Fibronectin is a multimodule protein with numerous functions. Our goal is to determine the three dimensional structure of the fragment consisting of the ninth and tenth type III modules. The tenth module contains the RGD cell attachment sequence while the ninth module contains the RGD synergy region. The fragment consisting of the ninth and tenth type III modules has full fibronectin binding activity to the specific integrin receptor, alpha5beta1. Initial attempts to study the human fragment were frustrated by protein aggregation at concentrations required to carry out NMR experiments. This problem was overcome by expressing the recombinant mouse protein, which yields good quality NMR spectra at concentrations of 0.8mM. Virtually complete signal assignments have been obtained of the two module mouse fragment, and the structures of both modules have been determined at moderate resolution. The available structure shows that the RGD region is on the protein surface and is highly flexible. In addition it is not close to the synergy sequence in the ninth module. Hence the enhanced activity of the module fragment is not due to a direct interaction between the RGD and synergy sequences. What remains to be determined is the order of the modules relative to one another. Relaxation measurements together with additional noe constraints are being analyzed to address this issue. The significance of the project resides in the insight that the structure of the RGD fibronectin fragment will provide about cell attachment and integrin recognition. This information should facilitate the design of more active cell attachment molecules which could improve treatment of diseases resulting from impaired cell adhesion.
