Inhibins and activins (1, 2) belong to a group of structurally related proteins that includes transforming growth factor β (TGF-β), Müllerian inhibiting substance (MIS) and bone morphogenetic proteins (BMPs) (reviewed in Ref. 3). Ovarian granulosa cells and testicular Sertoli cells are the main source of circulating inhibins. Inhibins are composed of an α-subunit and one of two β-subunits (αA or βB), joined by disulfide bonds. Both inhibin A (αβA) and B (αβB) preferentially inhibit FSH release from the pituitary, but dimers of the β-subunits (activins) increase pituitary FSH secretion. Activin A is a homodimer ofβA-subunits (βAβA), while activin B is a homodimer of βB-subunits (βBβB), and activin AB a heterodimer of β-subunits (βAβB). Each of the subunits (α, βA, βB) is encoded by separate genes, and the primary translation products are processed further before they join to form mature inhibins or activins.
Follistatin (also known as FSH-suppressing protein) is a