Alternately Spliced NH-terminal Immunoglobulin-like Loop I in the Ectodomain of the Fibroblast Growth Factor (FGF) Receptor 1 Lowers Affinity for both Heparin and FGF-1

doi:10.1074/jbc.270.17.10231

Alternately Spliced NH-terminal Immunoglobulin-like Loop I in the Ectodomain of the Fibroblast Growth Factor (FGF) Receptor 1 Lowers Affinity for both Heparin and FGF-1(*)

From the (1) Albert B. Alkek Institute of Biosciences and Technology, Department of Biochemistry and Biophysics, Texas A & M University, Houston, Texas 77030-3303

^¶ To whom correspondence should be addressed:
Albert B. Alkek Inst. of Biosciences and Technology, Texas A & M University, 2121 W. Holcombe Blvd., Houston, TX 77030-3303;
Tel.: 713-677-7522; Fax: 713-677-7512; E-mail: wmckeeha{at}ibt.tamu.edu.

↵^§ Current address: Howard Hughes Medical Inst., Program in Molecular Medicine, University of Massachusetts Medical Center, 373 Plantation St., Worcester, MA 01605.

Abstract

Alternate splicing of a single exon 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 exon coding for Loop I results in the functional differences between the FGFRα and FGFRβ variants.

Footnotes

↵* This work was supported by NIDDKD Public Health Service Grants DK35310 and DK38639 and NCI Grant CA59971, National Institutes of Health. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
↵¹ The abbreviations used are:

FGFR

fibroblast growth factor receptor

FGF

fibroblast growth factor

PCR

polymerase chain reaction

bp

base pair(s)

PBS

phosphate-buffered saline (pH 7.0).
↵²M. Kan and R. Owens, unpublished results.
↵³M. Kan and W. L. McKeehan, unpublished results.