Common and specific determinants for fibroblast: Growth factors in the ectodomain of the receptor kinase complex

The assembly and activation of oligomeric complexes of FGF, the transmembra ne receptor kinase (FGFR), and heparan sulfate transmit intracellular signa ls regulating growth and function of cells. An understanding of the structu ral relationships between the three subunits and their redundancy and speci ficity is essential for understanding the ubiquitous FGF signaling system i n health and disease. Previously, we reported that a primary heparin or hep aran sulfate binding site resides in a distinct sequence in immunoglobulin (Ig)-like module II of the three modules of FGFR. Here we report that in th e absence of flanking sequences, isolated Ig module II of FGFR1 supports th e binding of FGF-1, FGF-2, and FGF-7 in respective order of affinity. None of the three FGFs detectably bind Ig module I or the IIIb and IIIc splice v ariants of Ig module LII in the absence of flanking sequences. Ig module I and the C-terminus of Ig module III are dispensable for high-affinity bindi ng of FGF-1, FGF-2, and FGF-7. Alterations in highly conserved Ig module II in the heparin binding domain and substitution of individual sequence doma ins spanning the entire sequence of Ig module IT with those from Ig module I obliterated FGF binding. Addition of a specific number of FGFR sequences to the C-terminus of Ig module II resulted in a gain in affinity for FGF-7. Several site-specific alterations in the C-terminus of full-length FGFR1II Ic, an isoform that otherwise absolutely rejects FGF-7, resulted in gain of FGF-7 binding. These results suggest that a complex of Ig module If and he paran sulfate is the base common active core of the FGFR ectodomain and tha t flanking structural domains modify FGF affinity and determine specificity .