Isolation and Characterization of the Prochlorococcus Carboxysome Reveal the Presence of the Novel Shell Protein CsoS1D

Cyanobacteria, including members of the genusProchlorococcus , contain icosahedral protein microcompartments known as carboxysomes that encapsulate multiple copies of the CO2 -fixing enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) in a thin protein shell that enhances the catalytic performance of the enzyme in part through the action of a shell-associated carbonic anhydrase. However, the exact mechanism by which compartmentation provides a catalytic advantage to the enzyme is not known. Complicating the study of cyanobacterial carboxysomes has been the inability to obtain homogeneous carboxysome preparations. This study describes the first successful purification and characterization of carboxysomes from the marine cyanobacteriumProchlorococcus marinus MED4. Because the isolatedP. marinus MED4 carboxysomes were free from contaminating membrane proteins, their protein complement could be assessed. In addition to the expected shell proteins, the CsoS1D protein that is not encoded by the canonicalcso gene clusters of α-cyanobacteria was found to be a low-abundance shell component. This finding and supporting comparative genomic evidence have important implications for carboxysome composition, structure, and function. Our study indicates that carboxysome composition is probably more complex than was previously assumed based on the gene complements of the classicalcso gene clusters.