STRUCTURE OF THE TRANSITION-STATE FOR THE FOLDING UNFOLDING OF THE BARLEY CHYMOTRYPSIN INHIBITOR-2 AND ITS IMPLICATIONS FOR MECHANISMS OF PROTEIN-FOLDING/

The equilibrium and kinetics of folding of the single-domain protein c hymotrypsin inhibitor 2 conform to the simple two-state model. The str ucture of the rate-determining transition state has been mapped out at the resolution of individual side chains by using the protein enginee ring method on 74 mutants that have been constructed at 37 of the 64 r esidues. The structure contains no elements of secondary structure tha t are fully formed. The majority of interactions are weakened by >50% in the transition state, although most regions do have some very weak structure. The structure of the transition state appears to be an expa nded form of the native state in which secondary and tertiary elements have been partly formed concurrently. This is consistent with a ''glo bal collapse'' model of folding rather than a framework model in which folding is initiated from fully preformed local secondary structural elements. This may be a general feature for the folding of proteins la cking a folding intermediate and is perhaps representative of the earl y stages of folding for multidomain or multimodule proteins. The major transition state for the folding of barnase, for example, has some fu lly formed secondary and tertiary structural elements in the major tra nsition state, and barnase appears to form by a framework process. How ever, the fully formed framework may be preceded by a global collapse, and a unified folding scheme is presented.