The in vitro oxidative folding of the insulin superfamily.

Insulin and related proteins, which have been found not only in mammals, birds, reptiles, amphibians, fish, and cephalochordate, but also in mollusca, insects, and Caenorhabditis elegans, form a large protein family, the insulin superfamily. In comparing their amino acid sequences, a common sequence characteristic, the insulin structural motif, is found in all members of the superfamily. The structural motif is deduced to be the sequence basis of the identical disulfide linkages and similar three-dimensional structures of the superfamily. The insulin superfamily provides a series of disulfide-containing proteins for the studies of in vitro oxidative folding. The in vitro folding pathways of insulin-like growth factor-1 (IGF-1), porcine insulin precursor (PIP), human proinsulin, and Amphioxus insulin-like peptide (AILP) have been established by capture and analysis of the folding intermediates during their in vitro oxidative folding process. The family also provides an excellent system for study of the sequence structure relation: insulin and IGF-1 share high amino acid sequence homology, but they have evolved different folding behaviors. The sequence determinants of their different folding behaviors have been revealed by analyzing the folding behaviors of those global and local insulin/IGF-1 hybrids.