The HBS topology of FGF19 is completely different from that of classical, paracrine-acting FGFs. (A) Molecular surface and ribbon representation of the FGF19 crystal structure. The molecular surface is shown as transparent. The β strands of FGF19 are labeled according to the conventional strand nomenclature for FGF1 and FGF2. Note that FGF19 lacks the β11 strand present in classical, paracrine-acting FGFs. The HBS, consisting of the loop between β1 and β2 and the segment between β10 and β12, is in blue. A secondary structure element unique to the HBS of FGF19 is the α11 helix located in the β10-β12 segment. Note that α11 and the β1-β2 loop protrude from the β-trefoil like core domain of FGF19 (orange) and that there is a cleft between these two heparin-binding regions. Cysteines 58 (in β2) and 70 (in β3) form a disulfide bridge which stabilizes the altered conformation of the heparin-binding region between β10 and β12 (explained in more detail below). Sulfur atoms are in green. NT and CT, N and C termini of FGF19. (B) Superimposition of the Cα trace of the FGF19 β-trefoil like core onto the Cα trace of the FGF2 β-trefoil from the FGF2-FGFR1c-heparin structure (PDB ID, 1FQ9). A close-up view of the heparin-binding regions is shown on the right, and to aid the reader, a view of the whole structure is shown on the left. FGF19 and FGF2 are in orange and cyan blue, respectively. Black arrowheads mark leucines 145 and 162, at which the Cα trace of FGF19 diverges from that of FGF2 and converges again, respectively. Note that cysteines 58 and 70 of FGF19 form a disulfide bridge which packs against these two leucine residues, thereby stabilizing the altered conformation of the β10-β12 segment. Also note that there are no intramolecular interactions between the β1-β2 loop and the β10-β12 segment in FGF19 (indicated by a dashed orange line with arrowheads; see also the cleft illustrated in panel A), whereas in FGF2, these regions interact with one another (indicated by a cyan blue line with arrowheads). Black circles denote glycine and threonine residues of the GXXXXGXX(T/S) motif present in FGF2 and other classical FGFs (see also panel C). (C) Superimposition of the Cα traces of the β-trefoil core domain of classical, paracrine-acting FGFs onto one another. The view is from the top looking down into the β-trefoil core. A close-up view of the heparin-binding region encompassing β10 and β12 is shown on the right, and to orient the reader, a view of the whole structure is shown on the left. The FGF ligands are colored as follows: FGF1 (PDB ID, 1EVT), green; FGF2 (PDB ID, 1FQ9), cyan blue; FGF4 (PDB ID, 1IJT), red; FGF7 (PDB ID, 1QQL), blue; FGF9 (PDB ID, 1IHK), yellow; FGF10 (PDB ID, 1NUN), purple; and FGF8b (PDB ID, 2FDB), brown. Note that the Cα traces of these seven classical FGFs take nearly identical paths in the β10-β12 region. In panel B, FGF2 was chosen from this set of FGFs to illustrate the divergence of FGF19 at this region. Glycine and threonine/serine residues of the GXXXXGXX(T/S) motif conserved in these classical FGFs are marked by black arrows.