Unraveling K63 Polyubiquitination Networks by Sensor-Based Proteomics

The polybiquitination of proteins can take on different topologies depending on the residue from ubiquitin involved in the chain formation. Although the role of lysine-48 (K48) polyubiquitination in proteasome-mediated degradation is fairly well characterized, much less is understood about the other types of ubiquitin chains and proteasome-independent functions. To overcome this, we developed a K63 polyubiquitin-specific sensor-based approach to track and isolate K63 polyubiquitinated proteins in plants. Proteins carrying K63 polyubiquitin chains were found to be enriched in diverse membrane compartments as well as in nuclear foci. Using liquid chromatography-tandem mass spectrometry, we identified over 100 proteins from Arabidopsis (Arabidopsis thaliana) that are modified with K63 polyubiquitin chains. The K63 ubiquitinome contains critical factors involved in a wide variety of biological processes, including transport, metabolism, protein trafficking, and protein translation. Comparison of the proteins found in this study with previously published nonresolutive ubiquitinomes identified about 70 proteins as ubiquitinated and specifically modified with K63-linked chains. To extend our knowledge about K63 polyubiquitination, we compared the K63 ubiquitinome with K63 ubiquitination networks based on the Arabidopsis interactome. Altogether, this work increases our resolution of the cellular and biological roles associated with this poorly characterized posttranslational modification and provides a unique insight into the networks of K63 polyubiquitination in plants.