1H-n.m.r. studies of the fibronectin 13kDa collagen-binding fragment. Evidence for autonomous conserved type I and type II domain folds

A 1H-n.m.r. study of a 117-residue (13 kDa) gelatin-binding fragment of human fibronectin, which contains the sixth (from the N-terminus) type I domain and the first type II domain, was undertaken. The resolution of the 1H-n.m.r. spectrum indicates that the domains are independent and mobile relative to each other. Analysis of two-dimensional 1H-n.m.r. experiments recorded at 500 MHz afforded spin-system identifications for all aromatic and a number of aliphatic residues. Utilizing the fact that phenylalanine residues occur only in the type II portion of this fragment, many spin systems were localized to either the type I or the type II module via analysis of two-dimensional nuclear-Overhauser-effect (NOESY) experiments. This allowed unambiguous assignment of the two tryptophan residues, as they occur singly in each domain. Patterns of NOESY connectivities are found to be consistent with known type I and type II domain structures; this affords a number of tentative sequence-specific assignments. For both domains, evidence of conserved hydrophobic cores and secondary-structure elements is obtained. In addition, 1H-n.m.r.-monitored thermal-melting studies demonstrate conclusively that the domains are independently folded and that the type I domain has high thermal stability relative to the type II domain. This is consistent with the results of calorimetric studies, and also confirms the localization of spin systems determined from the NOESY data.