The three-dimensional solution structure of the ligand binding D2 domain of the fibroblast growth factor receptor (FGFR) is determined using multidimensional NMR techniques. The atomic root-mean-square distribution for the backbone atoms in the structured region is 0.64 A. Secondary structural elements in the D2 domain include 11 beta-strands arranged antiparallely into two layers of beta-sheets. The structure of the D2 domain is characterized by the presence of a short flexible helix that protrudes out of the layers of beta-sheets. Results of size exclusion chromatography and sedimentation velocity experiments show that the D2 domain exists in a monomeric state both in the presence and in the absence of bound sucrose octasulfate (SOS), a structural analogue of heparin. Comparison of the solution structure of the D2 domain with the crystal structure of the protein (D2 domain) in the FGF signaling complex reveals significant differences, suggesting that ligand (FGF) binding may induce significant conformational changes in the receptor. SOS binding sites in the D2 domain have been mapped on the basis of the 1H-15N chemical shift perturbation data. SOS binds to the positively charged residues located in beta-strand III and the flexible helix. Isothermal titration calorimetry data indicate that the ligand (hFGF-1) binds strongly (Kd approximately 10(-9) M) to the D2 domain even in the absence of SOS. Binding of SOS to either the D2 domain or hFGF-1 does not seem to be the driving force for the formation of the D2-hFGF-1 binary complex. The function of SOS binding appears to stabilize the preformed D2-FGF binary complex.