Biology:RPTOR

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Short description: Protein-coding gene in humans


A representation of the 3D structure of the protein myoglobin showing turquoise α-helices.
Generic protein structure example

Regulatory-associated protein of mTOR also known as raptor or KIAA1303 is an adapter protein that is encoded in humans by the RPTOR gene.[1][2][3] Two mRNAs from the gene have been identified that encode proteins of 1335 (isoform 1) and 1177 (isoform 2) amino acids long.

Gene and expression

The human gene is located on human chromosome 17 with location of the cytogenic band at 17q25.3.[3]

Location

RPTOR is highly expressed in skeletal muscle and is somewhat less present in brain, lung, small intestine, kidney, and placenta tissue. Isoform 3 is widely expressed and most highly expressed in the nasal mucosa and pituitary. The lowest levels occur in the spleen.[4] In the cell, RPTOR is present in cytoplasm, lysosomes, and cytoplasmic granules. Amino acid availability determines RPTOR targeting to lysosomes. In stressed cells, RPTOR associates with SPAG5 and accumulates in stress granules, which significantly reduces its presence in lysosomes.[5][6]

Function

RPTOR encodes part of a signaling pathway regulating cell growth which responds to nutrient and insulin levels. RPTOR is an evolutionarily conserved protein with multiple roles in the mTOR pathway. The adapter protein and mTOR kinase form a stoichiometric complex. The encoded protein also associates with eukaryotic initiation factor 4E-binding protein-1 and ribosomal protein S6 kinase. It upregulates S6 kinase, the downstream effector ribosomal protein, and it downregulates the mTOR kinase. RPTOR also has a positive role in maintaining cell size and mTOR protein expression. The association of mTOR and RPTOR is stabilized by nutrient deprivation and other conditions which suppress the mTOR pathway.[4] Multiple transcript variants exist for this gene which encode different isoforms.[3]

Structure

RPTOR is a 150 kDa mTOR binding protein that is part of the mammalian target of rapamycin complex 1 (mTORC1). This complex contains mTOR, MLST8, RPTOR, AKT1S1/PRAS40, and DEPTOR. mTORC1 both binds to and is inhibited by FKBP12-rapamycin. mTORC1 activity is upregulated by mTOR and MPAK8 by insulin-stimulated phosphorylation at Ser-863.[7][8] MAPK8 also causes phosphorylation at Ser-696, Thr-706, and Ser-863 as a result of osmotic stress.[9] AMPK causes phosphorylation in the event of nutrient starvation and promotes 14-3-3 binding to raptor, which downregulates the mTORC1 complex.[10] RPS6KA1 stimulates mTORC1 activity by phosphorylating at Ser-719, Ser-721, and Ser-722 as a response to growth factors.

Interactions

  • mTORC1 binds to and is inhibited by FKBP12-rapamycin
  • RPTOR binds to 4EBP1 and RPS6KB1 directly whether or not it is associated with mTOR[11]
  • RPTOR binds to poorly phosphorylated or non-phosphorylated EIF4EBP1 preferentially, which is important for mTOR to be able to catalyze phosphorylation.[2][11][12][13][14][15][16][17]
  • RPTOR interacts with ULK1. This interaction depends on nutrients and is reduced in the case of starvation.[18]
  • When RPTOR is phosphorylated by AMPK, it interacts with 14-3-3 protein and inhibits its activity.[10]
  • RPTOR interacts with SPAG5, which competes with mTOR for binding RPTOR and causes decreased mTORC1 formation.
  • RPTOR interacts with G3BP1. Oxidative stress increases the formation of the complex formed with RPTOR, G3BP1, and SPAG5[6]

RPTOR has also been shown to interact with:

Clinical significance

Signaling in cancer

The clinical significance of RPTOR is primarily due to its involvement in the mTOR pathway, which plays roles in mRNA translation, autophagy, and cell growth. Mutations in the PTEN tumor suppressor gene are the best known genetic deficiencies in cancer which affect mTOR signaling. These mutations are frequently found in a very large variety of cancers, including prostate, breast, lung, bladder, melanoma, endometrial, thyroid, brain, and renal carcinomas. PTEN inhibits the lipid-kinase activity of class I PtdIns3Ks, which phosphorylate PtdIns(4,5)P2 to create PtdIns(3,4,5)P3 (PIP3). PIP3 is a membrane-docking site for AKT and PDK1. In turn, active PDK1, along with mTORC1, phosphorylates S6K in the part of the mTOR pathway which promotes protein synthesis and cell growth.[35]

The mTOR pathway has also been found to be involved in aging. Studies with C. elegans, fruitflies, and mice have shown that the lifespan of the organism is significantly increased by inhibiting mTORC1.[36][37] mTORC1 phosphorylates Atg13 and stops it from forming the ULK1 kinase complex. This inhibits autophagy, the major degradation pathway in eukaryotic cells.[38] Because mTORC1 inhibits autophagy and stimulates cell growth, it can cause damaged proteins and cell structures to accumulate. For this reason, dysfunction in the process of autophagy can contribute to several diseases, including cancer.[39]

The mTOR pathway is important in many cancers. In cancer cells, astrin is required to suppress apoptosis during stress. Astrin recruits RPTOR to stress granules, inhibiting mTORC1 association and preventing apoptosis induced by mTORC1 hyperactivation. Because astrin is frequently upregulated in tumors, it is a potential target to sensitize tumors to apoptosis through the mTORC1 pathway.[6]

RPTOR is overexpressed in pituitary adenoma, and its expression increases with tumor staging. RPTOR could be valuable in the prediction and prognosis of pituitary adenoma due to this correlation between protein expression and the growth and invasion of the tumor.[40]

As a drug target

mTOR is found in two different complexes. When it associates with rapamycin-insensitive companion of mTOR (rictor), the complex is known as mTORC2 and it is insensitive to rapamycin. However, the complex mTORC1 formed by association with accessory protein RPTOR is sensitive to rapamycin. Rapamycin is a macrolide which is an immunosuppressant in humans that inhibits mTOR by binding to its intracellular receptor FKBP12. In many cancers, hyperactive AKT signaling leads to increased mTOR signaling, so rapamycin has been considered as an anti-cancer therapeutic for cancers with PTEN inactivation. Numerous clinical trials involving rapamycin analogs, such as CCI-779, RAD001, and AP23573, are ongoing. Early reports have been promising for renal-cell carcinoma, breast carcinomas, and non-small-cell lung carcinomas.[35]

See also

References

  1. ↑ "Prediction of the coding sequences of unidentified human genes. XVI. The complete sequences of 150 new cDNA clones from brain which code for large proteins in vitro". DNA Res. 7 (1): 65–73. Apr 2000. doi:10.1093/dnares/7.1.65. PMID 10718198. 
  2. ↑ 2.0 2.1 2.2 2.3 "Raptor, a binding partner of target of rapamycin (TOR), mediates TOR action". Cell 110 (2): 177–89. Aug 2002. doi:10.1016/S0092-8674(02)00833-4. PMID 12150926. 
  3. ↑ 3.0 3.1 3.2 "Entrez Gene: KIAA1303 raptor". https://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=57521. 
  4. ↑ 4.0 4.1 4.2 "mTOR interacts with raptor to form a nutrient-sensitive complex that signals to the cell growth machinery". Cell 110 (2): 163–75. 2002. doi:10.1016/S0092-8674(02)00808-5. PMID 12150925. 
  5. ↑ "Ragulator-Rag complex targets mTORC1 to the lysosomal surface and is necessary for its activation by amino acids". Cell 141 (2): 290–303. 2010. doi:10.1016/j.cell.2010.02.024. PMID 20381137. 
  6. ↑ 6.0 6.1 6.2 "Inhibition of mTORC1 by astrin and stress granules prevents apoptosis in cancer cells". Cell 154 (4): 859–74. 2013. doi:10.1016/j.cell.2013.07.031. PMID 23953116. 
  7. ↑ "Regulation of mTOR complex 1 (mTORC1) by raptor Ser863 and multisite phosphorylation". J. Biol. Chem. 285 (1): 80–94. 2010. doi:10.1074/jbc.M109.029637. PMID 19864431. 
  8. ↑ "Oncogenic MAPK signaling stimulates mTORC1 activity by promoting RSK-mediated raptor phosphorylation". Curr. Biol. 18 (17): 1269–77. 2008. doi:10.1016/j.cub.2008.07.078. PMID 18722121. 
  9. ↑ "Osmotic stress regulates mammalian target of rapamycin (mTOR) complex 1 via c-Jun N-terminal Kinase (JNK)-mediated Raptor protein phosphorylation". J. Biol. Chem. 287 (22): 18398–407. 2012. doi:10.1074/jbc.M111.326538. PMID 22493283. 
  10. ↑ 10.0 10.1 "AMPK phosphorylation of raptor mediates a metabolic checkpoint". Mol. Cell 30 (2): 214–26. 2008. doi:10.1016/j.molcel.2008.03.003. PMID 18439900. 
  11. ↑ 11.0 11.1 11.2 "Activation of mammalian target of rapamycin (mTOR) by insulin is associated with stimulation of 4EBP1 binding to dimeric mTOR complex 1". J. Biol. Chem. 281 (34): 24293–303. 2006. doi:10.1074/jbc.M603566200. PMID 16798736. 
  12. ↑ 12.0 12.1 "TOS motif-mediated raptor binding regulates 4E-BP1 multisite phosphorylation and function". Curr. Biol. 13 (10): 797–806. 2003. doi:10.1016/S0960-9822(03)00329-4. PMID 12747827. 
  13. ↑ 13.0 13.1 13.2 "PLD2 forms a functional complex with mTOR/raptor to transduce mitogenic signals". Cell. Signal. 18 (12): 2283–91. 2006. doi:10.1016/j.cellsig.2006.05.021. PMID 16837165. 
  14. ↑ 14.0 14.1 14.2 "The mammalian target of rapamycin (mTOR) partner, raptor, binds the mTOR substrates p70 S6 kinase and 4E-BP1 through their TOR signaling (TOS) motif". J. Biol. Chem. 278 (18): 15461–4. 2003. doi:10.1074/jbc.C200665200. PMID 12604610. 
  15. ↑ "Different roles for the TOS and RAIP motifs of the translational regulator protein 4E-BP1 in the association with raptor and phosphorylation by mTOR in the regulation of cell size". Genes Cells 11 (7): 757–66. 2006. doi:10.1111/j.1365-2443.2006.00977.x. PMID 16824195. 
  16. ↑ "Target of rapamycin (TOR)-signaling and RAIP motifs play distinct roles in the mammalian TOR-dependent phosphorylation of initiation factor 4E-binding protein 1". J. Biol. Chem. 278 (42): 40717–22. 2003. doi:10.1074/jbc.M308573200. PMID 12912989. 
  17. ↑ "Distinct signaling events downstream of mTOR cooperate to mediate the effects of amino acids and insulin on initiation factor 4E-binding proteins". Mol. Cell. Biol. 25 (7): 2558–72. 2005. doi:10.1128/MCB.25.7.2558-2572.2005. PMID 15767663. 
  18. ↑ "Nutrient-dependent mTORC1 association with the ULK1-Atg13-FIP200 complex required for autophagy". Mol. Biol. Cell 20 (7): 1981–91. 2009. doi:10.1091/mbc.E08-12-1248. PMID 19211835. 
  19. ↑ 19.0 19.1 "Mammalian TOR complex 2 controls the actin cytoskeleton and is rapamycin insensitive". Nat. Cell Biol. 6 (11): 1122–8. 2004. doi:10.1038/ncb1183. PMID 15467718. 
  20. ↑ 20.0 20.1 "Rictor, a novel binding partner of mTOR, defines a rapamycin-insensitive and raptor-independent pathway that regulates the cytoskeleton". Curr. Biol. 14 (14): 1296–302. 2004. doi:10.1016/j.cub.2004.06.054. PMID 15268862. 
  21. ↑ "Structure of S6 kinase 1 determines whether raptor-mTOR or rictor-mTOR phosphorylates its hydrophobic motif site". J. Biol. Chem. 280 (20): 19445–8. 2005. doi:10.1074/jbc.C500125200. PMID 15809305. 
  22. ↑ "Rheb binds and regulates the mTOR kinase". Curr. Biol. 15 (8): 702–13. 2005. doi:10.1016/j.cub.2005.02.053. PMID 15854902. 
  23. ↑ 23.0 23.1 "SIN1/MIP1 maintains rictor-mTOR complex integrity and regulates Akt phosphorylation and substrate specificity". Cell 127 (1): 125–37. 2006. doi:10.1016/j.cell.2006.08.033. PMID 16962653. 
  24. ↑ "mSin1 is necessary for Akt/PKB phosphorylation, and its isoforms define three distinct mTORC2s". Curr. Biol. 16 (18): 1865–70. 2006. doi:10.1016/j.cub.2006.08.001. PMID 16919458. 
  25. ↑ "Localization of Rheb to the endomembrane is critical for its signaling function". Biochem. Biophys. Res. Commun. 344 (3): 869–80. 2006. doi:10.1016/j.bbrc.2006.03.220. PMID 16631613. 
  26. ↑ "Farnesylthiosalicylic acid inhibits mammalian target of rapamycin (mTOR) activity both in cells and in vitro by promoting dissociation of the mTOR-raptor complex". Mol. Endocrinol. 19 (1): 175–83. 2005. doi:10.1210/me.2004-0305. PMID 15459249. 
  27. ↑ "Dissociation of raptor from mTOR is a mechanism of rapamycin-induced inhibition of mTOR function". Genes Cells 9 (4): 359–66. 2004. doi:10.1111/j.1356-9597.2004.00727.x. PMID 15066126. 
  28. ↑ "Vinculin: a novel marker for quiescent and activated hepatic stellate cells in human and rat livers". Virchows Arch. 443 (1): 78–86. 2003. doi:10.1007/s00428-003-0804-4. PMID 12719976. 
  29. ↑ "Two motifs in the translational repressor PHAS-I required for efficient phosphorylation by mammalian target of rapamycin and for recognition by raptor". J. Biol. Chem. 278 (22): 19667–73. 2003. doi:10.1074/jbc.M301142200. PMID 12665511. 
  30. ↑ "The mammalian target of rapamycin (mTOR) pathway regulates mitochondrial oxygen consumption and oxidative capacity". J. Biol. Chem. 281 (37): 27643–52. 2006. doi:10.1074/jbc.M603536200. PMID 16847060. 
  31. ↑ "Prolonged rapamycin treatment inhibits mTORC2 assembly and Akt/PKB". Mol. Cell 22 (2): 159–68. 2006. doi:10.1016/j.molcel.2006.03.029. PMID 16603397. 
  32. ↑ "Nutrients suppress phosphatidylinositol 3-kinase/Akt signaling via raptor-dependent mTOR-mediated insulin receptor substrate 1 phosphorylation". Mol. Cell. Biol. 26 (1): 63–76. 2006. doi:10.1128/MCB.26.1.63-76.2006. PMID 16354680. 
  33. ↑ "Redox regulation of the nutrient-sensitive raptor-mTOR pathway and complex". J. Biol. Chem. 280 (47): 39505–9. 2005. doi:10.1074/jbc.M506096200. PMID 16183647. 
  34. ↑ "Identification of Sin1 as an essential TORC2 component required for complex formation and kinase activity". Genes Dev. 20 (20): 2820–32. 2006. doi:10.1101/gad.1461206. PMID 17043309. 
  35. ↑ 35.0 35.1 "Defining the role of mTOR in cancer". Cancer Cell 12 (1): 9–22. July 2007. doi:10.1016/j.ccr.2007.05.008. PMID 17613433. 
  36. ↑ "TOR signaling and rapamycin influence longevity by regulating SKN-1/Nrf and DAF-16/FoxO". Cell Metab. 15 (5): 713–24. May 2012. doi:10.1016/j.cmet.2012.04.007. PMID 22560223. 
  37. ↑ "Rapamycin fed late in life extends lifespan in genetically heterogeneous mice". Nature 460 (7253): 392–5. July 2009. doi:10.1038/nature08221. PMID 19587680. Bibcode: 2009Natur.460..392H. 
  38. ↑ "Molecules and their functions in autophagy". Exp. Mol. Med. 44 (2): 73–80. February 2012. doi:10.3858/emm.2012.44.2.029. PMID 22257882. 
  39. ↑ "Autophagy and signaling: their role in cell survival and cell death". Cell Death Differ. 12 (Suppl 2): 1509–18. November 2005. doi:10.1038/sj.cdd.4401751. PMID 16247498. http://www.hal.inserm.fr/inserm-00172272. 
  40. ↑ "Expression of the mTOR pathway regulators in human pituitary adenomas indicates the clinical course". Anticancer Res. 33 (8): 3123–31. August 2013. PMID 23898069. 

Further reading



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