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Legend: BMP signaling in Drosophila Three different BMP ligand species exist in the early embryo. (A) Dpp homodimers, (B) Dpp/Scw heterodimers, and (C) Scw homodimers. (B) Dpp/Scw heterodimers are preferentially transported to the dorsal midline through the action of Sog/Tsg and Tld. At the dorsal midline, the heterodimer accumulates and is free for signaling as Tld processes Sog. This heterodimer binds to a heteromeric receptor complex, probably a tetramer located in the plasma membrane (PM), composed of two type II receptors (Punt), and one subunit each of the type I receptors Tkv and Sax. Punt activates Tkv and Sax by phosphorylating residues within their GS boxes (a glycine-serine rich segment near the membrane). Once activated, the type I receptors phosphorylate Mad. Mad then associates with the co-Smad Medea (probably in a trimeric complex
of uncharacterized subunit composition), and the complex translocates to the nucleus where it binds to and activates or represses target genes in conjunction with other transcription factors (TFs). At the midline, the Sax and Tkv receptors produce a synergistic signal that
results in the activation of high-threshold target genes, such as
race. In the lateral regions, homodimers of Scw and Dpp produce moderate and low levels signals, respectively, that can activate low-threshold response genes, such as pannier (pnr) (O'Connor 2006).
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