Similarities between the spectrin SH3 domain denatured state and its folding transition state

We have expanded our description of the energy landscape for folding of the SH3 domain of chicken alpha-spectrin by a detailed structural characteriza tion of its denatured state ensemble (DSE). This DSE is significantly popul ated under mildly acidic conditions in equilibrium with the folded state. E vidence from heteronuclear nuclear magnetic resonance (NMR) experiments on H-2,N-15-labeled protein suggests the presence of conformers whose residual structure bears some resemblence to the structure of the folding transitio n state of this protein. NMR analysis in a mutant with an engineered, non-n ative alpha-helical tendency shows a significant amount of local non-native structure in the mutant, while the overall characteristics of the DSE are unchanged. Comparison with recent theoretical predictions of SH3 domain fol ding reactions reveals an interesting correlation with the predicted early events. Based on these results and recent data from other systems, we propo se that the DSE of a protein will resemble the intermediate or transition s tate of its nearest rate-limiting step, as a consequence of simple energeti c and kinetic principles. (C) 2000 Academic Press.