Domain rotations between open, closed and bullet-shaped forms of the thermosome, an archaeal chaperonin

Three conformations of the thermosome, an archaeal group II chaperonin, hav e been determined by cryo-electron microscopy (EM). We describe an open for m of the double-ring oligomer, a closed form and a bullet-shaped form with one ring open and the other closed. Domain movements have been deduced by d ocking atomic coordinates into the EM maps. The subunit apical domains, bea ring the putative substrate binding sites, rotate about 30 degrees upwards and twist in the plane of the ring from the closed to the open conformation . The closed rings have their nucleotide binding pockets closed by the inte rmediate domains, but in the open rings, the pocket is accessible. (C) 2000 Academic Press.