Free polyubiquitin chains are cleaved by DUBs to produce monoubiquitin. The chains may be produced by the E1-E2-E3 machinery in the cell free from any substrate protein. Another source of free polyubiquitin is the product of ubiquitin-substrate cleavage. If DUBs cleave the base of the polyubiquitin chain that is attached to a protein, the whole chain will become free and needs to be recycled by DUBs.
