Folding of a beta-hairpin peptide derived from the N-terminus of ubiquitin- Conformational preferences of beta-turn residues dictate non-native beta-strand interactions

The role of the non-native beta-turn sequence (NPDG) in nucleating the fold ing of a beta-hairpin peptide derived from the N-terminus of ubiquitin, has been examined by NMR and CD spectroscopy. The NPDG sequence, while represe nting a common two-residue type I turn sequence in proteins, folds to give a G1-bulged type I turn in the context of a beta-hairpin peptide, to the ex clusion of other possible conformations. The turn conformation results in m isalignment of the two beta strands and a beta hairpin with non-native side chain interactions. A truncated 12-residue analogue of the hairpin, in whi ch the majority of residues in the N-terminal beta strand have been deleted , shows some weak propensity to fold into a G-bulged type I turn conformati on in the absence of interstrand stabilizing interactions. The NPDG turn se quence pays some of the entropic cost in initiating folding allowing inters trand interactions, which in this case arise from the non-native pairing of residue side chains, to stabilize a significant population of the folded s tate. Examination of the relative abundance of the Pro-Asp type I turn, wit h G in the +B1 position, vs. the type I G-bulged turn PXG, in a database of high resolution structures, reveals 48 instances of PXG bulged turns for w hich X = Asp is by far the most common residue with 20 occurrences. Strikin gly, there are no examples of a type I PD turn with G at the +B1 position, in good agreement with our experimental observations that the PDG G-bulged turn is populated preferentially in solution.