Secondary structure of a pair of fibronectin type 1 modules by two-dimensional nuclear magnetic resonance

The fourth and fifth type 1 module pair, corresponding to residues 151-244 from the amino terminus of human fibronectin, has been produced as a recombinant protein using a yeast expression system and studied by two-dimensional homonuclear 1H nuclear magnetic resonance (NMR) spectroscopy. The sequence-specific resonance assignment of the 1H NMR spectrum has been completed using a combination of 2D 1H nuclear Overhauser effect (NOE) spectroscopy, homonuclear Hartmann-Hahn, and correlated spectroscopy spectra recorded under a variety of pH and temperature conditions. Slow exchanging amide protons have been identified and estimates of many backbone 3JNH-C alpha H coupling constants were obtained by line shape fitting. The secondary structures of each module conform closely to the "consensus" fibronectin type 1 module structure determined previously for two other single type 1 modules. In the module pair described here, the two modules are linked by a short five-residue linker which appears to form a turn. The intermodule interface is defined by NOEs observed between a hydrophobic three-residue sequence from the fourth type 1 module and residues in the first double-stranded beta-sheet of the fifth type 1 module. The interaction is dominated by a tryptophan residue (unconserved in other type 1 sequences) within the fourth module, which causes large upfield ring current shifts for several proton resonances from the beta-sheet of the fifth module. The NMR data indicate that there is little or no relative reorientation of the two modules about the linker region but rather that the two modules combine with a fixed and intimate hydrophobic contact.