A snapshot of the Physcomitrella N-terminome reveals N-terminal methylation of organellar proteins

Post- or co-translational modifications of protein N-termini influence their half-life and mediate protein sorting to organelles via cleavable N-terminal sequences that are recognized by the respective translocation machinery. In the present study, we provide an overview on the current modification state of the N-termini of over 4500 proteins from the model moss species Physcomitrella (Physcomitrium patens) using a compilation of 24 N-terminomics datasets. Our data reveal distinct proteoforms and modification states and confirm predicted targeting peptide cleavage sites of 1144 proteins localized to plastids and the thylakoid lumen, to mitochondria, and to the secretory pathway, respectively. Additionally, we uncover extended N-terminal methylation of mitochondrial proteins. Moreover, we identified PpNTM1 (P. patens alpha N-terminal protein methyltransferase 1) as a promising candidate for protein methylation in plastids, mitochondria and the cytosol. These data can now be used to optimize computational targeting predictors, for customized protein fusions and their targeted localization in biotechnology, and it provides novel insights into potential dual targeting of proteins.