Extended Data Figure 9: The FGF19 and FGF21 co-receptor β-klotho is a non-enzymatic scaffold protein analogous to α-klotho.

Structure-based sequence alignment of α-klotho and β-klotho. The locations of the eight alternating β-strands and α-helices of the TIM fold are indicated above the alignment. Cyan, blue and yellow bars below the alignment mark the domain boundaries of KL1, KL2 and the KL1–KL2 linker. Asterisks denote sequence identity and dots denote sequence similarity. Scissor symbols mark the four proposed sites of α-klotho cleavage by ADAM proteases/secretases. Cleavage 1, which coincides with the end of the rigid core of KL2, results in shedding of the entire α-klotho ectodomain from the cell membrane. Although this cleavage product is a functional co-receptor, the α-klotho fragments generated by cleavages 2, 3 and 4 would be devoid of co-receptor activity. Black triangle denotes the site where alternative splicing replaces the C-terminal KL2 sequence with a 15-residue-long unrelated sequence. Glycan chain symbols denote seven predicted N-linked glycosylation sites. Zn²⁺-chelating residues of α-klotho are green, FGFR1c-binding residues are light purple, and FGF23-binding residues are red. Light purple box denotes β1α1 loop sequence in KL2 termed RBA. β-Klotho RBA is about as long as α-klotho RBA, and key FGFR-binding residues are conserved between these two RBAs, which is consistent with the similar FGFR-binding specificity of α-klotho and β-klotho^9,11,12. But α-klotho residues in the binding pockets for the FGF23 C-terminal tail are not conserved in β-klotho, conforming to major sequence differences between the C-terminal tails of FGF23, FGF19 and FGF21 (Extended Data Fig. 10a).