Ubiquitin: not just for proteasomes anymore

Ubiquitin is a small protein that can be covalently linked to itself or other proteins, either as single ubiquitin molecules or as chains of polyubiquitin. Addition of ubiquitin to a target protein requires a series of enzymatic activities (by ubiquitin-activating, -conjugating and -ligating enzymes). The first function attributed to ubiquitin was the covalent modification of misfolded cytoplasmic proteins, thereby directing proteasome-dependent proteolysis. More recently, additional functions have been ascribed to ubiquitin and ubiquitin-related proteins. Ubiquitin directs specific proteins through the endocytic pathway by modifying cargo proteins, and possibly also components of the cytoplasmic protein trafficking machinery.