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Activins, members of the TGF-beta superfamily, are disulfide-linked dimeric proteins originally purified from gonadal fluids as proteins that stimulated pituitary follicle stimulating hormone (FSH) release. Activin proteins have a wide range of biological activities, including mesoderm induction, neural cell differentiation, bone remodeling, hematopoiesis and roles in reproductive physiology. Activin isoforms and other members of the TGF-beta superfamily exert their biological effects by binding to heteromeric complexes of a type I and a type II serine-threonine kinase receptor, both of which are essential for signal transduction.
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 to date. 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).