Identical proline-->arginine gain-of-function mutations in fibroblast growth 
factor receptor (FGFR) 1 (Pro252Arg), FGFR2 (Pro253Arg) and FGFR3 (Pro250Arg), 
result in type I Pfeiffer, Apert and Muenke craniosynostosis syndromes, 
respectively. Here, we characterize the effects of proline-->arginine mutations 
in FGFR1c and FGFR3c on ligand binding using surface plasmon resonance and X-ray 
crystallography. Both Pro252Arg FGFR1c and Pro250Arg FGFR3c exhibit an 
enhancement in ligand binding in comparison to their respective wild-type 
receptors. Interestingly, binding of both mutant receptors to FGF9 was notably 
enhanced and implicates FGF9 as a potential pathophysiological ligand for mutant 
FGFRs in mediating craniosynostosis. The crystal structure, of Pro252Arg FGFR1c 
in complex with FGF2, demonstrates that the enhanced ligand binding is due to an 
additional set of receptor-ligand hydrogen bonds, similar to those 
gain-of-function interactions that occur in the Apert syndrome Pro253Arg 
FGFR2c-FGF2 crystal structure. However, unlike the Apert syndrome Pro253Arg 
FGFR2c mutant, neither the Pfeiffer syndrome Pro250Arg FGFR1c mutant nor the 
Muenke syndrome Pro250Arg FGFR3c mutant bound appreciably to FGF7 or FGF10. This 
observation provides a potential explanation for why the limb phenotypes, 
observed in type I Pfeiffer and Muenke syndromes, are less severe than the limb 
abnormalities observed in Apert syndrome. Hence, although analogous 
proline-->arginine mutations in FGFR1-3 act through a common structural 
mechanism to result in gain-of-function, differences in the primary sequence 
among FGFRs result in varying effects on ligand binding specificity.