Ubiquitination: RING for destruction?

Targeted protein proteolysis is increasingly understood to be an important general mechanism by which cells regulate protein levels — and consequently their functions at specific times. In eukaryotic cells, the main mechanism for such control involves the specific covalent modification by polyubiquitin, which labels target proteins for proteolysis and subsequent degradation. There are many known examples of such events, and ubiquitination is now recognised as a major mechanism for cellular regulation, perhaps even vying with phosphorylation in general importance. But how does the cell know which proteins to target?