Control of protein life-span by N-terminal methionine excision

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Abstract
Peptide deformylases (PDFs) have been discovered recently in eukaryotic genomes, and it appears that N‐terminal methionine excision (NME) is a conserved pathway in all compartments where protein synthesis occurs. This work aimed at uncovering the function(s) of NME in a whole proteome, using the chloroplast‐encoded proteins of both Arabidopsis thaliana and Chlamydomonas reinhardtii as model systems. Dis ruption of PDF1B in A.thaliana led to an albino phenotype, and an extreme sensitivity to the PDF‐ specific inhibitor actinonin. In contrast, a knockout line for PDF1A exhibited no apparent phenotype. Photosystem II activity in C.reinhardtii cells was substantially reduced by the presence of actinonin. Pulse–chase experiments revealed that PDF inhibi tion leads to destabilization of a crucial subset of chloroplast‐encoded photosystem II components in C.reinhardtii . The same proteins were destabilized in pdf1b . Site‐directed substitutions altering NME of the most sensitive target, subunit D2, resulted in similar effects. Thus, plastid NME is a critical mechanism specifically influencing the life‐span of photosystem II polypeptides. A general role of NME in modulating the half‐life of key subsets of proteins is suggested.