High-affinity binding and activation of a truncated FGF receptor by both aFGF and bFGF

We recently reported the cloning and overexpression of full-length forms of human fibroblast growth factor (FGF) receptors, bek and flg. These receptors contain three immunoglobulin (Ig)-like domains and an unusual acidic motif in the extracellular region, a single transmembrane segment and a protein tyrosine kinase cytoplasmic domain containing a 14 amino acid insert. Each of the related full-length gene products interacts at high affinity with both acidic FGF and basic FGF. We now report the isolation of cDNA clones encoding two variant forms of human bek. One variant form encodes a potentially secreted bek protein containing only the first Ig-like domain and acidic motif, whereas the other variant includes all of the full-length bek protein except the first Ig-like domain and acidic motif. Overexpression of the latter bek form in NIH3T3 cells has been used to demonstrate that the N-terminal Ig-like domain and acidic region are not required for binding or activation of bek by aFGF or bFGF.