Identification of the extracellular domains of Flt-1 that mediate ligand interactions.

Vascular Endothelial Growth Factor (VEGF) mediates its actions through the Flt-1 and KDR(Flk-1) receptor tyrosine kinases. To localize the extracellular region of Flt-1 that is involved in ligand interactions, we prepared secreted fusion proteins between various combinations of its seven extracellular IgG-like folds. Ligand binding studies show that in combination, domains one and two (amino acids 1-234) are sufficient to achieve VEGF165 interactions. Either domain alone is insufficient to achieve this effect. However, Scatchard analysis reveals that despite the binding capabilities of this construct, the Kd is five fold lower than ligand binding to the full extracellular domain. We find that addition of domain three to this minimal site restores high affinity receptor binding. Further, we show that domains one and two are sufficient to achieve interactions of Flt-1 with Placental Growth Factor (PIGF-1).