O-GlcNAcylation Dampens Dpp/BMP Signaling to Ensure Proper Drosophila Embryonic Development

BMP (bone morphogenetic protein) signaling activity is precisely controlled by both pathway agonists and antagonists. Here, we identify a previously unrecognized BMP signaling antagonist. We demonstrate that the Drosophila BMP type I receptor Sax (Saxophone) functions as a Dpp (Decapentaplegic) receptor in Drosophila embryos, but that its activity is normally inhibited by the O-linked glycosyltransferase Sxc (Super sex combs). In wild-type embryos, Sax activity is inhibited, and the BMP type I receptor Tkv (Thickveins) is the sole conduit for Dpp. In contrast, in sxc mutants, the Dpp signal is transduced by both Tkv and Sax, and elevated Dpp signaling results in embryonic lethality. We also demonstrate that Sxc O-glycosylates Sax and observe elevated Dpp signaling in response to maternal restriction of dietary sugar. These findings link fertility to nutritive environment and point to Sax signaling as the nutrient-sensitive branch of BMP signaling.