Protein and nonprotein targets of ubiquitin modification

Ubiquitin regulates a wide variety of biological functions by modifying diverse substrates, via many different conjugation types. Classically, the C-terminus of ubiquitin conjugates to protein substrates via an isopeptide or peptide bond. Recent studies revealed that ubiquitin can form an atypical oxyester bond, which can target protein and even nonproteinaceous substrates, including sugars and lipids. How nonprotein ubiquitination affects substrate and cellular functions is incompletely understood. This review covers recent discoveries in ubiquitination and its potential impacts on biology.