| TGF‑β receptor II (TGFBR2) is a transmembrane serine/threonine kinase of the TGF‑β receptor subfamily that forms, together with TGF‑β receptor I (TGFBR1), the high‑affinity signaling complex for the dimeric cytokines TGF‑β1, TGF‑β2 and TGF‑β3, and acts as the constitutively active upstream kinase that initiates canonical and non‑canonical TGF‑β signaling cascades. The receptor has an N‑terminal extracellular ligand‑binding ectodomain with a compact three‑finger toxin–like fold composed of nine β‑strands and a single helix stabilized by six intrachain disulfide bonds, followed by a transmembrane segment and a C‑terminal cytoplasmic kinase domain, and high‑resolution crystal structures show that the elongated first “finger” and specific surface charge and hydrophobic patterns shape picomolar‑affinity binding sites for the TGF‑β ligand dimer. Ligand engagement drives assembly of a heterotetrameric signaling complex containing two TGFBR2 and two TGFBR1 molecules symmetrically bound to the cytokine dimer; within this complex the constitutively active TGFBR2 kinase trans‑phosphorylates the GS domain of TGFBR1, switching on its catalytic activity and thereby triggering downstream signaling. Canonical signaling proceeds when activated TGFBR1 phosphorylates receptor‑regulated Smads (R‑Smads) such as Smad2 and Smad3 on C‑terminal SSXS motifs, promoting dissociation from the receptor and association with the common mediator Smad4; the Smad2/3–Smad4 complex then accumulates in the nucleus, where it binds DNA and cooperates with other transcription factors to regulate expression of TGF‑β‑responsive genes involved in cell‑cycle arrest, differentiation, extracellular matrix production, immunosuppression and tissue repair. In addition to this Smad‑dependent pathway, TGFBR2‑containing complexes can activate non‑canonical routes, including p38 MAPK, ERK, JNK, Rho GTPases and PI3K/AKT signaling, providing alternative outputs that modulate cytoskeletal organization, migration, survival and metabolic responses depending on cell type and context. Isoform diversity of TGFBR2, generated by alternative splicing, includes variants that differ in extracellular or cytoplasmic segments; reported isoforms can bind TGF‑β ligands with high affinity and either support full Smad activation or act as ligand traps that sequester TGF‑β1, TGF‑β2 and TGF‑β3 without permitting Smad2/3 phosphorylation, thereby dampening canonical signaling at the receptor level. Mutations in TGFBR2 are frequent in microsatellite‑unstable colorectal and gastric cancers and have been associated with loss of tumor‑suppressive TGF‑β responses, while germline TGFBR2 variants underlie heritable connective tissue and vascular disorders such as Loeys–Dietz aortic aneurysm syndrome and are linked to Marfan‑like phenotypes and hereditary hemorrhagic telangiectasia, reflecting the receptor’s central role in vascular and extracellular matrix homeostasis. |
