Stepwise release of Activin-A from its inhibitory prodomain is modulated by cysteines and requires furin coexpression to promote melanoma growth

The Activin-A precursor dimer can be cleaved by furin, but how this proteolytic maturation is regulated in vivo and how it facilitates access to signaling receptors is unclear. Here, analysis in a syngeneic melanoma grafting model shows that without furin coexpression, Activin-A failed to accelerate tumor growth, correlating with failure of one or both subunits to undergo cleavage in signal-sending cells, even though compensatory processing by host cells nonetheless sustained elevated circulating Activin-A levels. In reporter assays, furin-independent cleavage of one subunit enabled juxtacrine Activin-A signaling, whereas completion of proteolytic maturation by coexpressed furin or by recipient cells stimulated contact-independent activity, crosstalk with BMP receptors, and signal inhibition by follistatin. Mechanistically, Activin-A processing was modulated by allosteric disulfide bonds flanking the furin site. Disruption of these disulfide linkages with the prodomain enabled Activin-A binding to cognate type II receptors independently of proteolytic maturation. Stepwise proteolytic maturation is a novel mechanism to control Activin-A protein interactions and signaling.