GroEL-GroES cycling: ATP and nonnative polypeptide direct alternation of folding-active rings

The double-ring chaperonin GroEL mediates protein folding in the central cavity of a ring bound by ATP and GroES, but it is unclear how GroEL cycles from one folding-active complex to the next. We observe that hydrolysis of ATP within the cis ring must occur before either nonnative polypeptide or GroES can bind to the trans ring, and this is associated with reorientation of the trans ring apical domains. Subsequently, formation of a new cis-ternary complex proceeds on the open trans ring with polypeptide binding first, which stimulates the ATP-dependent dissociation of the cis complex (by 20- to 50-fold), followed by GroES binding. These results indicate that, in the presence of nonnative protein, GroEL alternates its rings as folding-active cis complexes, expending only one round of seven ATPs per folding cycle.