Structural characterization of the circulating soluble FGF receptors reveals multiple isoforms generated by secretion and ectodomain shedding

Soluble fibroblast growth factor receptors (FGFRs) have been identified in multiple biological fluids, including blood. Efforts to examine the biological properties of these proteins have been hampered by the incomplete chemical characterization of the receptors within the second half of the third immunoglobulin (Ig)-like domain, where alternative splicing leads to receptor variants with different ligand binding properties. Using mass spectrometry techniques, we have mapped the soluble FGFRs to the secreted receptor, FGFR1(IIIa), the two and three Ig-like domain isoforms of FGFR1(IIIc) and a carboxyl-terminal cleavage peptide from the two and three Ig-like domain isoforms of FGFR1(IIIb). The secreted FGFR is produced by the translation of an alternatively spliced transcript and the cleaved receptors are released by ectodomain shedding of the transmembrane receptors.