Folding of egg white proteins as a post-translational processing.

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Egg white proteins, ovotransferrin and ovalbumin, which are synthesized in hen oviducts under hormonal regulation, were investigated for their folding mechanisms using in vitro translation system as well as refolding systems of denatured forms.A two-step procedure was found to be useful for the efficient refolding of a complex protein, ovotransferrin. In the first step, the reduced and denatured form of the protein was incubated at a low temperature in a nondenaturing buffer containing reduced glutathione; in the second step, the reduced form was reoxidized at a higher temperature in the presence of oxidized glutathione. Under these conditions, the fully reduced forms of ovotransferrin and its half-molecules were almost quantitatively reoxidized to regain iron-binding abilities and conformations, very similar to the native form. The circular dichroism spectra revealed that at low temperatures the fully reduced forms have partially folded conformations, which are fluctuating like "molten globule" states. The reoxidization kinetics compared between whole ovotransferrin and the two half-molecules supported independent refolding of the N- and C-terminal domains.With respect to ovalbumin about 40% of urea-denatured protein was renatured to the native form after 18 hr incubation under non-denaturing conditions. Likewise, only a part of the mRNA-directed translation product in wheat germ translation system was found to take a native-like conformation.