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Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyses the fixation of atmospheric CO₂ in photosynthesis, but tends to form inactive complexes with its substrate ribulose 1,5-bisphosphate (RuBP). In plants, Rubisco is reactivated by the AAA⁺ (ATPases associated with various cellular activities) protein Rubisco activase (Rca), but no such protein is known for the Rubisco of red algae. Here we identify the protein CbbX as an activase of red-type Rubisco. The 3.0-Å crystal structure of unassembled CbbX from Rhodobacter sphaeroides revealed an AAA⁺ protein architecture. Electron microscopy and biochemical analysis showed that ATP and RuBP must bind to convert CbbX into functionally active, hexameric rings. The CbbX ATPase is strongly stimulated by RuBP and Rubisco. Mutational analysis suggests that CbbX functions by transiently pulling the carboxy-terminal peptide of the Rubisco large …