2A is a member of the Ras-related protein family.-Interactions:RAP2A has been shown to interact with RUNDC3A, RASSF5 and RALGDS.-Further reading:...
; RAP2B
RAP2B
Ras-related protein Rap-2b is a protein that in humans is encoded by the RAP2B gene. RAP2B belongs to the Ras-related protein family.-Further reading:...
; RAP2C; RASD1
RASD1
Dexamethasone-induced Ras-related protein 1 is a protein that in humans is encoded by the RASD1 gene.-Further reading:...
; RASD2
RASD2
GTP-binding protein Rhes is a protein that in humans is encoded by the RASD2 gene.-Further reading:...
; RASL10A; RASL10B; RASL11A; RASL11B
RASL11B
Ras-like protein family member 11B is a protein that in humans is encoded by the RASL11B gene.-Further reading:...
; RASL12; REM1; REM2; RERG; RERGL; RRAD
RRAD
GTP-binding protein RAD is a protein that in humans is encoded by the RRAD gene.-Interactions:RRAD has been shown to interact with CAMK2G and TPM2.-Further reading:...
; RRAS
RRAS
Ras-related protein R-Ras is a protein that in humans is encoded by the RRAS gene.-Interactions:RRAS has been shown to interact with RASSF5, NCK1, Bcl-2, ARAF and RALGDS.-Further reading:...
; RRAS2
RRAS2
Ras-related protein R-Ras2 is a protein that in humans is encoded by the RRAS2 gene.-Further reading:...
Ras in cancer
Mutations in the Ras family of proto-oncogenes (comprising H-Ras, N-Ras and K-Ras) are very common, being found in 20% to 30% of all human tumours. it is reasonable to speculate that a pharmacological approach that curtails Ras activity may represent a possible method to inhibit certain cancer types. Ras point mutations are the single most common abnormality of human proto-oncogenes.Ras inhibitor trans-farnesylthiosalicylic acid (FTS, Salirasib) exhibits profound anti-oncogenic effects in many cancer cell lines.
Inappropriate activation
Inappropriate activation of the gene has been shown to play a key role in signal transduction, proliferation and malignant transformation.Mutations in a number of different genes as well as RAS itself can have this effect. Oncogene
Oncogene
An oncogene is a gene that has the potential to cause cancer. In tumor cells, they are often mutated or expressed at high levels.An oncogene is a gene found in the chromosomes of tumor cells whose activation is associated with the initial and continuing conversion of normal cells into cancer...
s such as p210BCR-ABL or the growth receptor erbB are upstream of Ras, so if they are constitutively activated their signals will transduce through Ras.
The tumour suppressor gene NF1
Neurofibromin 1
Neurofibromin 1 also known as neurofibromatosis-related protein NF-1 is a protein that in humans is encoded by the NF1 gene. Mutations in the NF1 gene are associated with neurofibromatosis type I .- Function :NF1 encodes the protein neurofibromin, which appears to be a negative regulator of the...
encodes a Ras-GAP – its mutation in neurofibromatosis
Neurofibromatosis
Neurofibromatosis is a genetically-inherited disorder in which the nerve tissue grows tumors that may be benign or may cause serious damage by compressing nerves and other tissues...
will mean that Ras is less likely to be inactivated. Ras can also be amplified, although this only occurs occasionally in tumours.
Finally, Ras oncogenes can be activated by point mutations so that the GTPase reaction can no longer be stimulated by GAP – this increases the half life of active Ras-GTP mutants.
Constitutively active Ras
Constitutively active Ras (RasD) is one which contains mutations that prevent GTP hydrolysis, thus locking Ras in a permanently 'On' state.The most common mutations are found at residue G12 in the P-loop and the catalytic residue Q61.
- The glycine to valine mutation at residue 12 renders the GTPase domain of Ras insensitive to inactivation by GAP and thus stuck in the "on state". Ras requires a GAP for inactivation as it is a relatively poor catalyst on its own, as opposed to other G-domain-containing proteins such as the alpha subunit of heterotrimeric G proteins.
- Residue 61 is responsible for stabilizing the transition state for GTP hydrolysis. Because enzyme catalysis in general is achieved by lowering the energy barrier between substrate and product, mutation of Q61 to K necessarily reduces the rate of intrinsic Ras GTP hydrolysis to physiologically meaningless levels.
See also "dominant negative" mutants such as S17N and D119N.
Ras-targeted cancer treatments
Reovirus was noted to be a potential cancer therapeutic when early studies on reovirus suggested it reproduces well in certain cancer cell lines. It has since been shown to replicate specifically in cells that have an activated Ras pathway (a cellular signaling pathway that is involved in cell growth and differentiation). Reovirus replicates in and eventually kills Ras-activated tumour cells and as cell death occurs, progeny virus particles are free to infect surrounding cancer cells. This cycle of infection, replication and cell death is believed to be repeated until all tumour cells carrying an activated Ras pathway are destroyed. Activating mutations of the Ras protein and upstream elements of the Ras protein may play a role in more than two thirds of all human cancers, including most metastatic disease. ReolysinReolysin
Reolysin is a formulation of reovirus that Oncolytics Biotech is developing for the treatment of various cancers and cell proliferative disorders. Reolysin is classified as a oncolytic virus, a virus that preferentially lyses cancer cells...
, a formulation of reovirus, is currently in clinical trials for the treatment of various cancers.