ROLE OF A SUBDOMAIN IN THE FOLDING OF BOVINE PANCREATIC TRYPSIN-INHIBITOR

THE disulphide-bonded intermediates that accumulate in the oxidative folding of bovine pancreatic trypsin inhibitor (BPTI) were characterized some time ago1. Structural characterization of these intermediates would provide an explanation of the kinetically preferred pathways of folding for BPTI. When folding occurs under strongly oxidizing conditions, more than half the molecules become trapped in an intermediate, designated N*, which is similar to the native protein but lacks the 30-51 disulphide bond2-4. We have tested the hypothesis5 that the precursor to N* is the one-disulphide intermediate [5-55], which contains the most stable disulphide in BPTI, and present evidence here that this is the case. A peptide model of [5-55], corresponding to a subdomain of BPTI, seems to fold into a native-like conformation, explaining why [5-55] does not lead to native protein and why it folds rapidly5 to N*. A native-like subdomain structure in a peptide model6 of [30-51], the other crucial one-disulphide intermediate, may explain the route by which [30-51] folds to native protein. Thus, much of the folding pathway5 of BPTI can be explained by the formation of a native-like subdomain in these two early intermediates. This suggests that a large part of the protein folding problem can be reduced to identifying and understanding subdomains of native proteins. © 1990 Nature Publishing Group.