e It has been recently established that Klotho coreceptors associate with fibroblast growth factor (FGF) receptor tyrosine kinases (FGFRs) to enable signaling by endocrine-acting FGFs. However, the molecular interactions leading to FGF-FGFR-Klotho ternary complex formation remain incompletely understood. Here, we show that in contrast to ␣Klotho, ␤Klotho binds its cognate endocrine FGF ligand (FGF19 or FGF21) and FGFR independently through two distinct binding sites. FGF19 and FGF21 use their respective C-terminal tails to bind to a common binding site on ␤Klotho. Importantly, we also show that Klotho coreceptors engage a conserved hydrophobic groove in the immunoglobulin-like domain III (D3) of the "c" splice isoform of FGFR. Intriguingly, this hydrophobic groove is also used by ligands of the paracrine-acting FGF8 subfamily for receptor binding. Based on this binding site overlap, we conclude that while Klotho coreceptors enhance binding affinity of FGFR for endocrine FGFs, they actively suppress binding of FGF8 subfamily ligands to FGFR. Fibroblast growth factor (FGF) signaling plays pleiotropic roles in metazoan development and metabolism (5, 24). Based on sequence homology and phylogenetic and structural considerations, the 18 mammalian FGF ligands are grouped into five paracrine-acting subfamilies and one endocrine-acting subfamily comprising FGF19, FGF21, and FGF23 (24,43). Paracrine FGFs have high affinity for pericellular heparan sulfate (HS) glycosaminoglycans (3, 5) and form distinct morphogenetic gradients in the pericellular matrix (27, 39) to fulfill essential roles during embryonic development (7,24,65). In contrast, endocrine FGFs exhibit poor affinity for HS (3, 15) and thus are able to enter the blood circulation to regulate key metabolic processes, including bile acid homeostasis (18,20,36) and hepatic glucose and protein metabolism (30, 57) (FGF19), glucose and lipid metabolism (4, 21, 29, 58) (FGF21), and vitamin D and phosphate homeostasis (1, 59, 62) (FGF23). These hormone-like FGFs have taken center stage in drug discovery for a number of inherited and acquired metabolic disorders (5, 6).Mammalian FGFs signal through four FGF receptor (FGFR) tyrosine kinases (FGFR1 to FGFR4) and their alternatively spliced isoforms (23, 43). The extracellular domain of a prototypical FGFR consists of three immunoglobulin-like domains (D1 to D3), and structural and biochemical studies have established that the region including D2, D3, and the D2-D3 linker comprises the minimal ligand-binding domain (54,55,63). The ligand-binding specificity of FGFR1 to FGFR3 is primarily regulated by a tissuespecific splicing in D3 of these receptors that generates "b" and "c" isoforms (9,26,42,52). Structural studies have revealed that this splicing alters the primary amino acid sequence of key ligandbinding loops/pockets in D3 (50,74).In addition to mediating paracrine FGF gradient formation in the pericellular matrix, HS is required for enhancing the affinity of paracrine FGF for FGFR and promoting dimerization of ligan...

Genetic and epigenetic analyses of the human 9p21 locus indicate that modifications of ARF occur independently of p16 inactivation in human melanoma and suggest that ARF is more frequently inactivated than p16.