The interaction between two TGF-β type I receptors plays important roles in ligand binding, SMAD activation, and gradient formation

The goal of this report is to elucidate the contributions of the Drosophila TGF-β type I receptors TKV and SAX to the activity gradient formed by the two BMP family members DPP and GBB that play important roles in growth and patterning of imaginal discs. Binding studies display preferential interactions of DPP and GBB with homodimers of TKV or SAX, respectively, but also low affinities of both ligands to heterodimers. Inside the cell, constitutively activated forms of both TKV and SAX can ectopically phosphorylate the SMAD transcription factor MAD. However, MAD phosphorylated by homodimers of activated SAX or certain mutant forms of TKV localizes to the nucleus without changing the expression of downstream genes. Differences in signaling between SAX and TKV can be localized to amino acid residues within an area that has been shown to influence complexes formation between type I and type II receptors. The finding that the type II receptor PUT but not activated forms of SAX can enhance signaling of a pseudo-activated MAD-SDVD, which cannot be phosphorylated at the C-terminus, suggests a model, where activation of SMADs requires the presence of type II receptors and a second activation step in addition to C-terminal phosphorylation. Complete activation of MAD can only occur in tetrameric complexes of type II receptors in combination with SAX-TKV heterodimers or TKV homodimers but not SAX homodimers. Since TKV is not distributed equally in wing discs, heterodimers of SAX and TKV play an important role in extending the BMP activity gradient by facilitating DPP diffusion and assisting GBB signaling through functional complexes with type II receptors.