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Towards a better understanding of the regulation of chloroplast biogenesis and functions

We have identified new proteins in the chloroplast envelope. The functioning of this cellular organelle, the site of photosynthesis, is still very poorly understood. These data constitute a new resource to define the molecular principles and mechanisms controlling the fundamental aspects of biogenesis and chloroplast functions.

Published on 10 July 2019
Cells from higher plants and algae contain a compartment specific to the plant kingdom: the chloroplast. This organelle is the place of oxygen production and of synthesis of vitamins, amino acids, fatty acids, lipids, starch... i.e. most compounds essential for our diet and sources of bioenergy. To synthesize these molecules, the chloroplast only needs CO2, water, minerals and some metals. The energy used to catalyze all these reactions derives from the conversion of solar energy into chemical energy through photosynthesis. However, the mechanisms that regulate dynamic exchanges between the chloroplast and other cell compartments still remain poorly understood.

Current estimates suggest that the chloroplast contains about 3500 different proteins catalyzing above-cited reactions. Variations in environmental conditions (light, temperature, etc.) perceived by plants require constant adaptations of the chloroplast composition to optimize its functioning. However, the vast majority of these 3500 proteins must be imported into the chloroplast by mechanisms that, for some of them, remain unknown. These mechanisms are themselves catalyzed by yet unidentified proteins located in two biological membranes (the envelope) that surround the chloroplast. As these envelope proteins are very rare at the chloroplast and plant cell scales, their identification required the use of cell fractionation (to enrich them, Figure) and proteomics approaches (to identify them). During this study, we have in collaboration with the the Large Scale Biology Laboratory combined biochemical, imaging, mass spectrometry, bioinformatics and statistical approaches to reveal previously unidentified protein components present in these membranes.

Fractionation of the chloroplast and relative abundance of proteins present in each sub-compartment.

This study sheds new light on the composition of the chloroplast envelope membranes, and opens new perspectives in understanding mechanisms that regulate the dynamics of chloroplast biogenesis and functions. Efforts to characterize the role of novel proteins identified during these approaches are performed in the two above-cited IRIG's laboratories.

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