Characterization of the Extracellular Domain in Vascular Endothelial Growth Factor Receptor‐1 (Flt‐1 Tyrosine Kinase)

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Abstract
Flt‐1 tyrosine kinase, vascular endothelial growth factor (VEGF) receptor‐1, binds VEGF and a new VEGF‐related ligand, placenta growth factor, but KDR/Flk‐1 (VEGF receptor‐2) binds only VEGF. To characterize the functional regions in the Flt‐1 extracellular domain such as the ligand binding region and the dimer formation of the receptor, we constructed a series of mutants of the Flt‐1 extracellular domain as soluble forms in a baculovirus system. We found that a region carrying the N‐terminal 1st to 3rd immunoglobulin (Ig)‐like domains of Flt‐1 binds both ligands with high affinity. However, for dimer formation of soluble Flt‐1, a region further downstream in the Flt‐1 extracellular domain was required. Mutant Flt‐1 receptors expressed in COS cells confirmed the requirement of the 4th to 7th Ig region for the activation of Flt‐1 tyrosine kinase. Soluble Flt‐1 carrying the N‐terminal 1st to 3rd Ig region suppressed VEGF‐dependent endothelial proliferation in vitro to the same level as the larger forms of soluble Flt‐1, suggesting that the binding of one soluble Flt‐1 molecule to one subunit of the VEGF homodimer may be sufficient to block the VEGF activity.