Inhibins and activins are protein hormones that reciprocally modulate a
diversity of regulatory pathways. Competitive binding experiments
revealed that betaglycan, the type III TGF-.beta. receptor, also
functions as an inhibin receptor. Betaglycan augments the binding of
inhibin to the ActRII activin receptor. By augmenting inhibin binding to
ActRII, betaglycan effectively sequesters ActRII away from activin and
thereby reduces activin signaling. In addition, the ActRII-betaglycan
complex may generate novel signals distinct from those initiated by
activin signaling via ActRII and ALK4. Betaglycan is produced in discrete
nuclei of the rat brain and by specific cell types within the adult rat
pituitary, testis, and ovary. The presence of betaglycan within
inhibin-responsive tissues and cell types, together with the ability of
this protoglycan to bind inhibin and to confer inhibin sensitivity, is
consistent with a role of betaglycan as an inhibin-specific receptor
mediating inhibin responses within various tissues.