NMR structure of the first Ig module of mouse FGFR1

Fibroblast growth factor (FGF) receptors (FGFRs) regulate a multitude of cellular processes during embryogenesis and in the adult. The extracellular part of the prototypical FGFR consists of three Ig modules (Ig1 - Ig3), in which Ig2 and Ig3 determine affinity and specificity for FGF and heparin, while the Ig1 module is thought to have a regulatory function. The crystal structures of the Ig2 and Ig3 modules alone and in complex with FGF have previously been reported. The structure of the Ig1 module is unknown, and very little is known about the structural determinants for the regulatory function of this module. We describe here the NMR structure of the Ig1 module of mouse FGFR1. The three-dimensional fold of the module belongs to the intermediate Ig subgroup and can be described as a beta-barrel consisting of two beta-sheets. One sheet is formed by A', G, F, C, and C', and the other by A, B, B', E, and D beta-strands. The overall strand topology of the Ig1 module is similar to that of the Ig2 and Ig3 modules. However, the A/A' loop of the Ig1 module is much longer than that of the Ig2 and Ig3 modules. It contains eight extra residues compared to the Ig3 module, and five extra residues compared to Ig2.