N-linked oligosaccharides direct the differential assembly and secretion of inhibin α- and β_A-subunit dimers

The biosynthetic pathway governing inhibin heterodimer (α/β) and activin homodimer (β/β) assembly and secretion from ovarian granulosa cells is not fully understood. Here, we examined the role of inhibin subunit glycosylation in the assembly and secretion of mature inhibin A and activin A. Inhibition of subunit glycosylation by tunicamycin treatment of α-and β_A-expressing CHO cell lines reduced inhibin but not activin secretion. Dimeric inhibin A is preferentially secreted from parental isogenic wild-type (wt) cell lines (α^wtβ^wt). Mutation of a single glycosylation site at asparagine 268 (α ^Δ268β^wt) reduces inhibin secretion by 78% and permits β/β assembly and secretion. Conversely, gain of a glycosylation (GOG) site in the analogous region of the β_A- subunit (α_wtβ_GOG327) enhances inhibin A secretion. The present study demonstrates that N-linked glycan sites direct heterodimer vs. homodimer assembly, and prevention of glycosylation abrogates inhibin secretion. These data support a definitive role for sitespecific N-glycosylation in governing inhibin/activin dimer assembly and secretion.