GroEL/ES chaperonins protect interferon-gamma against physicochemical stress--study of tertiary structure formation by alpha-casein quenching and ELISA.

Interferon-gamma (IFN-gamma) is a structurally labile cytokine that rapidly denatures upon exposure to acid or heat. Here we show that both acid-denatured (pH 2) and thermally inactivated (50 degrees C) porcine IFN-gamma can be rescued with the Escherichia coli GroEL/ES chaperonin system and ATP, and reassembled into bioactive dimers. At 35 degrees C, spontaneous refolding of acid-denatured IFN-gamma was found to be dependent on the presence of guanidinium hydrochloride (0.15-0.25 M) or NaCl (0.1-0.2 M). Under non-permissive reaction conditions for regain of native structure (low-ionic-strength buffer at 35 degrees C), the yield of IFN-gamma refolded with GroEL/ES/ATP increased about 30-fold above the level of spontaneous refolding. In the absence of GroES, GroEL captured IFN-gamma in a folding-competent complex. Under these conditions, both ATP and alpha-casein induced release of IFN-gamma from GroEL but with the released protein tending to partition into sedimentable aggregates. Only in the presence of GroES, did ATP induce complete discharge of IFN-gamma from GroEL, with the released protein refolded into a conformation that is (a) immunoreactive/bio-active, (b) resistant to precipitation and (c) in a dimeric configuration. Chicken egg albumin and 90-kDa heat-shock protein were inactive in the exertion of any protective effect against physicochemical stress. The precise amount of protein refolded to the native state at different times of the folding reaction was determined by alpha-casein quenching and ELISA. The former is based on the conversion by excess alpha-casein of any population of unfolded IFN-gamma into one that escapes antibody recognition by subsequent ELISA. Since the native dimers, however, are not affected by alpha-casein quenching, immunoreactivity is directly proportional to the yield of correctly refolded protein. The validity of this approach was confirmed by measurement of biological activity. GroEL/ES-meditated reactivation amounted to > 80% both by ELISA and antiviral assay.