Structure of tetraubiquitin. Four ubiquitin molecules are linked by isopeptide bonds. Notice that each isopeptide bond is formed by the linkage of the carboxylate group at the end of the extended C terminus with the ϵ-amino group of a lysine residue. Dashed lines indicate the positions of the extended C-termini that were not observed in the crystal structure. This unit is the primary signal for degradation when linked to a target protein.
[Drawn from 1TBE. pdb.]