Ribulose bisphosphate carboxylase/oxygenase (Rubisco), small subunit and related proteins. Rubisco is a bifunctional enzyme catalyzes the initial steps of two opposing metabolic pathways: photosynthetic carbon fixation and the competing process of photorespiration. Rubisco Form I, present in plants and green algae, is composed of eight large and eight small subunits. The nearly identical small subunits are encoded by a family of nuclear genes. After translation, the small subunits are translocated across the chloroplast membrane, where an N-terminal signal peptide is cleaved off. While the large subunits contain the catalytic activities, it has been shown that the small subunits are important for catalysis by enhancing the catalytic rate through inducing conformational changes in the large subunits. This superfamily also contains specific proteins from cyanobacteria. CcmM plays a role in a CO2 concentrating mechanism, which cyanobacteria need to to overcome the low specificity of their Rubisco and fusions to Rubisco activase, a type of chaperone, which promotes and maintains the catalytic activity of Rubisco. CcmM contains an N-terminal carbonic anhydrase fused to four copies of the Rubisco-small subunit domain
Jiyao W et al.(2023). "The conserved domain database in 2023.",