Isolation and characterization of independently folding regions of the beta chain of Escherichia coli tryptophan synthetase.

It had been reported previously that the beta2 subunit of Escherichia coli tryptophan synthetase [L-serinehydrolyase (adding indole) EC 4.2.1.20] can be cleaved by trypsin into a nearly functional dimeric protein, the monomer of which consists of two large, nonoverlapping, polypeptide fragments. In the present paper, it is shown that these fragments can be separated after denaturation. Upon removal of the denaturing agent, the isolated fragments spontaneously refold into conformation, which, by various physical-chemical criteria, are shown to approximate the conformations of the corresponding fragments associated within the native protein. Furthermore, it is demonstrated that, upon mixing, these renatured fragments reassociate to form the renatured nicked protein which, by all the physical and functional criteria used, is indistinguishable from the native nicked protein. These results are taken as strong evidence that the isolated fragments can be considered as independently folding regions corresponding to intermediates in the folding of the intact protein.