A dynamic bundle of four adjacent hydrophobic segments in the denatured state of staphylococcal nuclease

In an earlier study of the denatured state of staphylococcal nuclease (Wang Y, Shortle D, 1995, Biochemistry 34: 15895-15905), we reported evidence of a three strand antiparallel beta sheet that persists at high urea concentrations and is stabilized by a local 'non-native' interaction with four large hydrophobic residues. Because the amide proton resonances for all of the involved residues are severely broadened, this unusual structure is not amenable to conventional NMR analysis and must be studied by indirect methods. In this report, we present data that confirm the important role of interactions involving four hydrophobic residues (Leu 36, Leu 37, Leu 38, and Val 39) in stabilizing the structure formed by the chain segments corresponding to β1-β2-β3-h, interactions that are not present in the native state. Glycine substitutions for each of these large hydrophobic residues destabilizes or disrupts this beta structure, as assessed by H(N) line sharpening and changes in the CD spectrum. The ¹³C resonances of the carbonyl carbon for several of the residues in this structure indicate conformational dynamics that respond in a complex way to addition of urea or changes in sequence. Studies of hydrogen exchange kinetics in a closely related variant of staphylococcal nuclease demonstrate the absence of the stable hydrogen bonding between the strands expected for a native-like three- strand beta sheet. Instead, the data are more consistent with the three beta strand segments plus the four adjacent hydrophobic residues forming a dynamic, aligned array or bundle held together by hydrophobic interactions.