ATP-induced structural change of the thermosome is temperature-dependent

Protein folding by chaperonins is powered by ATP binding and hydrolysis. AT Pase activity drives the folding machine through a series of conformational rearrangements, extensively described for the group I chaperonin GroEL fro m Escherichia coli but still poorly understood for the group II chaperonins . The latter-archaeal thermosome and eukaryotic TMUCCT-function independent ly of a GroES-like cochaperonin and are proposed to rely on protrusions of their own apical domains for opening and closure in an ATP-controlled fashi on. Here we use small-angle neutron scattering to analyze structural change s of the recombinant a-only and the native alpha beta -thermosome from Ther moplasma acidophilum upon their ATPase cycling in solution. We show that sp ecific high-salt conditions, but not the presence of MgATP alone, induce fo rmation of higher order thermosome aggregates. The mechanism of the open-cl osed transition of the thermosome is strongly temperature-dependent. ATP bi nding to the chaperonin appears to be a two-step process: at lower temperat ures an open state of the ATP-thermosome is predominant, whereas heating to physiological temperatures induces its switching to a closed state. Our da ta reveal an analogy between the ATPase cycles of the two groups of chapero nins and enable us to put forward a model of thermosome action. (C) 2001 Ac ademic Press.