Inhibin Biosynthesis and Activity Are Limited by a Prodomain-Derived Peptide

Gonadal-derived inhibin A and B are essential factors in mammalian reproduction, negatively regulating pituitary production of FSH. Inhibins are synthesized as heterodimers of α- and β-subunits, each comprising an N-terminal pro- and C-terminal mature domain. After dimerization, the inhibin α- and β-subunit prodomains are enzymatically cleaved from the mature domains at consensus RXXR sites (site1). Interestingly, the inhibin α-subunit is a unique TGF-β ligand, comprising a second cleavage site (site2) within its prodomain. Cleavage at site2 in the inhibin α-subunit prodomain releases a 43-amino acid proα-peptide. We aimed to determine the influence of the proα-peptide on inhibin synthesis and bioactivity. Blocking proα-peptide release by silencing cleavage site2 (Arg56-Arg61) inhibited both inhibin A and B synthesis. Ligand blot analysis and solid-phase binding assays indicated that the proα-peptide binds specifically to a mature 30-kDa inhibin (mean Kd 86 nM) but was unable to bind related activins. The proα-peptide suppressed inhibin A and B bioactivity in primary rat pituitary cell cultures. Mechanistically, the proα-peptide blocked inhibin A binding to its coreceptor, betaglycan (IC50 131 nM), and the subsequent sequestration of the activin type II receptor (IC50 156 nM), which underscores inhibin's biological activity. Based on the sequential mutations across the inhibin α-subunit, the proα-peptide binding site was localized to residues Arg341-Thr354, corresponding directly to the betaglycan binding region. Together our findings indicate that the proα-peptide limits the synthesis and bioactivity of inhibins.