Cyclisation of Lys48-linked diubiquitin in vitro and in vivo

► Commercial Lys48-linked diubiquitin contains cyclic diubiquitin. ► Cyclic Lys48-linked diubiquitin is unable to bind to the ZnF-UBP of isopeptidase-T. ► Endogenous cyclic Lys48-linked diubiquitin is present in mammalian cells.