Cryo-EM reveals an asymmetry in a novel single-ring viral chaperonin

Chaperonins are ubiquitously present protein complexes, which assist the proper folding of newly synthesized proteins and prevent aggregation of denatured proteins in an ATP-dependent manner. They are classified into group I (bacterial, mitochondrial, chloroplast chaperonins) and group II (archaeal and eukaryotic cytosolic variants). However, both of these groups do not include recently discovered viral chaperonins. Here, we solved the symmetry-free cryo-EM structures of a single-ring chaperonin encoded by the gene 246 of bacteriophage OBP Pseudomonas fluorescens, in the nucleotide-free, ATPγS-, and ADP-bound states, with a resolution of 4.3A, 5.0A, and 6A, respectively. The structure of OBP chaperonin reveals a unique subunit arrangement, with three subunits pairs and one unpaired subunit. Each pair combines subunits in two possible conformations, differing in nucleotide-binding affinity. Binding of nucleotides result in the increase of subunits’ conformational variability. Due to its unique structural and functional features, OBP chaperonin can represent a new group. 148 words