Scientists have discovered four Fibroblast Growth Factor (FGF) receptors, FGFR1 through FGFR4, which are highly conserved transmembrane proteins containing receptor tyrosine kinases. FGF receptors all comprise an extracellular ligand-binding domain, a transmembrane domain, and a kinase domain. The extracellular ligand-binding domain contains three immunoglobulin-like domains IgI, IgII, and IgIII which are important in receptor dimerisation. FGFR1–3 can alternatively use part of the IgIII domain, generating different isoforms that are expressed in different tissues and have distinct binding specificities; whereas the FGFR4 has only one possible form of its IgIII domain usage. The truncated protein coded by IgIIIa splice variant is secreted, and may play as an inhibitor for release FGFs. A specific FGF ligand can bind to several FGFRs, though it may have a preference for a particular one.
Fig 4. Structure of human FGFR-1
The dashed line indicates the 5' untranslated region (UTR), The asterisks shows the stop codons and arrows denote select locations of exon splicing.
Fig 5. FGF receptors generated by the use of splice variants
Solid oval represents premature truncation and hatched boxes represent alternate c-terminis.