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

The mechanism of beta-sheet formation remains a fundamental issue in our un derstanding of the protein folding process, but is hampered by the often en countered kinetic competition between folding and aggregation. The role of local versus nonlocal interactions has been probed traditionally by mutagen esis of both turn and strand residues. Recently, rigid organic molecules th at impose a correct chain reversal have been introduced in several small pe ptides to isolate the importance of the long-range interactions. Here, we p resent the incorporation of a well-studied beta-turn mimic, designated as t he dibenzofuran-based (DBF) amino acid, in the B1 domain of streptococcal p rotein G (BIG), and compare our results with those obtained upon insertion of the same mimic into the N-terminal beta-hairpin of BIG (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, w hereas the modified peptide does not adopt a native-like conformation. The nature of the DBF flanking residues in the modified BIG domain prevents the beta-tum mimic from acting as a strong beta-sheet nucleator, which reinfor ces the idea that the native beta-hairpin formation is not driven by the be ta-turn formation, but by tertiary interactions.