Ras (protein)
From Wikipedia, the free encyclopedia
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| Triphosphate Structure of H-Ras p21 GNBP in complex with GppNHp and Mg2+ (PDB Code 5p21). Important sequence elements are highlighted. | ||
| Identifiers | ||
|---|---|---|
| Symbol | Ras | |
| Pfam | PF00071 | |
| InterPro | IPR013753 | |
| PROSITE | PDOC00859 | |
| SCOP | 5p21 | |
| OPM protein | 1uad | |
| Available PDB structures:
2folA:13-174 1ukvY:10-171 1yznA:10-171 2bcgY:10-171 1g17B:22-182 1g16C:22-182 3rabA:24-185 1zbdA:24-185 2ew1A:11-172 1x3sA:10-171 1z0kA:10-171 2bmeA:10-171 2bmdA:10-171 1yu9A:10-171 2aedA:13-174 1z0fA:13-174 1z0aD:8-169 2a5jA:8-169 1oivB:13-172 1yzkA:13-174 1oixA:13-172 1oiwA:13-172 1z06A:35-201 1huqA:23-182 1z0dC:23-183 1z07A:23-182 1r2qA:22-183 1n6rA:22-183 1ek0A:9-173 1yvdA:7-168 1z0jA:7-168 2fg5A:7-168 1z08A:21-182 1yztB:21-182 1yzuB:21-182 1z0iA:21-182 1yzqA:15-176 1d5cA:13-172 1t91C:10-175 1vg8D:10-175 1vg9H:10-175 1vg1A:10-175 1vg0B:10-175 1ky3A:10-178 1ky2A:10-178 1yzlA:9-174 1wmsB:9-174 1s8fB:9-174 2f7sA:11-183 1z22A:11-171 1z2aA:11-171 1plj :5-165 1ctqA:5-165 1crp :5-165 821p :5-165 2eryB:16-177 2fn4A:31-192 1x1sA:15-177 1x1rA:15-177 1u90A:16-177 1u8zA:16-177 1u8yB:16-177 1uadB:16-177 2bovA:16-177 2a78A:16-177 3rapR:5-166 1kao :5-166 2rap :5-166 1c1yA:5-167 1guaA:5-167 1xtrA:8-169 1xtqA:8-169 1xtsA:8-169 2erxB:9-171 1a4rB:5-178 1kmqA:7-180 1tx4B:7-179 1cxzA:7-180 1m7bA:25-199 1gwnA:25-199 2bkuA:12-170 3ranA:12-170 1qg4B:12-170 1byuA:12-170 2atvA:8-169 |
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Ras is a family of genes encoding small GTPases that are involved in cellular signal transduction. Activation of Ras signalling causes cell growth, differentiation and survival. Ras is the prototypical member of the Ras superfamily of proteins which are all related in structure and regulate diverse cell behaviours.
Since Ras communicates signals from outside the cell to the nucleus, mutations in ras genes can permanently activate it and cause inappropriate transmission inside the cell even in the absence of extracellular signals. Because these signals result in cell growth and division, dysregulated Ras signaling can ultimately lead to oncogenesis and cancer.[1] Activating mutations in Ras are found in 20-25% of all human tumors and up to 90% in specific tumor types.[2]
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[edit] History
The ras genes were first identified as the transforming oncogenes,[3] responsible for the cancer-causing activities of the Harvey (the HRAS oncogene) and Kirsten (KRAS) sarcoma viruses, by Edward M. Scolnick and colleagues at the National Institutes of Health (NIH).[4] These viruses were discovered originally in rats during the 1960s by Jennifer Harvey[5] and Werner Kirsten,[6] 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,[7] Mariano Barbacid and Stuart A. Aaronson at the NIH[8] and by Robert Weinberg of MIT.[9] Subsequent studies identified a third human RAS gene, designated NRAS, for its initial identification in human neuroblastoma cells.
The three human RAS genes encode highly related 188 to 189 amino acid proteins, designated H-Ras, N-Ras and K-Ras4A and K-Ras4B (the two K-Ras proteins arise from alternative gene splicing).
[edit] The Ras superfamily
There are more than a hundred proteins in the Ras superfamily.[10] Based on structure, sequence and function, the Ras superfamily is divided into eight main families, each of which is further divided into subfamilies: Ras, Rho, Rab, Rap, Arf, Ran, Rheb, Rad and Rit. Miro is a recent contributor to the superfamily.
Each subfamily shares the common core G domain, which provides essential GTPase and nucleotide exchange activity.
The surrounding sequence helps determine the functional specificity of the small GTPase, for example the 'Insert Loop', common to the Rho subfamily, specifically contributes to binding to effector proteins such as IQGAP and WASP.
The Ras family is generally responsible for cell proliferation, Rho for cell morphology, nuclear transport for Ran and vesicle transport for Rab and Arf:[11]
The following is a list of human proteins belong to the Ras superfamily:[10]
Unclassified:
[edit] Structure
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Ras contains 6 beta sheets and 5 alpha helices:[12]
- 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, RCE1 and ICMT
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
- G3 - DXXG motif, aspartate-57 is specific for guanine rather than adenine
- G4
- 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 during activation
- switch I includes threonine-35
- switch II glycine-60 in DXXG motif
Ras also binds a magnesium ion which helps to coordinate nucleotide binding.
[edit] Function
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.
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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.[13]
[edit] Activation and deactivation
Ras is a G protein, 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 nucleotideguanosine 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 activated 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 factors (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. Thus, GAPs regulate 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[citation needed]) GTP predominantly re-enters the nucleotide binding pocket of Ras and reloads the spring. Thus GEFs facilitate Ras activation.[12] 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.
[edit] Membrane attachment
Ras is attached to the cell membrane by prenylation, and in health is a key component in many pathways which couple growth factor receptors to downstream mitogenic effectors involved in cell proliferation or differentiation.[14] The C-terminal CaaX box of Ras first gets farnesylated at its Cys residue in the cytosol and then inserted into the membrane of the endoplasmatic reticulum. The Tripeptide (aaX) is then cleaved from the C-terminus by a specific prenyl-protein specific endoprotease, the new C-terminus is then methylated by a methyltransferase. The so processed Ras is now transported to the plasma membrane. Most Ras forms are now further palmitoylated, while K-Ras with its long positively charged stretch interacts electrostaticly with the membrane.
[edit] 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.[15] it is reasonable to speculate that a pharmacological approach that curtails Ras activity may represent a possible method to inhibit certain cancer types. Ras inhibitor trans-farnesylthiosalicylic acid (FTS, salirasib) exhibits profound antioncogenic effects in many cancer cell lines.[16][17]
[edit] Inappropriate activation
Inappropriate activation of the gene has been shown to play a key role in signal transduction, proliferation and malignant transformation.[13]
Mutations in a number of different genes as well as RAS itself can have this effect. Oncogenes 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 its GTPase reaction can no longer be stimulated by GAP – this increases the half life of active Ras-GTP mutants.[14]
[edit] 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[18] 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.
[edit] References
- ^ Goodsell DS (1999). "The molecular perspective: the ras oncogene". Oncologist 4 (3): 263–4. PMID 10394594. http://theoncologist.alphamedpress.org/cgi/content/full/4/3/263.
- ^ Downward J (January 2003). "Targeting RAS signalling pathways in cancer therapy". Nat. Rev. Cancer 3 (1): 11–22. doi:. PMID 12509763.
- ^ Malumbres M, Barbacid M (June 2003). "RAS oncogenes: the first 30 years". Nat. Rev. Cancer 3 (6): 459–65. doi:. PMID 12778136.
- ^ Chang EH, Gonda MA, Ellis RW, Scolnick EM, Lowy DR (August 1982). "Human genome contains four genes homologous to transforming genes of Harvey and Kirsten murine sarcoma viruses". Proc. Natl. Acad. Sci. U.S.A. 79 (16): 4848–52. doi:. PMID 6289320.
- ^ Harvey JJ (December 1964). "An unidentified virus which causes the rapid production of tumours in mice". Nature 204: 1104–5. doi:. PMID 14243400.
- ^ Kirsten WH, Schauf V, McCoy J (1970). "Properties of a murine sarcoma virus". Bibl Haematol (36): 246–9. PMID 5538357.
- ^ Cooper GM (August 1982). "Cellular transforming genes". Science (journal) 217 (4562): 801–6. doi:. PMID 6285471.
- ^ Santos E, Tronick SR, Aaronson SA, Pulciani S, Barbacid M (July 1982). "T24 human bladder carcinoma oncogene is an activated form of the normal human homologue of BALB- and Harvey-MSV transforming genes". Nature 298 (5872): 343–7. doi:. PMID 6283384.
- ^ Parada LF, Tabin CJ, Shih C, Weinberg RA (June 1982). "Human EJ bladder carcinoma oncogene is homologue of Harvey sarcoma virus ras gene". Nature 297 (5866): 474–8. doi:. PMID 6283357.
- ^ a b Wennerberg K, Rossman KL, Der CJ (March 2005). "The Ras superfamily at a glance". J. Cell. Sci. 118 (Pt 5): 843–6. doi:. PMID 15731001.
- ^ a b c d e Munemitsu S, Innis M, Clark R, McCormick F, Ullrich A, Polakis P. (1990). "Molecular cloning and experssion of a G25K cDNA, the human homolog of the yeast cell cycle gene CDC42". Mol Cell Biol 10 (11): 5977–82. ISSN 0270-7306. PMID 2122236.
- ^ a b Vetter IR, Wittinghofer A (November 2001). "The guanine nucleotide-binding switch in three dimensions". Science (journal) 294 (5545): 1299–304. doi:. PMID 11701921.
- ^ a b Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J (2000). "Chapter 25, Cancer". Molecular cell biology (4th ed.). San Francisco: W.H. Freeman. ISBN 0-7167-3706-X.
- ^ a b Reuter C, Morgan M, Bergmann L (2000). "Targeting the Ras signaling pathway: a rational, mechanism-based treatment for hematologic malignancies?". Blood 96 (5): 1655–69. PMID 10961860.
- ^ Bos J (1989). "ras oncogenes in human cancer: a review". Cancer Res 49 (17): 4682–9. PMID 2547513.
- ^ Rotblat B, Ehrlich M, Haklai R, Kloog Y (2008). "The Ras inhibitor farnesylthiosalicylic acid (Salirasib) disrupts the spatiotemporal localization of active Ras: a potential treatment for cancer.". Methods Enzymol 439: 467–89. doi:. PMID 18374183.
- ^ Roy Blum, yoel kloog (2005). "Ras Inhibition in Glioblastoma Down-regulates Hypoxia-Inducible Factor-1, Causing Glycolysis Shutdown and Cell Death". Cancer Research 65: 999–1006. PMID 15705901.
- ^ Omim - Neuroblastoma Ras Viral Oncogene Homolog; Nras
[edit] External links
- "Brain tumour findings offer hope of new strategy Canadian Cancer Society says" at ncic.cancer.ca
- "Novel cancer treatment gets NCI support" at arstechnica.com
- MeSH ras+Proteins
- MeSH ras+Genes
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