ARAF explained

Serine/threonine-protein kinase A-Raf, or simply A-Raf, is an enzyme that in humans is encoded by the ARAF gene. It belongs to the Raf kinase family of serine/threonine-specific protein kinases, which also includes Raf-1 and B-Raf. A-Raf is involved in the MAPK/ERK pathway, where it contributes to cell signaling processes that regulate proliferation, survival, and differentiation. Compared to Raf-1 and B-Raf, A-Raf is less well studied and exhibits distinct structural and regulatory features, including low kinase activity and alternative splicing in cancer. In addition to its role in MAPK signaling, A-Raf has functions in apoptosis suppression, cancer metabolism, and endocytic trafficking.

Structure

A-Raf, a member of the Raf kinase family, shares a conserved domain architecture with B-Raf and C-Raf, comprising three conserved regions: CR1, CR2, and CR3.

• CR1 (Conserved Region 1): This N-terminal region contains the Ras-binding domain (RBD) and the cysteine-rich domain (CRD). The RBD facilitates interaction with activated Ras-GTP, anchoring A-Raf to the plasma membrane.^[1] The CRD, characterized by its zinc-binding motif, contributes to membrane association and protein-protein interactions^[2] Structural studies confirm the RBD and CRD function as a single entity during Ras binding.^[3]
• CR2 (Conserved Region 2): Positioned between CR1 and CR3, CR2 is a serine/threonine-rich regulatory segment containing phosphorylation sites (e.g., Ser259 in Raf-1) that modulate A-Raf's activity and interactions with 14-3-3 proteins.^[4] This region is critical for autoinhibition and activation dynamics.^[5]
• CR3 (Conserved Region 3): The C-terminal kinase domain exhibits the bilobal architecture characteristic of protein kinases, with an ATP-binding site between the N-terminal and C-terminal lobes.^[6] Structural analyses reveal similarities to tyrosine kinase-like (TKL) group members^[7]

The RBD adopts a ubiquitin-like fold critical for Ras-GTP interaction.^[8], while the CRD's zinc-binding motif stabilizes membrane association.^[9] A-Raf's activity is regulated by phosphorylation-dependent 14-3-3 binding.^[10] and isoform dimerization, which is essential for MAPK pathway activation.^{[11] [12]}

Function

A-Raf shares the canonical role of Raf kinases in the MAPK signaling cascade. Upon activation by Ras, A-Raf translocates from the cytosol to the plasma membrane, where it phosphorylates and activates MEK proteins. This activation leads to downstream ERK signaling and promotes cell cycle progression and proliferation.^[13]

Among the Raf isoforms, A-Raf exhibits the lowest kinase activity toward MEK proteins.^[14] This may be due to amino acid substitutions in a negatively charged region upstream of the kinase domain (the N-region), which result in low basal activity.^[15]

A-Raf is also the only Raf kinase known to be regulated by steroid hormones.^[16] In its inactive form, A-Raf is bound to 14-3-3 proteins in the cytosol; activation by Ras causes its translocation to the plasma membrane.

Beyond the MAPK pathway, A-Raf has additional functions. It inhibits MST2, a proapoptotic kinase, thereby suppressing apoptosis. This inhibitory activity is dependent on the expression of full-length A-Raf protein, which is maintained by the splicing factor hnRNP H.^[17]

A-Raf also regulates energy metabolism by interacting with pyruvate kinase M2 (PKM2), a key enzyme in cancer cell glycolysis. By promoting a conformational shift from the dimeric to the tetrameric form of PKM2, A-Raf enhances its enzymatic activity and shifts glucose utilization from biosynthesis toward energy production.^[18]

In addition, A-Raf has been implicated in endocytic membrane trafficking. Upon activation by receptor tyrosine kinases and Ras, A-Raf localizes to phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂)-rich membranes and signals to endosomes, leading to activation of ARF6, a key regulator of endocytosis.^[19]

Clinical significance

A-Raf may contribute to tumorigenesis through multiple mechanisms. In cancer cells, overexpression of hnRNP H enhances the production of full-length A-Raf, which inhibits MST2 and prevents apoptosis. The downregulation of hnRNP H, in contrast, leads to alternative splicing of the ARAF gene and loss of this anti-apoptotic activity.^[20]

A-Raf's regulation of PKM2 activity further links it to cancer metabolism. By promoting glycolytic flux toward pyruvate and lactate production, A-Raf may help sustain the high energy demands of rapidly proliferating tumor cells.^[21]

Because A-Raf modulates both apoptosis and metabolism—two critical hallmarks of cancer—it may represent a potential target for future cancer therapies.

Interactions

ARAF has been shown to interact with:

• EFEMP1,
• MAP2K2,
• PRPF6,
• RRAS,
• TIMM44, and
• TH1L.

References

1. Tran TH, Chan AH, Young LC, Bindu L, Neale C, Messing S, Dharmaiah S, Taylor T, Denson JP, Esposito D, Nissley DV, Stephen AG, McCormick F, Simanshu DK . KRAS interaction with RAF1 RAS-binding domain and cysteine-rich domain provides insights into RAS-mediated RAF activation . Nature Communications . 12 . 1 . 1176 . February 2021 . 33608534 . 10.1038/s41467-021-21422-x . 7895934 . 2021NatCo..12.1176T .
2. Mott HR, Carpenter JW, Zhong S, Ghosh S, Bell RM, Campbell SL . The solution structure of the Raf-1 cysteine-rich domain: a novel ras and phospholipid binding site . Proceedings of the National Academy of Sciences of the United States of America . 93 . 16 . 8312–8317 . August 1996 . 8710867 . 10.1073/pnas.93.16.8312 . free . 38667 . 1996PNAS...93.8312M .
3. Tran TH, Chan AH, Young LC, Bindu L, Neale C, Messing S, Dharmaiah S, Taylor T, Denson JP, Esposito D, Nissley DV, Stephen AG, McCormick F, Simanshu DK . KRAS interaction with RAF1 RAS-binding domain and cysteine-rich domain provides insights into RAS-mediated RAF activation . Nature Communications . 12 . 1 . 1176 . February 2021 . 33608534 . 10.1038/s41467-021-21422-x . 7895934 . 2021NatCo..12.1176T .
4. Light Y, Paterson H, Marais R . 14-3-3 antagonizes Ras-mediated Raf-1 recruitment to the plasma membrane to maintain signaling fidelity . Molecular and Cellular Biology . 22 . 14 . 4984–4996 . July 2002 . 12077328 . 10.1128/MCB.22.14.4984-4996.2002 . 139778 .
5. Defrise M, Kinahan PE, Townsend DW, Michel C, Sibomana M, Newport DF . Exact and approximate rebinning algorithms for 3-D PET data . IEEE Transactions on Medical Imaging . 16 . 2 . 145–158 . April 1997 . 9101324 . 10.1109/42.563660 .
6. Defrise M, Kinahan PE, Townsend DW, Michel C, Sibomana M, Newport DF . Exact and approximate rebinning algorithms for 3-D PET data . IEEE Transactions on Medical Imaging . 16 . 2 . 145–158 . April 1997 . 9101324 . 10.1109/42.563660 .
7. Motegi A, Fujimoto J, Kotani M, Sakuraba H, Yamamoto T . ALK receptor tyrosine kinase promotes cell growth and neurite outgrowth . Journal of Cell Science . 117 . Pt 15 . 3319–3329 . July 2004 . 15226403 . 10.1242/jcs.01183 .
8. Tran TH, Chan AH, Young LC, Bindu L, Neale C, Messing S, Dharmaiah S, Taylor T, Denson JP, Esposito D, Nissley DV, Stephen AG, McCormick F, Simanshu DK . KRAS interaction with RAF1 RAS-binding domain and cysteine-rich domain provides insights into RAS-mediated RAF activation . Nature Communications . 12 . 1 . 1176 . February 2021 . 33608534 . 10.1038/s41467-021-21422-x . 7895934 . 2021NatCo..12.1176T .
9. Mott HR, Carpenter JW, Zhong S, Ghosh S, Bell RM, Campbell SL . The solution structure of the Raf-1 cysteine-rich domain: a novel ras and phospholipid binding site . Proceedings of the National Academy of Sciences of the United States of America . 93 . 16 . 8312–8317 . August 1996 . 8710867 . 10.1073/pnas.93.16.8312 . free . 38667 . 1996PNAS...93.8312M .
10. Light Y, Paterson H, Marais R . 14-3-3 antagonizes Ras-mediated Raf-1 recruitment to the plasma membrane to maintain signaling fidelity . Molecular and Cellular Biology . 22 . 14 . 4984–4996 . July 2002 . 12077328 . 10.1128/MCB.22.14.4984-4996.2002 . 139778 .
11. Rimmer A . Overseas doctors must not be used just to fill rota gaps, says leading consultant . BMJ . 361 . k2654 . June 2018 . 29907696 . 10.1136/bmj.k2654 .
12. Rushworth LK, Hindley AD, O'Neill E, Kolch W . Regulation and role of Raf-1/B-Raf heterodimerization . Molecular and Cellular Biology . 26 . 6 . 2262–2272 . March 2006 . 16508002 . 10.1128/MCB.26.6.2262-2272.2006 . 1430271 .
13. Mercer K, Giblett S, Oakden A, Brown J, Marais R, Pritchard C . A-Raf and Raf-1 work together to influence transient ERK phosphorylation and Gl/S cell cycle progression . Oncogene . 24 . 33 . 5207–5217 . 2005-04-25 . 15856007 . 10.1038/sj.onc.1208707 . en . 0950-9232 . free .
14. Matallanas D, Birtwistle M, Romano D, Zebisch A, Rauch J, von Kriegsheim A, Kolch W . Raf Family Kinases Old Dogs Have Learned New Tricks . Genes & Cancer . 2 . 3 . 232–260 . 2011-03-01 . 21779496 . 3128629 . 10.1177/1947601911407323 . en . 1947-6019 .
15. Baljuls A, Mueller T, Drexler HC, Hekman M, Rapp UR . Unique N-region determines low basal activity and limited inducibility of A-RAF kinase: the role of N-region in the evolutionary divergence of RAF kinase function in vertebrates . The Journal of Biological Chemistry . 282 . 36 . 26575–26590 . 2007-09-07 . 17613527 . 10.1074/jbc.M702429200 . 0021-9258 . free .
16. Lee JE, Beck TW, Wojnowski L, Rapp UR . Regulation of A-raf expression . Oncogene . 12 . 8 . 1669–1677 . 1996-04-18 . 8622887 . 0950-9232 .
17. Rauch J, O'Neill E, Mack B, Matthias C, Munz M, Kolch W, Gires O . Heterogeneous Nuclear Ribonucleoprotein H Blocks MST2-Mediated Apoptosis in Cancer Cells by Regulating a-raf Transcription . Cancer Research . 70 . 4 . 1679–1688 . 2010-02-15 . 20145135 . 2880479 . 10.1158/0008-5472.CAN-09-2740 . en . 0008-5472 .
18. Mazurek S, Drexler HC, Troppmair J, Eigenbrodt E, Rapp UR . Regulation of Pyruvate Kinase Type M2 by A-Raf: A Possible Glycolytic Stop or Go Mechanism . Anticancer Research . 27 . 6B . 3963–3971 . 2007-11-01 . 18225557 . en . 0250-7005 .
19. Nekhoroshkova E, Albert S, Becker M, Rapp UR . A-RAF Kinase Functions in ARF6 Regulated Endocytic Membrane Traffic . PLOS ONE . 4 . 2 . e4647 . 2009-02-27 . 19247477 . 2645234 . 10.1371/journal.pone.0004647 . 1932-6203 . free . 2009PLoSO...4.4647N .
20. Rauch J, O'Neill E, Mack B, Matthias C, Munz M, Kolch W, Gires O . Heterogeneous Nuclear Ribonucleoprotein H Blocks MST2-Mediated Apoptosis in Cancer Cells by Regulating a-raf Transcription . Cancer Research . 70 . 4 . 1679–1688 . 2010-02-15 . 20145135 . 2880479 . 10.1158/0008-5472.CAN-09-2740 . en . 0008-5472 .
21. Christofk HR, Vander Heiden MG, Harris MH, Ramanathan A, Gerszten RE, Wei R, Fleming MD, Schreiber SL, Cantley LC . The M2 splice isoform of pyruvate kinase is important for cancer metabolism and tumour growth . Nature . 452 . 7184 . 230–233 . 2008-03-13 . 18337823 . 10.1038/nature06734 . 2008Natur.452..230C . en . 0028-0836 . 16111842 .

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Further reading

• Belanger BF, Williams WJ, Yin TC . A flexible renewal process simulator for neural spike trains . IEEE Transactions on Bio-medical Engineering . 23 . 3 . 262–266 . 1976 . 1262038 . 10.1109/TBME.1976.324641 . 1976ITBE...23..262B . 2411492 .
• Popescu NC, Mark GE . Localization of the pKs gene, a raf related sequence on human chromosomes X and 7 . Oncogene . 4 . 4 . 517–519 . 1989 . 2717185 .
• Beck TW, Huleihel M, Gunnell M, Bonner TI, Rapp UR . The complete coding sequence of the human A-raf-1 oncogene and transforming activity of a human A-raf carrying retrovirus . Nucleic Acids Research . 15 . 2 . 595–609 . 1987 . 3029685 . 340454 . 10.1093/nar/15.2.595 .
• Papin C, Eychène A, Brunet A, Pagès G, Pouysségur J, Calothy G, Barnier JV . B-Raf protein isoforms interact with and phosphorylate Mek-1 on serine residues 218 and 222 . Oncogene . 10 . 8 . 1647–1651 . 1995 . 7731720 .
• Lee JE, Beck TW, Brennscheidt U, DeGennaro LJ, Rapp UR . The complete sequence and promoter activity of the human A-raf-1 gene (ARAF1) . Genomics . 20 . 1 . 43–55 . 1994 . 8020955 . 10.1006/geno.1994.1125 .
• Alessi DR, Saito Y, Campbell DG, Cohen P, Sithanandam G, Rapp U, Ashworth A, Marshall CJ, Cowley S . Identification of the sites in MAP kinase kinase-1 phosphorylated by p74raf-1 . The EMBO Journal . 13 . 7 . 1610–1619 . 1994 . 8157000 . 394991 . 10.1002/j.1460-2075.1994.tb06424.x .
• Gardner AM, Vaillancourt RR, Johnson GL . Activation of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase by G protein and tyrosine kinase oncoproteins . Journal of Biological Chemistry . 268 . 24 . 17896–17901 . 1993 . 8394352 . 10.1016/S0021-9258(17)46789-5 . free .
• Andersson B, Wentland MA, Ricafrente JY, Liu W, Gibbs RA . A "double adaptor" method for improved shotgun library construction . Analytical Biochemistry . 236 . 1 . 107–113 . 1996 . 8619474 . 10.1006/abio.1996.0138 .
• Wu X, Noh SJ, Zhou G, Dixon JE, Guan KL . Selective activation of MEK1 but not MEK2 by A-Raf from epidermal growth factor-stimulated Hela cells . Journal of Biological Chemistry . 271 . 6 . 3265–3271 . 1996 . 8621729 . 10.1074/jbc.271.6.3265 . free .
• Boldyreff B, Issinger OG . A-Raf kinase is a new interacting partner of protein kinase CK2 beta subunit . FEBS Letters . 403 . 2 . 197–199 . 1997 . 9042965 . 10.1016/S0014-5793(97)00010-0 . free . 1997FEBSL.403..197B .
• Yu W, Andersson B, Worley KC, Muzny DM, Ding Y, Liu W, Ricafrente JY, Wentland MA, Lennon G, Gibbs RA . Large-scale concatenation cDNA sequencing . Genome Research . 7 . 4 . 353–358 . 1997 . 9110174 . 139146 . 10.1101/gr.7.4.353 .
• Yuryev A, Ono M, Goff SA, Macaluso F, Wennogle LP . Isoform-specific localization of A-RAF in mitochondria . Molecular and Cellular Biology . 20 . 13 . 4870–4878 . 2000 . 10848612 . 85938 . 10.1128/MCB.20.13.4870-4878.2000 .
• King TR, Fang Y, Mahon ES, Anderson DH . Using a phage display library to identify basic residues in A-Raf required to mediate binding to the Src homology 2 domains of the p85 subunit of phosphatidylinositol 3'-kinase . Journal of Biological Chemistry . 275 . 46 . 36450–36456 . 2000 . 10967104 . 10.1074/jbc.M004720200 . free .
• Fang Y, Johnson LM, Mahon ES, Anderson DH . Two phosphorylation-independent sites on the p85 SH2 domains bind A-Raf kinase . Biochemical and Biophysical Research Communications . 290 . 4 . 1267–1274 . 2002 . 11812000 . 10.1006/bbrc.2002.6347 . 2002BBRC..290.1267F .
• Yin XL, Chen S, Yan J, Hu Y, Gu JX . Identification of interaction between MEK2 and A-Raf-1 . Biochimica et Biophysica Acta (BBA) - Molecular Cell Research . 1589 . 1 . 71–76 . 2002 . 11909642 . 10.1016/S0167-4889(01)00188-4 . free .
• Yin XL, Chen S, Gu JX . Identification of TH1 as an interaction partner of A-Raf kinase . Molecular and Cellular Biochemistry . 231 . 1–2 . 69–74 . 2002 . 11952167 . 10.1023/A:1014437024129 . 19362635 .
• Yuryev A, Wennogle LP . Novel raf kinase protein-protein interactions found by an exhaustive yeast two-hybrid analysis . Genomics . 81 . 2 . 112–125 . 2003 . 12620389 . 10.1016/S0888-7543(02)00008-3 .
• Liu W, Shen X, Yang Y, Yin X, Xie J, Yan J, Jiang J, Liu W, Wang H, Sun M, Zheng Y, Gu J . Trihydrophobin 1 is a new negative regulator of A-Raf kinase . Journal of Biological Chemistry . 279 . 11 . 10167–10175 . 2004 . 14684750 . 10.1074/jbc.M307994200 . free .

External links

• • PDBe-KB provides an overview of all the structure information available in the PDB for Human Serine/threonine-protein kinase A-Raf