Rubisco plays a central role in the process of carbon dioxide fixation within biological systems. When Rubisco encounters inhibition by sugar phosphates, including its own substrate RuBP, it relies on the assistance of the rubisco activase machinery, acting as repair chaperones. Remarkably, there exist three distinct groups of rubisco activases that, despite their distant evolutionary relationships, have independently evolved to serve the same essential biochemical function.
In recent years we discovered a novel type of rubisco activase, known as the CbbQO-type, originating from chemolithoautotrophic bacteria. CbbQ is a AAA+ protein under the MoxR class and is composed of a hexameric CbbQ component alongside an adaptor molecule, CbbO, which contains a von Willebrand factor A (VWA) domain. Our lab has obtained a cryoEM structure of CbbQO-type rubisco activase.
Based on the extensive structural and biochemical data we have gathered, our findings suggest a unique mechanism employed by the CbbQO-type activase. Other AAA proteins, such as the green and red type rubisco activases (Rca) thread the Rubisco large subunit through the central pore of the hexamer. Instead, the CbbQ hexamer forms a complex with CbbO, whose finger domain is inserted into the central pore.
AF2 multimer prediction indicates that the N-terminal domain of CbbO binds to the inhibited Rubisco. The middle domain of CbbO is bound to the rim of the concave side of the CbbQ hexamer, while the finger domain (FD) extends through the pore at the centre of the hexamer. Additionally, the VWA domain is centrally located within the structure of CbbQ to interact with rubisco via an acidic residue.