Characterization of the backbone dynamics of folded and denatured states of an SH3 domain

Measurements of 15N NMR relaxation parameters have been used to characterize the backbone dynamics of folded and denatured states of the N-terminal SH3 domain from the adapter protein drk, in high salt or guanidinium chloride, respectively. Values of the spectral density function evaluated at a number of frequencies are compared. The levels of backbone dynamics in the folded protein show little variation across the molecule and are of similar magnitude to those determined previously for the folded state of the protein in exchange with an unfolded state at low salt concentrations [Farrow et al. (1995) Biochemistry 34, 868-878]. The denatured state of the domain exhibits both more extensive and more heterogeneous dynamics than the folded state. In particular the profile of the spectral density function evaluated at zero-frequency for the unfolded state of the domain indicates that residues in the middle of the protein sequence are considerably less mobile than those at the termini. These data suggest that the molecule is not behaving as an extended polymer and that concerted motions of the central portions of the molecule are occurring, consistent with a reasonably compact conformation in this region. The backbone dynamics of the folded and unfolded states were studied at two temperatures. The level of high-frequency motions in the folded molecule is largely unaffected by changes in temperature, whereas an increase in temperature results in increased high-frequency motion in the unfolded state.