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The crystal structures of the group II chaperonins consisting of the α subunit with amino acid substitutions of G65C and/or I125T from the hyperthermophilic archaeum Thermococcus strain KS-1 were determined. These mutants have been shown to be active in ATP hydrolysis but inactive in protein folding. The structures were shown to be double-ring hexadecamers in an extremely closed form, which was consistent with the crystal structure of native α₈β₈-chaperonin from Thermoplasma acidophilum. Comparisons of the present structures with the atomic structures of the GroEL₁₄–GroES₇–(ADP)₇ complex revealed that the deficiency in protein-folding activity with the G65C amino acid substitution is caused by the steric hindrance of the local conformational change in an equatorial domain. We concluded that this mutant chaperonin with G65C substitution is deprived of the smooth conformational change in the …