Human Activin B MAb (Clone 146807)

Activins and inhibins, members of the TGF-beta superfamily, are disulfide-linked dimeric proteins that were originally purified from gonadal fluids as proteins that stimulated or inhibited, respectively, pituitary follicle stimulating hormone (FSH) release. These proteins have since been shown to have a wide range of biological activities including: mesoderm induction, neural cell differentiation, bone remodeling, hematopoiesis and reproductive physiology. Activins/inhibins are produced as precursor proteins with an amino-terminal propeptide that is cleaved to release the carboxy-terminal bioactive ligands. Activins are homodimers or heterodimers of the various beta subunit isoforms, while inhibins are heterodimers of a unique alpha  subunit and one of the various beta  subunits. Five beta subunits (mammalian beta _A, beta _B, beta _C, beta _E and  Xenopus beta _D) have been cloned. The activin/inhibin nomenclature reflects the subunit composition of the proteins: activin A ( beta _A­‑ beta _A), activin B ( beta _B­‑ beta _B), activin AB ( beta _A‑ beta _B), inhibin A ( alpha ­‑ beta _A) and inhibin B ( alpha ­‑ beta _B). At present, little is known about the contribution of the other beta  subunits to activin or inhibin formation and biology. At the amino acid sequence level, the mature human beta B subunit is greater than 98% identical to mouse beta B, while the human and mouse alpha  subunits share approximately 80% identity. Similarly to other TGF-beta  family members, activins exert their biological activities through binding to the heterodimeric complex composed of two membrane spanning serine-threonine kinases designated as type I and type II. Two forms of activin receptor type I (Act RI-A and Act RI-B) and two forms of activin receptor type II (Act RII-A and Act RII-B) have been identified. Activin binds directly to Act RII, the complex then associates with Act RI and initiates signaling. Besides activins, Act RII has been shown to bind certain other TGF-beta superfamily members. Inhibin A has been shown to bind with low-affinity to Act RII. The existence of a distinct inhibin-specific receptor and/or signal transduction pathway has been hypothesized.