In vivo homopropargylglycine incorporation enables sampling, isolation and characterization of nascent proteins from Arabidopsis thaliana.

Summary: Determining which proteins are actively synthesized at a given point in time and extracting a representative sample for analysis is important to understand plant responses. Here we show that the methionine (Met) analogue homopropargylglycine (HPG) enables Bio‐Orthogonal Non‐Canonical Amino acid Tagging (BONCAT) of a small sample of the proteins being synthesized 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 amino acid sequences and correlation with independent studies of protein labelling with 15N verified the data. We provide evidence that HPG‐based BONCAT tags a better sample of nascent plant proteins than azidohomoalanine (AHA)‐based BONCAT in Arabidopsis and show that the AHA induction of Met metabolism and greater inhibition of cell growth rate than HPG probably limits AHA incorporation at Met sites in Arabidopsis. We show HPG‐based BONCAT provides a verifiable method for sampling, which plant proteins are being synthesized at a given time point and enriches a small portion of new protein molecules from the bulk protein pool for identification, quantitation and subsequent biochemical analysis. Enriched nascent polypeptides samples 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 post‐translational modifications in plants. Significance Statement: Non‐canonical amino acids can be incorporated as proteins are made and enable a sample of newly synthesized proteins to be identified and extracted from plant cells through click chemistry, allowing rapid plant cell responses and the ageing of proteins in vivo to be investigated. We show the non‐canonical amino acid homopropargylglycine enables these aims in Arabidopsis thaliana and its use can enrich newly synthesized proteins with few post‐translational modifications. [ABSTRACT FROM AUTHOR]