A linear 23-residue peptide reveals a propensity to form an unusual native-like conformation

To gain insight into the earliest events of protein folding, a 23-residue peptide with a sequence corresponding to the 38-60 fragment of the beta-subunit of human chorionic gonadotropin (hCG beta) was studied by NMR. In aqueous solution the majority of the peptide residues adopted an extended polyproline II (PII) conformation similar to those in mature, fully folded hCG beta. The finding that the isolated protein fragment may acquire native-like structural motifs, even without alpha-helices or beta-structures, extends the possibility of using free peptides as model systems to better understand the protein folding mechanisms. It was shown that the PII-rich structural motif can be determined efficiently by NMR spectroscopy. The observation that in the absence of extensive medium- and long-range interactions the majority of amino acid residues may adopt the PII conformation suggests that the PII-rich structural motifs may play an important role in early events of protein folding.