Chaperonin action is controlled by cycles of nucleotide binding and hydrolysis. Here, we examine the effects of nucleotide binding on an archaeal group 2 chaperonin. In contrast to the ordered apo state of the group 1 chaperonin GroEL, the unliganded form of the homo-16-mer Methanococcus maripaludis group 2 chaperonin is very open and flexible, with intersubunit contacts only in the central double belt of equatorial domains. The intermediate and apical domains are free of contacts and deviate significantly from the overall 8-fold symmetry. Nucleotide binding results in three distinct, ordered 8-fold symmetric conformations--open, partially closed, and fully closed. The partially closed ring encloses a 40% larger volume than does the GroEL-GroES folding chamber, enabling it to encapsulate proteins up to 80 kDa, in contrast to the fully closed form, whose cavities are 20% smaller than those of the GroEL-GroES chamber.