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Imen Bouchnak

Chloroplast biogenesis: Alternative protein targeting pathways

Published on 1 October 2018
Thesis presented October 01, 2018

Chloroplasts are a major component of plant cells. Their origin traces back to a cyanobacterial ancestor that was engulfed by an ancient eukaryotic cell and eventually integrated as an organelle during evolution. As a result, more than 95% of the ancestral cyanobacterial genes were transferred to the host cell nucleus. Proteins encoded by these relocated genes need to return to internal chloroplast compartments. This import is mainly achieved by the general TOC/TIC machinery located at the chloroplast surface. Until recently, all proteins destined to chloroplast were believed to possess an N-terminal and cleavable chloroplast targeting peptide, and to engage the TOC/TIC machinery. However, recent studies have revealed the existence of several non-canonical preproteins, lacking cleavable transit peptides. The first evidence for such ‘non-canonical’ chloroplast proteins was provided by our team studying the Arabidopsis chloroplast envelope proteome, leading to the identification of a quinone oxidoreductase homologue termed « ceQORH ». Furthermore, a few such proteins were demonstrated to use alternative targeting pathways, independent of the TOC/TIC machinery. To better characterize components of such alternative targeting machineries, a targeted study combining affinity purification and mass spectrometry aiming to identify alternative receptors at the chloroplast surface has been performed. This study allowed us to identify new “partner” involved in the control of chloroplast targeting of ceQORH protein. Alternatively, we also revisited the chloroplast envelope proteome composition and initiated a gene candidate approach. In addition, some non-canonical proteins are shared by plastids and other cell compartments. However, molecular mechanisms controlling subcellular localization of these non-canonical plastid proteins remain unknown. In order to explain the variable subcellular localization of ceQORH protein, our team hypothesized a probable interaction of ceQORH with a cytosolic partner. In the last part of this study, we validated the interaction between ceQORH and its partner in planta by a genetic approach analyzing the impact of the absence of the cytosolic partner on the regulation of the sub-cellular localization of ceQORH protein.

Chloroplast, Arabidopsis, envelope, proteome, alternative, targeting, pathways, non-canonical, protein.

On-line thesis.

Chloroplast, Arabidopsis, envelope, proteome, alternative, targeting, pathways, non-canonical, proteins