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Fig. 1. 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 midline through the action of Sog/Tsg
and Tld. At the 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) (for details, see
Shimmi et al., 2005b).
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