Phosphoproteomic analysis of nuclei-enriched fractions from Arabidopsis thaliana.

Phosphorylation is a ubiquitous regulatory mechanism, that governs the activity, subcellular localisation and molecular interactions of proteins. To identify a broad range of nuclear phosphoproteins from Arabidopsis thaliana, we enriched for nuclei from suspension cell cultures and seedlings before extensive fractionation and identification of phosphopeptides by mass spectrometry. We identified 416 phosphopeptides from 345 proteins with high confidence. Our data show that sub-cellular fractionation is an effective strategy for identifying nuclear phosphoproteins, two thirds of our dataset are known or predicted to be nuclear localised and one half of the nuclear localised proteins have novel phosphorylation sites. We identified novel phosphorylation sites on transcription factors, chromatin remodelling proteins, RNA silencing components and the spliceosome. Intriguingly, we also identified phosphorylation sites on several proteins associated with Golgi vesicle trafficking such as the exocyst complex, and speculate that these may be involved in cell plate formation during cytokinesis.