Eukaryotic cells contain many compartments and this compartmentalization actively participates in the control of biological processes. An essential compartment of the plant cell is the chloroplast, the place where, among other functions, photosynthesis occurs. In order to achieve this compartmentalization, the cell has established precise mechanisms to specifically address a protein encoded by a nuclear gene in a targeted cell compartment. Sometimes, however, the function catalyzed by a protein is required in more than one cell compartment. In this case, the cell usually produces several isoforms of this protein, which can be addressed to separate cell compartments. A new mechanism has recently been discovered that allows a single protein to be addressed to multiple compartments.

In plant cells, it is accepted that the regulation of the targeting of proteins to the chloroplast is essential to control the main functions of this organelle (photosynthesis, synthesis of vitamins, sugars, lipids, amino acids...). When environmental conditions fluctuate (light, temperature, hygrometry, etc...), plant cells adapt the protein composition of the chloroplast in order to optimize its functioning. Researchers at our institute have shown that the dynamics of the protein content of the chloroplast is not only the result of variations in the abundance of each plastid protein, but also of the control of the localization of specific proteins within the cell. Some chloroplast proteins can indeed be trapped in another cell compartment (the cytosol) through interaction with a partner protein (a calmodulin). The abundance of this calmodulin therefore regulates the ability of the targeted plastid proteins to reach, or not, the chloroplast.

This study sheds new light on the control of chloroplast biogenesis and opens up new perspectives in the understanding of the mechanisms that regulate its functioning. More generally, this work raises the question of whether the plant cell uses this process to control the composition of other cell compartments, or even, whether this mechanism is present in other eukaryotic kingdoms to orchestrate protein targeting to other cell compartments since calmodulins are proteins conserved in all eukaryotes.


Differential localization of the same chloroplastic protein according to its ability to bind its cytosolic partner (calmodulin). Red: autofluorescence of the chlorophyll present in the chloroplasts. Green: fluorescence of the GFP (Green Fluorescence Protein) fused to the plastid protein analyzed. On the left, the protein (green fluorescence) is located in the cytosol. Chloroplasts appear in red. On the right, the orange fluorescence indicates that the protein is addressed to the chloroplast where the red fluorescence of the chlorophyll and the green fluorescence of the protein overlap.