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Abstract

The cytoplasm of eukaryotes contains a heteromeric toroidal chaperonin assembled from the t-complex TCP-1 and several other related polypeptides. The structure of the TCP-1 cytoplasmic chaperonin and that of the binary complex formed between this chaperonin and unfolded beta-actin have been studied using electron microscopy and image processing techniques. Two-dimensional averaging of front views reveals a circular stain-excluding mass surrounding a central stain-penetrating region in which the stain is excluded upon actin binding. Sections of a three-dimensional reconstruction of the chaperonin show that the inner core is an empty channel that becomes filled upon binary complex formation with unfolded beta-actin. Upon incubation with Mg-ATP, the beta-actin:chaperonin complex discharges the actin such that the chaperonin central cavity reappears. Side views from different forms of TCP-1 reveals that upon Mg-ATP binding, the cytoplasmic chaperonin undergoes a structural rearrangement that is confirmed using a new classification method.