CHARACTERIZATION OF THE BACKBONE DYNAMICS OF FOLDED AND DENATURED STATES OF AN SH3 DOMAIN

Measurements of N-15 NMR relaxation parameters have been used to chara cterize the backbone dynamics of folded and denatured states of the N- terminal SH3 domain from the adapter protein drk, in high salt or guan idinium chloride, respectively. Values of the spectral density functio n evaluated at a number of frequencies are compared. The levels of bac kbone dynamics in the folded protein show little variation across the molecule and are of similar magnitude to those determined previously f or 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 a nd more heterogeneous dynamics than the folded state. In particular th e profile of the spectral density function evaluated at zero-frequency for the unfolded state of the domain indicates that residues in the m iddle of the protein sequence are considerably less mobile than those at the termini. These data suggest that the molecule is not behaving a s an extended polymer and that concerted motions of the central portio ns 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-fr equency motions in the folded molecule is largely unaffected by change s in temperature, whereas an increase in temperature results in increa sed high-frequency motion in the unfolded state.