ATPase cycle of an archaeal chaperonin

Recent structural data imply differences in allosteric behavior of the grou p I chaperonins, typified by GroEL from Escherichia coli, and the group II chaperonins, which comprise archaeal thermosome and eukaryotic TRiC/CCT. Th erefore, this study addresses the mechanism of interaction of adenine nucle otides with recombinant alpha-only and native alpha beta-thermosomes from T hermoplasma acidophilum acidophilum, which also enables us to analyze the r ole of the heterooligomeric composition of the natural thermosome. Although all subunits of the alpha-only thermosome seem to bind nucleotides tightly and independently, the native chaperonin has two different classes of ATP- binding sites. Furthermore, for the alpha-only thermosome, the steady-state ATPase rate is determined by the cleavage reaction itself, whereas, for th e alpha beta-thermosome, the rate-limiting step is associated with a post-h ydrolysis isomerisation into a non-covalent ADP*P-i species prior to the re lease of the gamma-phosphate group. After half-saturation with ATP, a negat ive cooperativity in hydrolysis is observed for both thermosomes. The effec t of Mg2+ and K+ nucleotide cycling is documented. We conclude that archaea l chaperonins have unique allosteric properties and discuss them in the lig ht of the mechanism established for the group I chaperonins. (C) 2000 Acade mic Press.