THE 2ND IMMUNOGLOBULIN-LIKE DOMAIN OF THE VEGF TYROSINE KINASE RECEPTOR FLT-1 DETERMINES LIGAND-BINDING AND MAY INITIATE A SIGNAL-TRANSDUCTION CASCADE

Vascular endothelial growth factor (VEGF) is an angiogenic inducer tha t mediates its effects through two high affinity receptor tyrosine kin ases, Flt-1 and KDR. Flt-1 is required for endothelial cell morphogene sis whereas KDR is involved primarily in mitogenesis, Flt-1 has an alt ernative ligand, placenta growth factor (PIGF). Both Flt-1 and KDR hav e seven immunoglobulin (Ig)-like domains in the extracellular domain. The significance and function of these domains for ligand binding and receptor activation are unknown. Here we show that deletion of the sec ond domain of Flt-1 completely abolishes the binding of VEGF. Introduc tion of the second domain of KDR into an Flt-1 mutant lacking the homo logous domain restored VEGF binding, However, the ligand specificity w as characteristic of the KDR receptor. We then created chimeric recept ors where the first three or just the second Ig-like domains of Flt-1 replaced the corresponding domains in Flt-4, a receptor that does not bind VEGF, and analyzed their ability to bind VEGF. Both swaps conferr ed upon Flt-4 the ability to bind VEGF with an affinity nearly identic al to that of wildtype Flt-1. Furthermore, transfected cells expressin g these chimeric Flt-4 receptors exhibited increased DNA synthesis in response to VEGF or PIGF. These results demonstrate that a single Ig-l ike domain is the major determinant for VEGF-PIGF interaction and that binding to this domain may initiate a signal transduction cascade.