Biology:ARAF

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Short description: Protein-coding gene in humans
For other uses, see Araf (disambiguation).

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

Serine/threonine-protein kinase A-Raf, or simply A-Raf, is an enzyme that in humans is encoded by the ARAF gene.[1] It belongs to the Raf kinase family of serine/threonine-specific protein kinases, which also includes Raf-1 and B-Raf.[2] 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.[3] The CRD, characterized by its zinc-binding motif, contributes to membrane association and protein-protein interactions[4] Structural studies confirm the RBD and CRD function as a single entity during Ras binding.[5]
  • 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.[6] This region is critical for autoinhibition and activation dynamics.[7]
  • 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.[8] Structural analyses reveal similarities to tyrosine kinase-like (TKL) group members[9]

The RBD adopts a ubiquitin-like fold critical for Ras-GTP interaction.[10], while the CRD's zinc-binding motif stabilizes membrane association.[11] A-Raf's activity is regulated by phosphorylation-dependent 14-3-3 binding.[12] and isoform dimerization, which is essential for MAPK pathway activation.[13][14]

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.[15]

Among the Raf isoforms, A-Raf exhibits the lowest kinase activity toward MEK proteins.[16] 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.[17]

A-Raf is also the only Raf kinase known to be regulated by steroid hormones.[18] 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.[19]

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.[20]

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.[21]

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.[22]

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.[23]

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:

References

  1. ↑ "Entrez Gene: ARAF V-raf murine sarcoma 3611 viral oncogene homolog". https://www.ncbi.nlm.nih.gov/gene?Db=gene&Cmd=ShowDetailView&TermToSearch=369. 
  2. ↑ "Pks, a raf-related sequence in humans". Proceedings of the National Academy of Sciences of the United States of America 83 (17): 6312–6316. September 1986. doi:10.1073/pnas.83.17.6312. PMID 3529082. Bibcode: 1986PNAS...83.6312M. 
  3. ↑ "KRAS interaction with RAF1 RAS-binding domain and cysteine-rich domain provides insights into RAS-mediated RAF activation". Nature Communications 12 (1). February 2021. doi:10.1038/s41467-021-21422-x. PMID 33608534. Bibcode: 2021NatCo..12.1176T. 
  4. ↑ "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. doi:10.1073/pnas.93.16.8312. PMID 8710867. Bibcode: 1996PNAS...93.8312M. 
  5. ↑ "KRAS interaction with RAF1 RAS-binding domain and cysteine-rich domain provides insights into RAS-mediated RAF activation". Nature Communications 12 (1). February 2021. doi:10.1038/s41467-021-21422-x. PMID 33608534. Bibcode: 2021NatCo..12.1176T. 
  6. ↑ "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. doi:10.1128/MCB.22.14.4984-4996.2002. PMID 12077328. 
  7. ↑ "Exact and approximate rebinning algorithms for 3-D PET data". IEEE Transactions on Medical Imaging 16 (2): 145–158. April 1997. doi:10.1109/42.563660. PMID 9101324. 
  8. ↑ "Exact and approximate rebinning algorithms for 3-D PET data". IEEE Transactions on Medical Imaging 16 (2): 145–158. April 1997. doi:10.1109/42.563660. PMID 9101324. 
  9. ↑ "ALK receptor tyrosine kinase promotes cell growth and neurite outgrowth". Journal of Cell Science 117 (Pt 15): 3319–3329. July 2004. doi:10.1242/jcs.01183. PMID 15226403. 
  10. ↑ "KRAS interaction with RAF1 RAS-binding domain and cysteine-rich domain provides insights into RAS-mediated RAF activation". Nature Communications 12 (1). February 2021. doi:10.1038/s41467-021-21422-x. PMID 33608534. Bibcode: 2021NatCo..12.1176T. 
  11. ↑ "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. doi:10.1073/pnas.93.16.8312. PMID 8710867. Bibcode: 1996PNAS...93.8312M. 
  12. ↑ "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. doi:10.1128/MCB.22.14.4984-4996.2002. PMID 12077328. 
  13. ↑ "Overseas doctors must not be used just to fill rota gaps, says leading consultant". BMJ 361. June 2018. doi:10.1136/bmj.k2654. PMID 29907696. 
  14. ↑ "Regulation and role of Raf-1/B-Raf heterodimerization". Molecular and Cellular Biology 26 (6): 2262–2272. March 2006. doi:10.1128/MCB.26.6.2262-2272.2006. PMID 16508002. 
  15. ↑ "A-Raf and Raf-1 work together to influence transient ERK phosphorylation and Gl/S cell cycle progression" (in en). Oncogene 24 (33): 5207–5217. 2005-04-25. doi:10.1038/sj.onc.1208707. ISSN 0950-9232. PMID 15856007. 
  16. ↑ "Raf Family Kinases Old Dogs Have Learned New Tricks" (in en). Genes & Cancer 2 (3): 232–260. 2011-03-01. doi:10.1177/1947601911407323. ISSN 1947-6019. PMID 21779496. 
  17. ↑ "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. doi:10.1074/jbc.M702429200. ISSN 0021-9258. PMID 17613527. 
  18. ↑ "Regulation of A-raf expression". Oncogene 12 (8): 1669–1677. 1996-04-18. ISSN 0950-9232. PMID 8622887. 
  19. ↑ "Heterogeneous Nuclear Ribonucleoprotein H Blocks MST2-Mediated Apoptosis in Cancer Cells by Regulating a-raf Transcription" (in en). Cancer Research 70 (4): 1679–1688. 2010-02-15. doi:10.1158/0008-5472.CAN-09-2740. ISSN 0008-5472. PMID 20145135. 
  20. ↑ "Regulation of Pyruvate Kinase Type M2 by A-Raf: A Possible Glycolytic Stop or Go Mechanism" (in en). Anticancer Research 27 (6B): 3963–3971. 2007-11-01. ISSN 0250-7005. PMID 18225557. http://ar.iiarjournals.org/content/27/6B/3963. 
  21. ↑ "A-RAF Kinase Functions in ARF6 Regulated Endocytic Membrane Traffic". PLOS ONE 4 (2). 2009-02-27. doi:10.1371/journal.pone.0004647. ISSN 1932-6203. PMID 19247477. Bibcode: 2009PLoSO...4.4647N. 
  22. ↑ "Heterogeneous Nuclear Ribonucleoprotein H Blocks MST2-Mediated Apoptosis in Cancer Cells by Regulating a-raf Transcription" (in en). Cancer Research 70 (4): 1679–1688. 2010-02-15. doi:10.1158/0008-5472.CAN-09-2740. ISSN 0008-5472. PMID 20145135. 
  23. ↑ "The M2 splice isoform of pyruvate kinase is important for cancer metabolism and tumour growth" (in en). Nature 452 (7184): 230–233. 2008-03-13. doi:10.1038/nature06734. ISSN 0028-0836. PMID 18337823. Bibcode: 2008Natur.452..230C. 
  24. ↑ 24.0 24.1 24.2 24.3 24.4 "Novel raf kinase protein-protein interactions found by an exhaustive yeast two-hybrid analysis". Genomics 81 (2): 112–125. February 2003. doi:10.1016/S0888-7543(02)00008-3. PMID 12620389. 
  25. ↑ "Identification of interaction between MEK2 and A-Raf-1". Biochimica et Biophysica Acta (BBA) - Molecular Cell Research 1589 (1): 71–76. February 2002. doi:10.1016/S0167-4889(01)00188-4. PMID 11909642. 
  26. ↑ 26.0 26.1 26.2 "Isoform-Specific Localization of A-RAF in Mitochondria". Molecular and Cellular Biology 20 (13): 4870–4878. July 2000. doi:10.1128/MCB.20.13.4870-4878.2000. PMID 10848612. 
  27. ↑ "Identification of TH1 as an interaction partner of A-Raf kinase". Molecular and Cellular Biochemistry 231 (1–2): 69–74. February 2002. doi:10.1023/A:1014437024129. PMID 11952167. 

Further reading

  • Human ARAF genome location and ARAF gene details page in the UCSC Genome Browser.
  • PDBe-KB provides an overview of all the structure information available in the PDB for Human Serine/threonine-protein kinase A-Raf



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