Synthesis, folding, and structure of the β-turn mimic modified B1 domain of streptococcal protein G

Abstract

The mechanism of β-sheet formation remains a fundamental issue in our understanding of the protein folding process, but is hampered by the often encountered kinetic competition between folding and aggregation. The role of local versus nonlocal interactions has been probed traditionally by mutagenesis of both turn and strand residues. Recently, rigid organic molecules that impose a correct chain reversal have been introduced in several small peptides to isolate the importance of the long-range interactions. Here, we present the incorporation of a well-studied β-turn mimic, designated as the dibenzofuran-based (DBF) amino acid, in the B1 domain of streptococcal protein G (B1G), and compare our results with those obtained upon insertion of the same mimic into the N-terminal β-hairpin of B1G (O Melnyk et al., 1998, Lett Pept Sci 5:147–150). The DBF-B1G domain conserves the structure and the functional and thermodynamical properties of the native protein, whereas the modified peptide does not adopt a native-like conformation. The nature of the DBF flanking residues in the modified B1G domain prevents the β-turn mimic from acting as a strong β-sheet nucleator, which reinforces the idea that the native β-hairpin formation is not driven by the β-turn formation, but by tertiary interactions.