Cryo-EM structure provides insights into the unusual heptameric assembly of rice (Oryza sativa L.) ClpB1 AAA+ ATPase

Heat stress disrupts the protein homeostasis leading to the accumulation of toxic aggregated proteins in the cell. ClpB disaggregase belonging to the AAA+ ATPase superfamily removes the aggregated toxic proteins. ClpB is present ubiquitously in bacteria, yeast, protozoans and plants and plays a role in acquired heat tolerance. This study was focused on cytoplasmic ClpB1 from rice which is the staple food for more than half of world's population. In bacteria and yeast, ClpB forms a hexameric assembly for carrying out the disaggregase function, however, none of the plant ClpB isoforms have been structurally characterized. Here, we report the cryo-EM structure of ClpB1 from rice (Oryza sativa L.; OsClpB1) at 4 Å resolution. The structure reveals that OsClpB1 assembles as an unusual heptameric ring, possibly representing a non-processive open conformation. Our results point to the structural plasticity of OsClpB1 since it exists in different oligomeric forms. Analytical ultracentrifugation studies confirmed OsClpB1 exist as a heptamer in solution as well, suggesting the presence of the heptameric form of OsClpB1 within the cellular milieu of the rice plant.