Molecular mechanisms controlling the biogenesis of the TGF-β signal Vg1

Abstract

The TGF-beta signals Vg1 (Dvr1/Gdf3) and Nodal form heterodimers to induce vertebrate mesendoderm. The Vg1 proprotein is a monomer retained in the endoplasmic reticulum (ER) and is processed and secreted upon heterodimerization with Nodal, but the mechanisms underlying Vg1 biogenesis are largely elusive. Here we clarify the mechanisms underlying Vg1 retention, processing, secretion and signaling, and introduce a Synthetic Processing (SynPro) system that enables the programmed cleavage of ER-resident and extracellular proteins. First, we find that Vg1 can be processed by intra- or extracellular proteases. Second, Vg1 can be processed without Nodal but requires Nodal for secretion and signaling. Third, Vg1-Nodal signaling activity requires Vg1 processing, whereas Nodal can remain unprocessed. Fourth, Vg1 employs exposed cysteines, glycosylated asparagines, and BiP chaperone-binding motifs for monomer retention in the ER. These observations suggest two mechanisms for rapid mesendoderm induction: chaperone-binding motifs help store Vg1 as an inactive but ready-to-heterodimerize monomer in the ER, and the flexibility of Vg1 processing location allows efficient generation of active heterodimers both intra- and extracellularly. These results establish SynPro as a new in vivo processing system and define molecular mechanisms and motifs that facilitate the generation of active TGF-beta heterodimers. Significance: The TGF-beta family members Nodal and Vg1 are the major inducers of mesendoderm formation during vertebrate embryogenesis. We previously established that the Vg1 proprotein is retained in the endoplasmic reticulum (ER), and that Nodal and Vg1 form heterodimers to pattern the early embryo. However, the mechanisms underlying the retention, processing, secretion, and signaling of Vg1 have been unclear. We found two mechanisms that embryos use to efficiently generate active Nodal-Vg1 heterodimers: (1) Vg1 employs its chaperone-binding motifs to ensure its retention as a ready-to-heterodimerize monomer in the ER; (2) Using a newly devised Synthetic Processing (SynPro) System, we found that Vg1 must be processed for signaling to occur, but its processing location is flexible.