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

The double-ring chaperonin GroEL mediates protein folding in the central ca vity of a ring bound by ATP and GroES, but it is unclear how GroEL cycles f rom one folding-active complex to the next. We observe that hydrolysis of A TP within the cis ring must occur before either nonnative polypeptide or Gr oES can bind to the trans ring, and this is associated with reorientation o f the trans ring apical domains. Subsequently, formation of a new cis-terna ry 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-activ e cis complexes, expending only one round of seven ATPs per folding cycle.