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Ubiquitin-Specific Proteases as Druggable Targets.

In much the same way as kinases and phosphatases attach and remove, respectively, phosphate groups from proteins to modulate their activity, there are a series of enzymes (E1, E2, E3) that add one or more ubiquitins onto a protein, as well as enzymes that remove them (deubiquitinases), thereby regulating their activity, location and/or rate of degradation 1. Ubiquitin is a 76 amino acid protein that is added onto lysines in the target protein through the C-terminal residue of ubiquitin: one can be added (monoubiquitination) or as many as ten can be added (polyubiquitination). For polyubiquitinated proteins these can be linear or branched chains of ubiquitin, with the complexity of branching reminiscent of the complexity of protein glycosylation 1, 2. Ubiquitin contains seven lysine residues (Lys6, Lys11, Lys27, Lys29, Lys33, Lys48, and Lys63), and the N-terminal methionine that can be ubiquitinated. Common linkages include isopeptide bonds through the K48 and K63 on ubiquitin. K48 polyubiquitinated proteins are often targeted to the proteasome for protein degradation and recycling of the ubiquitin 1. Ubiquitination of a protein can also control its activity/function, such as K63 linkages that regulate DNA damage response or cell signaling 1, 3.