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Abstract
Chaperonins are double-ring protein assemblies with a central cavity that provides
a sequestered environment for in vivo protein folding. Their reaction cycle is thought
to consist of a nucleotide-regulated alternation between an open substrate-acceptor
state and a closed folding-active state. The cavity of ATP-charged group I chaperonins,
typified by Escherichia coli GroEL [1], is sealed off by a co-chaperonin, whereas
group II chaperonins--the archaeal thermosome and eukaryotic TRiC/CCT [2]--possess
a built-in lid [3-5]. The mechanism of the lid's rearrangements requires clarification,
as even in the absence of nucleotides, thermosomes of Thermoplama acidophilum appear
open in vitrified ice [6] and closed in crystals [4]. Here we analyze the conformation
of the thermosome at each step of the ATPase cycle by small-angle neutron scattering.
The apo-chaperonin is open in solution, and ATP binding induces its further expansion.
Closure seems to occur during ATP hydrolysis and before phosphate release, and represents
the rate-limiting step of the cycle. The same closure can be triggered by the crystallization
buffer. Thus, the allosteric regulation of group II chaperonins appears different
from that of their group I counterparts.