Substitution of putative half-cystine residues in heparin-binding fibroblast growth factor receptors: Loss of binding activity in both two and three loop isoforms

Alternate use of an exon coding for an 89-residue NH₂ terminal immunoglobulin-like disulfide loop results in isoforms of the heparin-binding fibroblast growth factor receptor (FGF-R) with three (FGF-Rα) and two (FGF-Rβ) Ig-like loops in the extracellular domain. Both FGF-Rα and FGF-Rβ isoforms exhibit qualitatively similar ligand-binding activities. In this report, we show by site-directed mutagenesis and analysis of ligand-binding activity in transfected cells that substitution of a cysteine that potentially forms an intra-loop disulfide in either juxtamembrane Loop II or III disrupted maturation and formation of the ligand-binding site in both FGF-Rα and FGF-Rβ isoforms. Neither three loop FGF-Rα constructions coding for intact Loops I and II adjacent to defective Loop III nor intact Loops I and III separated by defective Loop II exhibited ligand-binding activity. In addition, a two-loop molecule of tandem Loops I and III was inactive. The results suggest that single Loops I, II, or III of FGF-R are insufficient to form a ligand-binding site. Loop I does not form an independent ligand-binding site with either Loop II or III, but interacts with a common ligand-binding site formed by Loops II and III (Xu, J., Nakahara, M., Crabb, J. W., Shi, E., Matuo, Y., Fraser, M., Kan, M., Hou, J., and McKeehan, W. L. (1992) J. Biol. Chem. 267, 17792-17803, 1992).