ARAF
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 : This N-terminal region contains the Ras-binding domain and the cysteine-rich domain. The RBD facilitates interaction with activated Ras-GTP, anchoring A-Raf to the plasma membrane. The CRD, characterized by its zinc-binding motif, contributes to membrane association and protein-protein interactions Structural studies confirm the RBD and CRD function as a single entity during Ras binding.
- CR2 : Positioned between CR1 and CR3, CR2 is a serine/threonine-rich regulatory segment containing phosphorylation sites that modulate A-Raf's activity and interactions with 14-3-3 proteins. This region is critical for autoinhibition and activation dynamics.
- CR3 : 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. Structural analyses reveal similarities to tyrosine kinase-like group members
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.Among the Raf isoforms, A-Raf exhibits the lowest kinase activity toward MEK proteins. This may be due to amino acid substitutions in a negatively charged region upstream of the kinase domain, which result in low basal activity.
A-Raf is also the only Raf kinase known to be regulated by steroid hormones. 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.
A-Raf also regulates energy metabolism by interacting with pyruvate kinase M2, 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.
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 -rich membranes and signals to endosomes, leading to activation of ARF6, a key regulator of endocytosis.
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.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.
Because A-Raf modulates both apoptosis and metabolism—two critical hallmarks of cancer—it may represent a potential target for future cancer therapies.