| A‑Raf (ARAF) is a serine/threonine protein kinase of the Raf family that shares the canonical Ras–Raf–MEK–ERK organization with B‑Raf and C‑Raf but is distinguished by low MEK kinase activity and prominent kinase‑independent scaffolding functions that modulate apoptosis, metabolism and endocytic trafficking. The protein displays the conserved Raf domain architecture with an N‑terminal region containing a Ras‑binding domain and cysteine‑rich domain that cooperate to bind Ras‑GTP and phospholipid membranes, a central serine/threonine‑rich regulatory segment harboring phosphorylation sites and 14‑3‑3 binding motifs that control autoinhibition and activation, and a C‑terminal catalytic domain with the bilobal kinase fold responsible for MEK phosphorylation when A‑Raf is engaged in MAPK signaling. Within the MAPK cascade, A‑Raf translocates from cytosol to the plasma membrane in response to activated Ras, contributes to MEK activation in side‑by‑side Raf dimers, and participates in transmitting mitogenic and differentiation cues; however, comparative studies show that A‑Raf has the lowest MEK‑directed kinase activity among Raf isoforms, and emerging data indicate that it often acts as a modulator that tempers ERK output rather than serving as the dominant MEK activator. Beyond its classical role, A‑Raf binds directly to the proapoptotic kinase MST2 and inhibits MST2‑mediated signaling independently of its own kinase activity, establishing a noncanonical anti‑apoptotic function in multiple cancer entities and positioning A‑Raf as a regulator of the MST2 tumor suppressor pathway. This MST2 interaction is controlled by subcellular localization: in proliferating normal and tumor cells A‑Raf localizes mainly to mitochondria, where it efficiently sequesters MST2, whereas in differentiated epithelial cells A‑Raf is redistributed to the plasma membrane, loses its ability to retain MST2, and thereby permits MST2 activation and apoptosis, with KSR2 acting as a scaffold that governs A‑Raf compartmentalization during epithelial differentiation. A‑Raf also associates with metabolic regulators: interaction with pyruvate kinase M2 (PKM2) promotes a conformational shift toward the tetrameric, high‑activity state, linking A‑Raf to control of glycolytic flux in cancer cells and to broader regulation of energy metabolism. Additional work implicates A‑Raf in endocytic membrane trafficking, where Ras and receptor tyrosine kinase–driven recruitment of A‑Raf to phosphatidylinositol 4,5‑bisphosphate–rich membranes allows signaling to endosomes and activation of ARF6, a small GTPase that regulates endocytosis, further extending A‑Raf’s role as a scaffold at membrane trafficking hubs. In cancer biology, A‑Raf contributes to tumorigenesis through combined suppression of MST2-dependent apoptosis and modulation of ERK signaling: full‑length A‑Raf maintained by hnRNP H–controlled splicing inhibits MST2, while changes in A‑Raf expression or localization influence ERK activation and can participate in resistance to Raf inhibitors by reshaping Raf dimer composition and downstream MEK–ERK signaling. |
