Structural characterization of folded and unfolded states of an SH3 domain in equilibrium in aqueous buffer.

The isolated N-terminal Src homology 3 (SH3) domain of Drosophila drk exists in equilibrium between folded and unfolded states in aqueous buffer near neutral pH. Nuclear magnetic resonance spectra recorded on both states simultaneously exhibit an approximate 1:1 ratio of protein conformations. The folded form is similar to other known SH3 structures, especially the N-terminal SH3 domain of the mammalian homologue GRB2. A stretch of sequential amide-amide nuclear Overhauser effect cross-peaks for resonances of the unfolded state is observed in a region corresponding to beta-strands in the folded state. The results suggest that turn-like conformations may be preferentially sampled in the folding pathway for this predominantly beta-structured SH3 domain. In addition, a stable turn at Leu-28 is observed in the unfolded but not in the folded state. Comparison of this unfolded form with a denatured state in 2 M guanidine hydrochloride shows that, while both are highly disordered, these states are not identical and more residual structure is present under nondenaturing conditions.