Interpretation of a BMP activity gradient in Drosophila embryos depends on synergistic signaling by two type I receptors, SAX and TKV

Graded activity of the DPP signaling pathway is critical for specification of dorsal embryonic cell fates in Drosophila. We present evidence that a second BMP ligand, SCW, potentiates DPP activity. Using dominant-negative forms of the type I receptors SAX and TKV, we demonstrate that SAX mediates the SCW signal, white TKV is required for both DPP and SCW activity. We find that while DPP/TKV signaling is obligatorily required, SCW/SAX activity is necessary but not sufficient for dorsal patterning. SAX and TKV act synergistically, suggesting a mechanism for integration of the SCW and DPP signals. Further, we show that the extracellular protein SOG can antagonize SCW, thus limiting its ability to augment DPP signaling in a graded manner.