A peripheral carboxysome component regulates cyanobacterial glycogen deposition.

In cyanobacteria, carbohydrate products of the Calvin cycle can be utilized as precursors for cellular biosynthesis or stored as glucose polymers in glycogen granules. Cyanobacterial carbon fixation occurs in the carboxysome, while glycogen is synthesized in the cytosol via GlgC to support respiration and survival in the dark. Here, we identify a carboxysome-associated protein, Carboxysome Interacting Protein G (CipG), that appears to physically link these processes. CipG coassociates in cyanobacterial genomes with peripheral carboxysome structural proteins and localizes to carboxysomes in vivo via a process that requires conserved tryptophan residues. CipG coprecipitates with GlgC and is required for the recruitment of a subpopulation of GlgC to the surface of the carboxysome. CipG mutants display a glycogen deficiency, while CipG overexpression increases glycogen accumulation ~sixfold compared to wild type. Taken together, our results highlight a role for CipG in cyanobacterial energy metabolism and suggest an additional capacity for the carboxysome to scaffold key enzymes related to carbon partitioning in cyanobacteria.