The role of the adaptor protein CbbO in the Rubisco activation system of chemoautotrophic bacteria

Ribulose 1, 5-bisphosphate carboxylase/oxygenase (Rubisco) is responsible for almost all biological CO2 assimilation, but has a tendency to form dead-end inhibition complexes with sugar phosphates, including its own substrate ribulose 1,5-bisphosphate (RuBP). AAA+ molecular chaperones Rubisco activases (Rca) remodel inhibited Rubisco to effectively release the inhibitors. Recently, a new class of Rca from the chemolithoautotrophic bacteria Acidithiobacillus ferrooxidans was characterized. This new Rubisco activation system is unique in requiring the participation of both the AAA+ motor CbbQ, and a VWA (von Willebrand factor type A) domain-containing CbbO as an adaptor to be functional. The CbbO protein is thus far poorly characterized and its role in Rubisco activation is unclear. In this work, biochemical conditions that allow the isolation of a stabilized Rubisco-CbbQO complex through chemical cross-linking were established. Cryo-electron microscopy revealed that, unlike other activases studied so far, the convex surface of the CbbQ hexamer is not involved in Rubisco remodeling. Instead, mechanical force for disruption of the active site is likely transmitted to Rubisco via the flexible adaptor CbbO featured on the concave face of CbbQ. Interaction of CbbO with the C-terminus of the Rubisco large subunit, which governs the accessibility of the active site would trigger a sequence of conformational changes that would ultimately result in liberation of the inhibitor.Doctor of Philosoph