Small-Angle Neutron Scattering Studies of the Solution Structures of Archaeosome, a Chaperonin from Sulfolobus Shibatae. Chwen-Yuan Ku', Elsie Quaite-Randall", P. Thiyagarajan' and Andrzej Joachimiak", 'Intense Pulsed Neutron Source, "Center for Mechanistic Biology and Biotechnology, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439

The major heat shock proteins in the archaeon Sulfolobus shibatae are similar to the cytosolic eukaryotic chaperonin and form an 18-subunit bitoroidal complex (double-ring structure). Two sequence-related subunits constitute a functional complex, named the archaeosome. The archaeosome exists in two distinct conformational states (open and closed ends of the double-ring structure) that are part of the chaperonin functional cycle. The closed archaeosome complex binds with ATP and forms an open complex. Small-Angle Neutron Scattering (SANS) of archaeosomes shows the difference in the solution structure with and without ATP. In addition, the presence of Mg in the solution also induces changes in the structure. Detailed SANS data analysis indicates that the archaeosomes form aggregates in the solution. The double-rings bind to each other along the axial direction of the cylindrical coordinate. The double-ring structure remains intact at higher temperature (75deg.C) only in the presence of ATP and Mg,