Regulation of CO2 assimilation in Chlamydomonas reinhardtii by a conditionally disordered chloroplast protein: CP12

The chloroplast protein CP12 is involved in CO2 assimilation by regulating the Calvin-Benson-Bassham (CBB) cycle. In the dark, CP12 binds to glyceraldehyde-3-phosphate dehydrogenase and phosphoribulokinase, thereby inhibiting their activity and turning off the CBB cycle. In these oxidizing conditions the protein is partially structured. However, in the light, under reducing conditions, when the protein is totally disordered, recent studies revealed that CP12 might have other functions beyond the regulation of the CBB cycle. To decipher other potential roles in Chlamydomonas reinhardtii, the unique CP12 gene was knocked out to prevent its expression. While growth rates of both wild-type and CP12 mutant strains were identical, relative quantitative proteomics revealed that the lack of CP12 re-routes the metabolism in a major way. Numerous proteins involved in redox homeostasis and stress responses were more abundant than in the wild-type strain. These results highlight CP12 as a moonlighting protein in Chlamydomonas reinhardtii with additional functions beyond its well-known regulatory role in carbon dioxide metabolism.
The diversity of these functions could be understood through a deeper characterization of its structural properties in these reducing conditions. To this day, CP12 has only been studied when isolated in solvent. However, the real environment of the chloroplast is heterogeneous and crowded by many macromolecules influencing its conformational and thermodynamical properties. To mimic the chloroplast environment, different complex media have been used such as crowding agents and sonicated cells. Our preliminary results show that crowding contributes less to the folding than others parameters such as pH or redox conditions. CP12, as a conditionally disordered protein, is a good candidate to show the influence of physicochemical parameters on proteins conformation and regulation of metabolic pathways.