How is a protein targeted to the proteasome?

Ubiquitin (Ub) is a conserved 76 amino acid protein found in all eukaryotic cells. Cytosolic proteins destined for proteasomal degradation are usually marked for destruction by covalent addition of a chain of Ub molecules (polyubiquitylation) allowing them to be bound by the 19S RP. A chain of four Ub proteins means a doomed protein: tetra-Ub is a degradation signal.