Conformational rearrangements of an archaeal chaperonin upon ATPase cycling

Chaperonins are double-ring protein assemblies with a central cavity that p rovides a sequestered environment for in vivo protein folding. Their reacti on cycle is thought to consist of a nucleotide-regulated alternation betwee n an open substrate-acceptor state and a closed folding-active state. The c avity of ATP-charged group I chaperonins, typified by Escherichia coil GroE L [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 dosed in crystals [4]. Here we analyze the conformation of the thermosome at each step of the ATPase cycle by sma ll angle neutron scattering. The apo-chaperonin is open in solution, and AT P binding induces Its further expansion. Closure seems to occur during ATP hydrolysis and before phosphate release, and represents the rate limiting s tep of the cycle. The same closure can be triggered by the crystallization buffer. Thus, the allosteric regulation of group II chaperonins appears dif ferent from that of their group I counterparts. (C) 2000 Elsevier Science L td. All rights reserved.