A robust method for identification of plant SUMOylation substrates in a library-based reconstitution system

Dear Editor,SUMOylation,which transfers a small ubiquitin-like protein to the lysine residues of target proteins,is an important type of posttranslational modification in eukaryotic cells.This modification plays a critical role in plant development and stress responses(Miura and Hasegawa,2010);thus,the efficient identification of SUMOylation substrates in plant cells is a fundamental issue in the field.In Arabidopsis,hundreds of SUMOylated proteins have been identified by a mass spectrometry(MS)approach in which modified targets in transgenic plants harboring an H89R variant of SUMO1 are enriched via affinity chromatography and a four-residue footprint is left on the substrates after trypsin digestion for MS analysis(Miller et al.,2010).Given that antibodies generated against SUMO molecules are not very specific,it is impractical to catch endogenous SUMO moieties via antibody-based one-step purification.Thus,it is difficult to obtain SUMOylation substrates via existing approaches in plant species that cannot be efficiently transformed.Given that SUMOylation is highly reversible(Yates et al.,2016),another problem is that the modification may be removed quickly during sample preparation.In addition,based on the efficiency and sensitivity of affinity enrichment and MS,substrates with low protein levels may be missed in the analyses.