The search for local native-like nucleation centers in the unfolded state of beta -sheet proteins.

An approach involving the systematic computational conformational analysis of all overlapping hexapeptide segments in the protein sequence has found fragments with the higher than average propensity to adopt the native-like three-dimensional structure and other regular nonrandom structures in the unfolded states of four beta-sheet proteins, namely IFABP (intestinal fatty acid-binding protein), ILBP (ileal fatty acid-binding protein), CRABP I (cellular retinoic acid-binding protein), and CRBP II (cellular retinal binding protein). The native three-dimensional structures of these four proteins are very similar even though they possess as little as approximately 30% sequence similarity. The computational results were validated by comparison with the experimental data of the heteronuclear sequential quantum correlation NMR spectroscopy obtained earlier for IFABP at high urea concentrations. On this basis, a molecular model of the unfolded state of IFABP has been developed. The model presumes a dynamic equilibrium between various nonrandom structures (including the native-like structure) and random coil in the local segments of the protein sequence. The model explains experimental observations obtained earlier for folding of several mutants of IFABP, as well as the observed differences in molecular mechanisms of folding for the four beta-sheet proteins. Because the computational approach itself does not employ any experimentally derived information in advance, it is not necessarily limited to the beta-sheet proteins.