Subdomain-size proteolytic fragments of Escherichia coli trp repressor have been produced that assemble in defined order to regenerate fully native dimers. By characterization of the secondary and tertiary structures of isolated and recombined fragments, the structure of assembly intermediates can be correlated with the kinetic folding pathway of the intact repressor deduced from spectroscopic measurement of folding rates. The nativelike structure of these intermediates provides further evidence that protein folding pathways reflect the stabilities of secondary structural units and assemblies found in the native state. The proteolytic method should be generally useful in adding structural detail to spectroscopically determined folding mechanisms.
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