THE STRUCTURE OF THE TRANSITION-STATE FOR THE ASSOCIATION OF 2 FRAGMENTS OF THE BARLEY CHYMOTRYPSIN INHIBITOR-2 TO GENERATE NATIVE-LIKE PROTEIN - IMPLICATIONS FOR MECHANISMS OF PROTEIN-FOLDING

Possible early events in protein folding may be studied by dissecting proteins into complementary fragments. Two fragments of chymotrypsin i nhibitor 2 [CI2-(20-59) and CI2-(60-83)] associate to form a native-li ke structure in a second-order reaction that combines collision and re arrangement. The transition state of the reaction, analyzed by the pro tein engineering method on 17 mutants, is remarkably similar to that f or the folding of the intact protein-a structure that resembles an exp anded version of the folded structure with most interactions significa ntly weakened. The exception is that the N-terminal region of the sing le alpha-helix (the N-capping box) is completely formed in the transit ion state for association of the fragments, whereas it is reasonably w ell formed for the intact protein. Preliminary evidence on the structu res of the individual fragments indicates that both are mainly nonnati ve, lacking native secondary structure and having regions of nonnative buried hydrophobic clusters. The association reaction does not result from the collision of a subpopulation of two fully native-like fragme nts but involves a considerable rearrangement of structure.