How is the protein targeted to the proteasome? (pt.2)

Ubiquitin is normally added covalently to the side chain of an available LYSINE residue on the target molecule. The process is repeated using side chains of lysine residues in ubiquitin, until a chain of at least 4 Ub is completed. This multi-ubiquitin tag is a degradation signal. Polyubiquitylated proteins can be bound by the proteasomal 19S RP.