Protein arginylation is a post-translational modification at the N-terminus of proteins and is crucial for viability and physiology in higher eukaryotes. The lack of arginylation causes severe developmental defects in plants and embryo lethality in Drosophila and in mice. Although several studies investigated impact and function of the responsible enzyme, the arginyl-tRNA protein transferase (ATE) in plants, identification of arginylated proteins by mass spectrometry was not hitherto achieved. In the present study, we report the identification of targets and interaction partners of ATE in the model plant Physcomitrella patens by mass spectrometry employing two different immuno-affinity strategies and a recently established transgenic ATE:GUS reporter line (Schuessele et al., 2015 New Phytol., DOI: 10.1111/nph.13656). Here we use a commercially available antibody against the fused reporter protein (GUS) to pull down ATE and its interacting proteins and validate the in vivo interaction with a class I small heatshock protein via Förster resonance energy transfer (FRET).