Cryo-EM of relaxed myosin filament reveals unique molecular motor conformation and unprecedented details of the filament backbone.

SUMMARY Thermosomes are group II chaperonins responsible for protein refolding in an ATP-dependent manner. Little is known regarding the conformational changes of thermosomes during their functional cycle due to lack of high-resolution structure in open state. Here we report the first complete crystal structure of thermosome (rATcpnβ) in open state from Acidianus tengchongensis. There is a ~30° rotation of the apical and lid domains compared to the previous closed structure. Besides, the structure reveals a conspicuous hydrophobic patch in the lid domain and residues locating in this patch are conserved across species. Both the closed and open forms of rATcpnβ were also reconstructed by electron microscopy (EM). Structural fitting revealed the detailed conformational change from open to closed state. Structural comparison as well as protease K digestion indicated only ATP binding without hydrolysis does not induce chamber closure of thermosome.

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