Backbone dynamics of homologous fibronectin type III cell adhesion domains from fibronectin and tenascin.

BACKGROUND: Fibronectin type III domains are found as autonomously-folded domains in a large variety of multidomain proteins, including extracellular matrix proteins. A subset of these domains employ an Arg-Gly-Asp (RGD) tripeptide motif to mediate contact with cell-surface receptors (integrins). This motif mediates protein-protein interactions in a diverse range of biological processes, such as in tissue development, would healing and metastasis. The molecular basis for affinity and specificity of cell adhesion via type III domains has not been clearly established. The tenth type III domain from fibronectin (FNfn10) and the third type III domain from tenascin-C (TNfn3) have 27% sequence identity and share the same overall protein fold, but present the RGD motifs in different structural contexts. The dynamical properties of the RGD motifs may affect the specificity and affinity of the FNfn10 and TNfn3 domains. Structure-dynamics correlations for these structurally homologous proteins may reveal common molecular features which are important to the dynamical properties of proteins.
RESULTS: The intramolecular dynamics of the protein backbones of FNfn10 and TNfn3 have been studied by 15N nuclear spin relaxation. The FG loop in FNfn10, which contains the RGD motif, exhibits extensive flexibility on picosecond to nanosecond timescales, but motions on microsecond to millisecond timescales are not observed. The equivalent region in TNfn3 is as rigid as regular elements of secondary structure. The CC' loop also is more flexible on picosecond-nanosecond timescales in FNfn10 than in TNfn3. Conformational exchange, reflecting flexibility on microsecond-millisecond timescales, is observed in beta strands A and B of both FNfn10 and TNfn3.
CONCLUSIONS: Comparison of the structures of the FNfn10 and TNfn3 reveals several features related to their different dynamical properties. The larger amplitude motions of loops in FNfn10 are consistent with the hypothesis that flexibility of these regions facilitates induced-fit recognition of fibronectin by multiple receptors. Similarly, the more rigid loops of TNfn3 may reflect greater specificity for particular integrins. The correlations observed between structural features and dynamical properties of the homologous type III domains indicate the influence of hydrogen bonding and hydrophobic packing on dynamical fluctuations in proteins.