From early proteomics developments to protein N-terminal acetylation in plant cells

Proteomics is a relatively young science that focuses on characterising the proteins contained in an organelle, cell or tissue at a given time. While this technique was initially based on the separation of proteins on polyacrylamide gel followed by their identification by Edman sequencing, this approach has gained in importance with the development of the mass spectrometry instrumentation. My thesis project on the 'Molecular Scanner' is an example of this fact which required the development of new experimental, analytical and signal processing techniques. My work subsequently led me to develop techniques for characterising the N-terminal sequences of proteins at high throughput and, in particular, to quantify the level of acetylation using the SILProNAQ technique. These results have led to the creation of an open-access database containing experimentally characterised protein N-termini for several species, including humans and the model plant A. thaliana. This unique resource has led to a better understanding and modelling of protein N-terminal maturation phenomena and, in particular, the cleavage site of transit peptides for nuclear proteins targeting the chloroplast or mitochondria. I am also interested in the enzymes responsible for N-terminal acetylation both in the cytosol in A. thaliana and H. sapiens, and in organelles of which the chloroplast. Several collaborative projects have demonstrated the involvement of this modification in the response of plants to biotic and abiotic stresses. A prospective study enabled to identify several N-terminal acetylases specific to the chloroplast. The development of the GAP assay has made it possible to determine the specificity of recombinant acetylases in cellulo, although their biological justification is not yet known. During this investigation, it became apparent that these acetylases target both the N-termini of proteins and internal lysines, which runs counter to the current paradigm. While N-terminal acetylation has long been associated with protein stability over time, it has never been possible to compare the N-terminal status of proteins with their half-life on a large scale, particularly in plants. Building on the knowledge acquired and accumulated on N-terminal acetylation in several species, we now need to develop an approach for determining the half-life of proteins in plants using a labelling technique based on the use of stable isotopes. This investigation will provide a better understanding of the influence of this modification on the fate of proteins and its implication in the regulation of stresses such as hydric stress or the phenotypic influence on the N-terminome of cultivated species in adapting to climate change.