(A) Comparison of the orientation of αC helix and the conformation of the DFGLAR motif at the start of the A-loop between the unphosphorylated autoinhibited WT FGFR1K (PDB ID: 1FGK [Mohammadi et al., 1996], in green) and A-loop tyrosine phosphorylated WT FGFR1K (PDB ID: 3GQI [Bae et al., 2009], in blue) structures. The side chains of the phenylalanine of the DFG motif are rendered in sticks. The dashed lines are used to illustrate the steric clashes that prevent the αC helix from rotating downward in the unphosphorylated autoinhibited FGFR1K structure. The portion of the A-loop in the unphosphorylated WT FGFR1K that becomes β9 strand upon A-loop tyrosine phosphorylation is highlighted in red. (B) Two close-range hydrogen bonds, depicted by dashed lines, between the phosphate moiety of the phosphorylated A-loop tyrosine and conserved arginine (R649 in FGFR1K) stabilize the β9-β6 sheet between the A-loop and the catalytic loop. (C–I) Zoomed-in views of the DFG latch environments observed in the crystal structures of the unphosphorylated WT FGFR1K (PDB ID: 1FGK [Mohammadi et al., 1996], in green), phosphorylated WT FGFR1K (PDB ID: 3GQI [Bae et al., 2009], in blue), unphosphorylated WT FGFR3K (Chen unpublished, in cyan), K650E gain-of-function mutant of FGFR3K (PDB ID: 4K33 [Huang et al., 2013], in brown), unphosphorylated FGFR4K (PDB ID: 4QQT [Huang et al., 2015], in orange), unphosphorylated WT FGFR2K (PDB ID: 2PSQ [Chen et al., 2007], in dark green), and phosphorylated WT FGFR2K (PDB ID 2PVF [Chen et al., 2007], in purple). Distances between the DFG phenylalanine and neighboring leucine and isoleucine residues are indicated by dashed red (autoinhibited kinase) or black (activated kinase) lines with the distance given in Å. Atom colorings are the same as in Figure 1.