Structures of Activin and Inhibin Prototypes and Precursors

Activins and inhibins, members of the TGF-β superfamily, are disulfide linked dimeric proteins. These proteins have a wide range of biological activities, including mesoderm induction, neural cell differentiation, bone remodeling, hematopoiesis, and reproductive physiology. Similar to other TGF-β family members, activins exert their biological activities through binding to the heterodimeric complex composed of tow membrane spanning serine-threonine kinases designated type I and type II.

Five β subunits (mammalian βA, βB, βC, βE, and Xenopus βD) have been cloned. 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 the amino acid sequence level, the mature human βA subunit is 100% identical to mouse βA, while the human and mouse α subunits share approximately 95% identity.

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.

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