Background:

Activin B is a TGF-β family member that exhibits a wide range of biological activities including regulation of embryogenesis, osteogenesis, hematopoiesis, reproductive physiology and hormone secretion from the hypothalamic, pituitary and gonadal glands. Activin B, like certain other members of the TGF-β family, signals through the ActRII receptor (Activin Receptor type II).

Activins are homodimers or heterodimers of the various β subunit isoforms, while inhibins are heterodimers of a unique α subunit and one of the various β subunits. Five β subunits (mammalian βA, βB, βC, βE and Xenopus βD) have been cloned to date. The activin/inhibin nomenclature reflects the subunit composition of the proteins: activin A (βA - βA), activin B (βB - βB), activin AB (βA - βB), inhibin A (α - βA) and inhibin B (α - βB). At present, little is known about the contribution of the other β subunits to activin or inhibin formation and biology. At the amino acid sequence level, the mature human βB subunit is greater than 98% identical to mouse βB, while the human and mouse α subunits share approximately 80% identity. Similarly to other TGF-β family members, activins exert their biological activities through binding to the heterodimeric complex composed of two membrane spanning serine-threonine kinases designated 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-β 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.