C-terminal Tail of FGF19 Determines Its Specificity toward Klotho Co-receptors

FGF19 subfamily proteins (FGF19, FGF21, and FGF23) are unique members of fibroblast growth factors (FGFs) that regulate energy, bile acid, glucose, lipid, phosphate, and vitamin D homeostasis in an endocrine fashion. Their activities require the presence of alpha or beta Klotho, two related single-pass transmembrane proteins, as co-receptors in relevant target tissues. We previously showed that FGF19 can bind to both alpha and beta Klotho, whereas FGF21 and FGF23 can bind only to either beta Klotho or alpha Klotho, respectively in vitro. To determine the mechanism regulating the binding and specificity among FGF19 subfamily members to Klotho family proteins, chimeric proteins between FGF19 subfamily members or chimeric proteins between Klotho family members were constructed to probe the interaction between those two families. Our results showed that a chimera of FGF19 with the FGF21 C-terminal tail interacts only with beta Klotho and a chimera with the FGF23 C-terminal tail interacts only with alpha Klotho. FGF signaling assays also reflected the change of specificity we observed for the chimeras. These results identified the C-terminal tail of FGF19 as a region necessary for its recognition of Klotho family proteins. In addition, chimeras between alpha and beta Klotho were also generated to probe the regions in Klotho proteins that are important for signaling by this FGF subfamily. Both FGF23 and FGF21 require intact alpha or beta Klotho for signaling, respectively, whereas FGF19 can signal through a Klotho chimera consisting of the N terminus of alpha Klotho and the C terminus of beta Klotho. Our results provide the first glimpse of the regions that regulate the binding specificity between this unique family of FGFs and their co-receptors.