Cyanobacterial α-carboxysome carbonic anhydrase is allosterically regulated by the Rubisco substrate RuBP

Cyanobacterial CO2 concentrating mechanisms (CCMs) sequester a globally significant proportion of carbon into the biosphere. Proteinaceous microcompartments, called carboxysomes, play a critical role in CCM function, housing two enzymes to enhance CO2 fixation: carbonic anhydrase (CA) and Rubisco. Despite its importance, our current understanding of the carboxysomal CAs found in [a]-cyanobacteria, CsoSCA, remains limited, particularly regarding the regulation of its activity. Here, we present the first structural and biochemical study of CsoSCA from the cyanobacterium Cyanobium PCC7001. Our results show that the Cyanobium CsoSCA is allosterically activated by the Rubisco substrate ribulose-1,5-bisphosphate (RuBP), and forms a hexameric trimer of dimers. Comprehensive phylogenetic and mutational analyses are consistent with this regulation appearing exclusively in cyanobacterial [a]-carboxysome CAs. These findings clarify the biologically relevant oligomeric state of -carboxysomal CAs and advance our understanding of the regulation of photosynthesis in this globally dominant lineage. One-Sentence SummaryThe carboxysomal carbonic anhydrase, CsoSCA, is allosterically activated by the Rubisco substrate RuBP, revealing a novel mechanism controlling key enzyme activity in cyanobacterial -carboxysomes.