In vivo homopropargylglycine incorporation enables nascent protein tagging, isolation and characterisation from Arabidopsis thaliana

Determining which proteins are actively synthesised at a given point in time and extracting them for analysis is important to understand plant responses. Here we show that the methionine (Met) analogue homopropargylglycine (HPG) enables BONCAT (Bio-Orthogonal Non-Canonical Amino acid Tagging) of proteins being synthesised in Arabidopsis plants or cell cultures, facilitating their click-chemistry enrichment for analysis. The sites of HPG incorporation could be confirmed by peptide mass spectrometry at Met-sites throughout protein AA sequences and correlation with independent studies of protein labelling with 15N verified the data. We provide evidence that HPG-based BONCAT tags nascent plant proteins more efficiently than azidohomoalanine (AHA)-based BONCAT in Arabidopsis and show that AHAs induction of Met metabolism and greater inhibition of cell growth rate than HPG likely limits AHA incorporation at Met sites in Arabidopsis. We show HPG-based BONCAT provides a verifiable method for determining which plant proteins are being synthesised at a given time point and enriches new protein molecules from the bulk protein pool for identification, quantitation and subsequent biochemical analysis. Enriched nascent polypeptides were found to contain significantly fewer common post-translationally modified residues than the same proteins from whole plant extracts, providing evidence for age-related accumulation of PTMs in plants.