Mass Spectrometry Characterization of Post-Translational Modifications on the Arabidopsis Growth Regulator Protein RGA

The world population has increased several-fold in the past century, but food production has increased at a greater rate, thanks to scientific developments like the Green Revolution. The Green Revolution included the development of sturdy semi-dwarf crops, which were able to bear heavy heads of grain without falling over in wind or rain. These plants were later found to have mutations in the gibberellin signaling pathway, including DELLA proteins. These master regulator proteins integrate multiple hormonal and environmental signaling pathways in order to repress plant growth. Gibberellin represses DELLA activity and causes its proteasomal degradation. DELLA proteins are also affected by post-translational modifications (PTMs). PTMs like phosphorylation have varied and important effects on proteins, influencing their stability, structure, activity, and interactions with other proteins. Mass spectrometry is the premier technique for studying protein PTMs; it can determine the precise sites and levels of PTMs on a protein. This work used tandem mass spectrometry to study REPRESSOR OF GA1-3 (RGA), a DELLA protein in the model plant Arabidopsis thaliana. RGA has two poly-S/T stretches that are heavily modified with different PTMs. Phosphorylation, O-GlcNAcylation, O-hexosylation, and O-fucosylation levels and sites on RGA were characterized in its native Arabidopsis. Different Arabidopsis mutants were examined, including the gibberellin biosynthesis enzyme GA1, the adaptor subunit of the SCF E3 ubiquitin ligase complex SLY1, the O-GlcNAcyltransferase SEC, and the O-fucosyltransferase SPY. A new modification of RGA was discovered: acetylation. This is some of the first acetylation seen on any DELLA protein in any organism. Additionally, phosphorylation and acetylation levels were correlated and may influence RGA stability.