Uncovering DUB Selectivity Through Ion-Mobility-Based Assessment of Ubiquitin Chain Isomers

Ubiquitination is a reversible posttranslational modification that maintains cellular homeostasis and regulates protein turnover. Deubiquitinases (DUBs) are a large family of proteases that catalyze the removal of ubiquitin (Ub) along with the dismantling and editing of Ub chains. Assessing the activity and selectivity of DUBs is critical for defining physiological function. Despite numerous methods for evaluating DUB activity, none are capable of assessing activity and selectivity in the context of multicomponent mixtures of native, unlabeled ubiquitin conjugates. Here we report on an ion mobility (IM)-based approach for measuring DUB selectivity in the context of unlabeled mixtures of Ub chains. We show that IM-MS can be used to assess the selectivity of DUBs in a time-dependent manner. Moreover, using the branched Ub chain selective DUB UCH37/UCHL5 along with a mixture of Ub trimers, a strong preference for branched Ub trimers bearing K6 and K48 linkages is revealed. Our results demonstrate that IM coupled with mass spectrometry (IM-MS) is a powerful method for evaluating DUB selectivity under conditions more physiologically relevant than single component mixtures.