Insights into ubiquitin chain architecture using Ub-clipping

Protein ubiquitination is a multi-functional post-translational modification that affects all cellular processes. Its versatility arises from architecturally complex polyubiquitin chains, in which individual ubiquitin moieties may be ubiquitinated on one or multiple residues, and/or modified by phosphorylation and acetylation(1-3). Advances in mass spectrometry have enabled the mapping of individual ubiquitin modifications that generate the ubiquitin code; however, the architecture of polyubiquitin signals has remained largely inaccessible. Here we introduce Ub-clipping as a methodology by which to understand polyubiquitin signals and architectures. Ub-clipping uses an engineered viral protease, Lb(pro)*, to incompletely remove ubiquitin from substrates and leave the signature C-terminal GlyGly dipeptide attached to the modified residue; this simplifies the direct assessment of protein ubiquitination on substrates and within polyubiquitin. Monoubiquitin generated by Lb(pro)* retains GlyGly-modified residues, enabling the quantification of multiply GlyGly-modified branch-point ubiquitin. Notably, we find that a large amount (10-20%) of ubiquitin in polymers seems to exist as branched chains. Moreover, Ub-clipping enables the assessment of co-existing ubiquitin modifications. The analysis of depolarized mitochondria reveals that PINK1/parkin-mediated mitophagy predominantly exploits mono- and short-chain polyubiquitin, in which phosphorylated ubiquitin moieties are not further modified. Ub-clipping can therefore provide insight into the combinatorial complexity and architecture of the ubiquitin code.