WW: An isolated three-stranded antiparallel beta-sheet domain that unfoldsand refolds reversibly; evidence for a structured hydrophobic cluster in urea and GdnHCl and a disordered thermal unfolded state

The objective of this study was to evaluate the suitability of the WW domai n as a desirable model system to understand the folding and stability of an isolated three-stranded antiparallel beta-sheet structure. The WW domain w as subjected to thermal and chaotropic denaturation/reconstitution utilizin g a variety of biophysical methods. This three-stranded sheet folds reversi bly and cooperatively utilizing both urea and GdnHCl as denaturants; howeve r, the denatured state retains structure in the form of a hydrophobic clust er involving at least one aromatic side chain. In contrast to chaotropic de naturation, thermal denaturation appears to be more complete and may be a t wo state process. The suitability of the WW domain for future studies aimed at understanding the kinetics and thermodynamics of antiparallel beta-shee t folding clearly emerges from this initial study. The most exciting and si gnificant result in this manuscript is the finding that the chaotropic dena tured state of WW has a hydrophobic cluster as discerned by near-UV CD evid ence. The role that the denatured state plays in the folding and stability of a three-stranded beta-sheets, and its capacity for preventing aggregatio n may be particularly important and is the subject of ongoing studies.