Alternately spliced NH2-terminal immunoglobulin-like loop I in the ectodomain of the fibroblast growth factor (FGF) receptor 1 lowers affinity for both heparin and FGF-1

Abstract

Alternate splicing of a single exert encoding an NH2-terminal immunoglobulin (Ig) disulfide loop in the ectodomain of the fibroblast growth factor receptor (FGFR) types 1 and 2 results in α and β isoforms that exhibit 3- and 2-Ig loops, respectively. Previously we demonstrated that alternately spliced Loop I has no independent ligand binding activity but is sufficiently interactive with the ligand- and heparin-binding site formed by Loops II and III to lower affinity for the same fibroblast growth factor (FGF) ligand. Here we show that a lower affinity of FGFR1α for heparin parallels the lower affinity for FGF-1. A mutant of FGFR1α in which the sequence between Loops I and II was deleted exhibits high affinity for both FGF-1 and heparin and other properties of the FGFR1β isoform, which include resistance to degradation by trypsin and display of specific antibody epitopes. This suggests that the interloop sequence facilitates the interaction of Loop I with Loops II and III. Lack of expression of both exons coding for Loop I and the sequence between Loops I and II in the FGFR2 gene characterizes rat prostate tumor cells, which exhibit a loss of the low affinity class of FGF receptors. Although the exon coding for the sequence between Loops I and II is alternately spliced in the FGFR2β isoform, coordinate expression with the oxen coding for Loop I results in the functional differences between the FGFRα and FGFRβ variants.