FST / Follistatin Protein Description
Follistatin, also known as Activin-binding protein and FST, is a secreted protein. Follistatin / FST is an autocrine glycoprotein that is expressed in nearly all tissues of higher animals. Follistatin / FST contains 3 follistatin-like domains, 3 Kazal-like domains and 1 TB (TGF-beta binding) domain. Follistatin / FST is produced by folliculostellate (FS) cells of the anterior pituitary. FS cells make numerous contacts with the classical endocrine cells of the anterior pituitary including gonadotrophs. Isoform 1 of Follistatin / FST is the predominant isoform in serum but is undetectable in follicular fluid. Follistatin / FST is a specific inhibitor of the biosynthesis and secretion of pituitary follicle stimulating hormone (FSH). Follistatin / FST binds directly to activin and functions as an activin antagonist. Follistatin / FST was initially isolated from follicular fluid and was identified as a protein fraction that inhibited follicle-stimulating hormone (FSH) secretion from the anterior pituitary, and so was known as FSH-suppressing protein (FSP). Follistatin / FST and BMPs are known to play a role in folliculogenesis within the ovary. The main role of Follistatin / FST in the oestrus/menstrus ovary appears to be progression of the follicle from early antral to antral/dominant, and importantly the promotion of cellular differentiation of the estrogen producing granulosa cells (GC) of the dominant follicle into the progesterone producing large lutein cells (LLC) of the corpus luteum.